Tapping the Potential of Urban Rooftops

Transcription

Final Repor t
Ta p p i n g t h e P o t e n t i a l o f U r b a n R o o f t o p s
Roof top R e so u rce s N e igh borhood A s s e s s m e nt
October 31, 2007
D E S I G N ,
C O M M U N I T Y
&
E N V I R O N M E N T
Final Repor t
Ta p p i n g t h e P o t e n t i a l o f U r b a n R o o f t o p s
Roof top R e so u rce s N e igh borhood A s s e s s m e nt
Bay Localize is an Oakland-based organization that catalyzes
a shift from a globalized, fossil fuel-based economy to a localized
green economy that strengthens all Bay Area communities. Bay
Localize is a nonprofit project of the Earth Island Institute.
This report is generously supported by the Community
Foundation Silicon Valley, Laurence Levine Charitable Fund, San
Francisco Foundation, Ollie Fund, and Bay Localize supporters.
It was prepared by Brian Holland and Sarah Sutton of Design,
Community and Environment, Kate Stillwell of Holmes Culley, and
Ingrid Severson and Kirsten Schwind of Bay Localize.
For more information, contact:
Bay Localize
436 14th Street, Ste 1127
Oakland, CA 94612
510-834-0420
www.baylocalize.org
October 31, 2007
D E S I G N ,
C O M M U N I T Y
&
E N V I R O N M E N T
TABLE OF CONTENTS
1.
EXECUTIVE SUMMARY/INTRODUCTION ........................................................ 1-1
2.
EXISTING CONDITIONS .............................................................................. 2-1
3.
ROOFTOP RESOURCE PROTOTYPES ............................................................. 3-1
4.
FINDINGS .................................................................................................. 4-1
Appendices
Appendix A: Assumptions and Methodology
i
B
T
R
T
A Y L O C A L I Z E
A P P I N G T H E P O T E N T I A L O F U R B A N R O O F T O P S
O O F T O P R E S O U R C E S N E I G H B O R H O O D A S S E S S M E N T
A B L E O F C O N T E N T S
List of Figures
Figure 2-1. Aerial view of buildings in study area with existing
rooftop resources....................................................................... 2-4
Figure 2-2. Aerial view of study area indicating distribution of
building types. ......................................................................... 2-14
Figure 3-1. Cross-section of Extensive Green Roof Prototype. .................. 3-3
Figure 3-2. Cross-section of Intensive Green Roof—Vegetables
prototype................................................................................... 3-9
Figure 3-3. Cross-section of intensive Green Roof—Herbs
prototype................................................................................. 3-14
Figure 3-4. Cross-section of Rooftop Hydroponic Garden
prototype................................................................................. 3-18
Figure 3-5. Diagram of assembly of rainwater catchment
system using 50-gallon drum. .................................................. 3-24
Figure 3-6. Diagram of integrated Rainwater Harvesting and
Solar Photovoltaics prototypes. .............................................. 3-25
Figure 4-1. Aerial view of study area with buildings assigned
rooftop resources prototypes. ................................................... 4-3
List of Tables
Building Typology-Typical Characteristics ............................ 2-12
Table 2-1
Table 2-2
Building Type Distribution..................................................... 2-13
Table 3-1
Prototype Characteristics.......................................................... 3-2
Table 4-1
Prototype Assignment and Productivity ................................ 4-10
ii
1
EXECUTIVE SUMMARY/INTRODUCTION
“Built-out” is a phrase often used in planning and development fields to describe dense, urban communities that have few remaining vacant buildable
parcels. As the Bay Area adopts smart growth and transit-oriented development policies emphasizing high-density housing, neighborhoods throughout
San Francisco, the East Bay, Peninsula, and South Bay are becoming increasingly built-out. This density presents a challenge in identifying available land
for important uses such as open space, community gardens, and stormwater
and energy infrastructure. In cities across the country, however, a new landscape is being discovered where building rooftops meet the sky.
Previously regarded as unusable space, the landscape of rooftops is being reclaimed for productive and sustainable purposes. Whereas in the past, roofs
have been a liability—emitting heat into the urban atmosphere, shedding pollutants into the watershed, requiring costly repair and replacement—some
cities are transforming roofs into assets. They are being used as catchment
areas for irrigation water, renewable energy platforms, recreational open
space, food and educational gardens, reduction of stormwater surges, and aesthetic improvement. In short, rooftops are being harnessed to improve cities
and enhance the quality of life of inhabitants.
A rooftop resource development philosophy is emerging and taking root in
the Bay Area. Building owners and developers are looking at the options of
solar power, rainwater catchment and living roofs to maximize their buildings’ efficiency and function. Designers and planners are coming together to
map out strategies for green roof implementation. Public works departments
and utilities are stimulating adoption of solar photovoltaic systems. And citizens are seeking ways to better utilize rooftops for energy, food and community empowerment.
I.
PROJECT OBJECTIVES
Information on green roofs, solar technologies, and rainwater harvesting is
available in abundance. This study seeks to fill gaps in that knowledge, par-
1-1
Rooftop garden atop St. Simon Stock
Catholic School Bronx, New York.
Source: St. Simon Stock Catholic
School.
B A Y L O C A L I Z E
T A P P I N G T H E P O T E N T I A L O F U R B A N R O O F T O P S
R O O F T O P R E S O U R C E S N E I G H B O R H O O D A S S E S S M E N T
E X E C U T I V E S U M M A R Y / I N T R O D U C T I O N
ticularly with regard to low-cost strategies on existing buildings and the potential productivity for future developments. The study analyzes rooftop
resource implementation and benefits for the Eastlake district in Oakland.
Objectives include:
♦ Analysis of the suitability of rooftop resource strategies in different built
contexts, highlighting retrofits to existing buildings without structural
improvement;
♦ Design of conceptual rooftop resource prototypes that are feasible for existing buildings;
♦ Analysis of productivity for edible garden designs on future development
in the area; and
♦ Quantification of the productivity benefits of rooftop gardens, renewable
energy, and rainwater catchment technologies.
Several unique contributions are addressed in this study, including:
♦ Focus on Existing Buildings. Most informational resources for green
roof development focus on new construction; therefore, less information
is available for building owners and policymakers to use when considering the potential for green roof retrofits on existing buildings.
♦ Regional Context. Much of the available information on green roofs
was developed in different social, political, economic, environmental and
meteorological contexts, from Chicago to Germany to Portland to Montreal. Also, while rooftop resource development in cities across the US
and the globe is supported with public financial incentives, the Bay Area
and the state of California fall short in implementing many of these policies.
♦ Urban Agriculture. This study also differs from many existing documents in that an emphasis is placed on rooftop vegetable gardening as a
strategy for intensifying urban agriculture activities, which can improve
nutrition and food security in urban neighborhoods while reducing dependence on an energy-intensive global food economy.
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B A Y L O C A L I Z
T A P P I N G T H E P O T E N T I A L O F U R B A N R O O F T O P
R O O F T O P R E S O U R C E S N E I G H B O R H O O D A S S E S S M E N
E X E C U T I V E S U M M A R Y / I N T R O D U C T I O
E
S
T
N
♦ Neighborhood Scale. This study looks beyond the analysis of green
roof benefits at the building scale to focus primarily on projecting outcomes at the neighborhood scale.
II.
PROJECT APPROACH
As detailed in later chapters, this assessment analyzes the potential for green
roofs, rooftop gardens, solar photovoltaics, and rainwater harvesting on existing buildings and future developments, and identifies possible benefits to the
Eastlake neighborhood in Oakland. A model was developed for this study to
produce quantitative estimates of rooftop productivity.
Buildings in the Study Area were categorized into types to generalize their
characteristics, including the weight-bearing capacity of the roof structure.
Rooftop resource prototypes were then designed to serve as test retrofits,
providing data on loading characteristics. The prototypes were tailored to
meet the special needs of existing buildings and were correlated with productivity estimates per square foot. Prototypes were then assigned to each building based on their suitability. Vacant lots were categorized as “opportunity
sites” that could hold intensive, edible roof gardens. Finally, the total area
and productivity estimates of each prototype were used to determine aggregate benefits to the Eastlake Study Area.
III.
PROJECT FINDINGS
The findings of the assessment demonstrate a great deal of potential for harvesting food, energy, and water on Bay Area roofs. Rooftop gardens, solar
photovoltaic systems, and rainwater harvesting technologies can all be fitted
on existing buildings. There are clear opportunities and constraints to each
strategy as well as some surprising benefits. In addition to well-documented
benefits such as water quality and energy efficiency improvements, provision
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of open space amenities, and aesthetic appeal, rooftop resources in the Study
Area can provide:
♦ Clean, renewable electricity satisfying approximately 25 percent of demand;
♦ Fresh, leafy-green vegetables for all area residents; and
♦ Supplemental rainwater for irrigation for approximately 83 percent of
the area’s buildings
These benefits are attainable, but not without significant effort invested by
State and local government, the private sector, communities and individual
households.
IV.
REPORT STRUCTURE
The report is organized into four chapters: Introduction, Documentation of
Existing Conditions, Description of the Rooftop Resource Prototypes, and
Study Findings. Methodological approaches and assumptions are described in
the text or footnoted, and also described in greater depth in Appendix A.
Figures are distributed throughout the text to provide accessible graphic illustration of concepts.
V.
ACKNOWLEDGEMENTS
Preparation of this study was aided by several professional advisors and
community volunteers. Deserving of special acknowledgement are:
♦ American Soil and Stone
♦ Andrea Solk, Sustaining Ourselves Locally
♦ Association of Bay Area Governments
♦ Babak Tondre
♦ Center For Sustainable Economy
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B A Y L O C A L I Z
E X E C U T I V E S U M M A R Y / I N T R O D U C T I O
♦
♦
♦
♦
♦
♦
♦
♦
♦
♦
♦
♦
♦
E
S
T
N
City Slicker Farms
Community Foundation Silicon Valley
Institute for Simplified Hydroponics
Intertribal Friendship House
Laurence Levine Charitable Trust
Mark Richmond, Practica Consulting
Natylie Baldwin
Rana Creek
REC Solar
San Francisco Planning and Urban Research Association (SPUR)
Stewart Winchester, Merritt College
Tufani Mayfield
United Nations Food and Agriculture Organization
Building Survey Volunteers
Aaron Lehmer
Andrea Mann
Bob Strayer
Carolyn Bush
Charles Hardy
David Jaber
Debbie Collins
Dominic Porrino
Ellen Doudna
Inga Sheffield
Kelley Lake
Kirsten Schwind
Lisa Katz
Maija Dzenis
Mark McBeth
Nelson Chick
Oliver Lear
Paula White
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Sarah Kennedy
Sharon Kutz
VI.
CONCLUSION
This Neighborhood Assessment conclusively demonstrates that rooftop resources can be developed on existing buildings in the Bay Area without structural improvements. Furthermore, future developments would gain considerable benefits by planning for intensive, edible roof gardens. Hydroponic
rooftop gardens and solar photovoltaics show the most promise for existing
buildings, while intensive and extensive green roofs and rainwater harvesting
present additional challenges, some of which may be overcome in time if
greater investment is warranted. Today, it is possible for building owners to
install rooftop technologies and improve water quality, save energy, grow
fresh produce, generate clean electricity, and contribute to greater community
resilience and livability. The promise of a healthier environment and greater
resource security makes it imperative that we begin planning and implementing for these sustainable rooftop systems now.
Education and leadership can bring about the kinds of benefits that so many
cities have successfully demonstrated. Policy and government support are
essential keys to fostering the implementation of these systems. Rooftops are
currently untapped resources and a package of appropriate design, development incentives, and public support is crucial to fulfilling their great potential.
1-6
2
EXISTING CONDITIONS
This chapter describes the current state of rooftop resource implementation
in the Bay Area and specifically in the Study Area. It also documents the architectural history and existing demographic and regulatory setting of the
Eastlake neighborhood in order to identify dynamics that may affect rooftop
resource development. The latter half of the chapter presents the Building
Typologies that were developed for the purposes of the assessment and describes their distribution in the Study Area.
I.
ROOFTOP UTILIZATION
The role of rooftops has historically been a peripheral consideration in the
development of urban infrastructure and largely remains an afterthought in
water, food and energy systems planning. Roofs have been used for collecting
water or insulating homes for millennia, but widely-held perceptions dismiss
these traditionally “low-tech” strategies as being old-fashioned or only applicable in rural contexts. While solar thermal and photovoltaic technologies
have been applied on roofs for decades, these practices have yet to gain widespread adoption. However, new interest in green building is once again focusing attention on rooftops. Green roofs, rainwater harvesting systems, and
rooftop photovoltaics are being installed at an increasing rate while California
remains a national leader in solar electricity generation.
A. Rooftop Utilization in the Bay Area
The Bay Area is well-known for its focus on environmental sustainability and
for good reason. With regards to rooftop resource strategies, the region is
ahead of the curve but far from taking full advantage of its resources.
2-1
Buildings within study area. Source: Ingrid
Severson.
E X I S T I N G C O N D I T I O N S
1. Solar Photovoltaic Installations
In the nine-county Bay Area, over 5,000 photovoltaic systems have been installed.1 The largest include the 675 kilowatt (kW) installation on the City of
San Francisco’s Moscone Center, a 766 kW system at the Rodney Strong
Winery in Healdsburg, and Alameda County’s 1,180 kW installation on the
Santa Rita Jail in Dublin. In addition, much of the region’s photovoltaic capacity exists in smaller systems under 15 kW, many of which serve residential
buildings. In seven counties of the Bay Area (excluding Napa and Solano
Counties), these systems comprise approximately 18,000 kW, or 18 megawatts (mW) of electricity generating capacity.2
2. Green Roofs and Rooftop Gardens
Despite a number of high-profile green roof projects in the Bay Area, the
green roof trend has been somewhat slow to take hold in the region. An outstanding exception is the Gap Headquarters in San Bruno, which was constructed with a 69,000 square-foot extensive green roof in 1997. The California Academy of Sciences building under construction in San Francisco’s
Golden Gate Park will also have a large, extensive green roof. Intensive green
roofs and rooftop gardens and parks have also been built, including park environments atop parking garages at Civic Center, Yerba Buena Gardens and
the North Beach Place mixed-use project in San Francisco, and at the Kaiser
Center office complex in downtown Oakland.
Nevertheless, a number of cities have consistently outperformed Bay Area
locations in terms of green roof implementation, including Chicago, Washington D.C., New York City and Portland, Oregon.3 As far as could be de-
Liz Merry, “Status of Photovoltaic Installations in California,” Solar Energy Resource
Guide, NorCal Solar, 2007.
1
2
Ibid.
Green Roofs for Healthy Cities, “Green Roof Industry Survey Final Report,”
http://www.greenroofs.org/storage/2006grhcsurveyresults.pdf (accessed April 14,
2007).
3
2-2
termined, no municipalities in the Bay Area region have green roof incentive
programs as do Chicago and Washington, D.C.
3. Rainwater Harvesting
Regional attention to harvesting rainwater has fluctuated with concerns over
environmental conditions or scarcity. Many residents discovered rainwater
harvesting, for example, in the drought of 1976-77, when reservoirs across the
region were dangerously drawn down and mandatory restrictions were imposed on water use. While data on regional rainwater catchment implementation is unavailable, it is likely that a limited number of residential buildings
are fitted with rainwater harvesting systems, and that this number is increasing, albeit very slowly, with elevated awareness of California’s water resource
and sustainability challenges.
B. Rooftop Utilization in the Study Area
A limited level of rooftop utilization is already occurring in the Study Area.
There are at least six rooftop solar water heating installations, all on apartment buildings. There are no green roofs on occupied buildings in the area,
but a vegetated plaza sits atop an underground parking structure. The plaza is
planted with a variety of trees, grasses, and shrubs, providing an attractive
semi-public space with stormwater retention benefits. It is possible that some
rainwater harvesting systems are in use but none were identified through aerial photograph analysis or the field survey. Figure 2-1 illustrates existing
rooftop resources in the area.
II.
REGULATORY AND POLICY SETTING
The Study Area is within the jurisdictional boundary of the City of Oakland
and is subject to a number of State and City regulations pertaining to rooftop
uses. This section introduces these regulations and their applicability to the
study.
2-3
Volunteers identifying building types. Source:
DC&E.
BAY LOCALIZE
ROOFTOP RESOURCES NEIGHBORHOOD ASSESSMENT
Pa
rk
Blv
d.
E. 19th St.
E. 18th St.
E. 1
8 th
St .
E. 17th St.
Lake Merritt
La
ke
sh
or
e
Bl
vd
.
Foothill Blvd.
8th Ave.
7th Ave.
6th Ave.
5th Ave.
4th Ave.
3rd Ave.
2nd Ave.
1st Ave.
E. 15th St.
International Blvd.
Clinton
Square
Park
E. 12th St.
E. 11th St.
E. 10th St.
0
250
500 Feet
Figure 2-1. Aerial view of buildings in study area with existing rootop resources.
Intensive Green Roof
No Resource
Solar Water Heating
Study Area
A. Zoning Code
The Zoning Code is contained in Title 17 of the City of Oakland’s Municipal
Code. The Code classifies, regulates, restricts and segregates land uses, building characteristics, and population densities according to the land use goals
established by the community in the General Plan. Minimum requirements
for usable open space are established for residential uses. In Chapter 17.126,
the Code sets minimum standards for usable open space on residential parcels,
including rooftop uses.
Residential parts of the Study Area are mostly zoned R-50 (Medium Density
Residential) or R-60 (Medium-High Density Residential), with the remainder Mixed use building. Source: DC&E.
zoned at higher densities. Usable open space requirements for these classifications range from 150 to 200 square feet per dwelling unit. Rooftop areas can
satisfy a maximum of 20 percent of this required open space, or 30 to 40
square feet per dwelling unit.
B. California Building Code
The State Building Code is contained in Title 24, Part 2 of the California
Code of Regulations. The Code regulates the construction and function of
buildings to ensure fire and life safety and adequate structural design. Pertinent sections of the code include Chapter 5, Section 509 (Guardrails), Chapter
10 (Means of Egress), Chapter 13, Section 1301 (Solar Energy Collectors),
Chapter 15 (Roofing and Roof Structures), and Chapter 16 (Structural Design
Requirements). The following considerations will affect the extent to which
usable rooftop spaces can be created.
1. Occupancy Load and Means of Egress
Since construction of an accessible space on a rooftop alters the use of the
roof, the municipal Building Department will ensure that Building Code requirements are met when reviewing plans for the improvement. Code requirements will vary depending on how the occupancy of the roof space is
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defined and on the maximum number of occupants expected and allowed to
use the space, which is the “occupant load.”
The most relevant example is with regard to means of egress.4 Accessible roof
spaces that accommodate many occupants will be required to provide more
than one exit, while spaces intended for ten or fewer occupants are adequately
served by one exit. This is a critical variable for rooftop gardening since very
few buildings have two exits from the roof. Therefore, for rooftop gardening
to be possible, the Building Department must ensure safety by either calling
for two exits or determining that the rooftop garden’s occupancy load will be
ten or less, rendering one exit sufficient.
The Building Department is responsible for assigning an occupancy load to
the rooftop space, in accordance with the following direction from Chapter
10, Section 1003 of the California Building Code:
♦ Areas with fixed seats. Occupant load for areas with fixed seats is determined by assigning one occupant per seat provided in the area. For
example, an area with 12 seats has an occupant load of 12.
♦ Areas without fixed seats. Here the occupant load is determined by dividing the occupied square footage by an “occupant load factor” in Table
10-A of the California Building Code. For uses not included in the table,
such as gardening, a factor for a similar type of use will be used. Speculatively, a case could be made that gardening is similar in intensity of use to
such uses as manufacturing or a commercial kitchen, where a limited
number of people are involved in a productive activity over a large area.
If these factors are used, as much as 2,000 square feet can be occupied for
gardening without exceeding the maximum desirable occupant load of 10.
Because rooftop gardening is a relatively rare phenomenon in the region, no
interpretation of the Code with regards to occupancy has been established. It
Means of egress are Code-compliant exits. Any occupiable space, such as a rooftop
garden, must have at least one Code-compliant means of egress.
4
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is possible that Code officials will be wary of rooftop uses more intensive
than gardening and will consider the space a gathering place, thereby requiring an additional exit. The outcome will depend on the municipality and on
assurances that can be made to limit the number of occupants.
2. Guardrails
Chapter 5 of the Building Code requires a guardrail around habitable space in
order to protect life safety. Some buildings in the Study Area have fixed parapets lining the perimeter of the roof area and extending as high as a few feet.
Others have no perimeter barrier at all. In any case, a code-compliance barrier that extends 42 inches in height is required.
C. Accessibility
Local, State, and federal governments address accessibility for the mobilityimpaired through several codes and laws. At the federal level, the Americans
with Disabilities Act (ADA) requires that equal access be provided for the
mobility-impaired when alterations to public spaces are made. Chapter 11 of
the California Building Code also sets forth stipulations for accessibility,
which are enforced by municipal Building Departments. Both the ADA and
Chapter 11 must be satisfied.
1. Americans with Disabilities Act Compliance
ADA requirements for building alterations do not apply to buildings that are
used for strictly residential purposes; only buildings considered “public accommodations”—such as restaurants, hotels, theaters, doctors’ offices, pharmacies, retail stores, museums, libraries, parks, private schools, and day care
centers—are subject to ADA rules. Some rooftop garden retrofits that are
accessible to the public would fall under ADA and would need to include
accessibility features to the roof, in the form of either elevators or ramps. It is
likely that these features would prove prohibitively expensive to install and
would create a major disincentive for creating accessible rooftop spaces on
existing buildings.
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ADA requirements apply only to public accommodations and do not address
residential environments. Even for public accommodations, elevators would
not be required in many cases. According to the Department of Justice, elevators are generally not required in facilities under three stories or with fewer
than 3,000 square feet per floor, unless the building is a shopping center or
mall; the professional office of a health care provider; a terminal, depot, or
other public transit station; or an airport passenger terminal.5 In addition,
accessibility requirements may be waived as an undue hardship if accessibility
features cost more than 20 percent of the total alteration cost, which would
apply in the case of rooftop gardens.
2. California Building Code, Chapter 11
Accessibility requirements in the State Code are similar to ADA requirements, but also include residential uses in their scope. Like the ADA, the
Code allows for exemptions based on “unreasonable hardship,” which waives
accessibility requirements when the cost of accessibility features exceeds 20
percent of the total alteration cost, and the total alteration cost is less than
$120,000 (both of which are true for rooftop gardens). Installation of a new
elevator in an existing building in order to access a new garden on the roof
may be acknowledged as unreasonable hardship, particularly if the structure
is not a major commercial or institutional building.
Every effort should be made to provide universal accessibility to rooftop gardens when feasible. The Americans with Disabilities Act and California
Building Code require that these improvements be made whenever feasible,
but may provide flexibility when the costs of accessibility improvements are
unreasonably high, as with elevator installation in existing residential structures and other small buildings.
US Department of Justice, “Americans with Disabilities Act Questions and Answers,” http://www.usdoj.gov/crt/ada/qandaeng.htm.
5
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III.
STUDY AREA CHARACTERISTICS
Comprising roughly one-quarter of a square mile, the Study Area consists of
the Eastlake commercial district and surrounding residential neighborhoods
southeast of Lake Merritt in Oakland. The area provides a fertile testing
ground for rooftop resource feasibility due to its great diversity, both in its
socioeconomic conditions and its built environment.
A. Demographics
Mixed-use building on 2nd Avenue.
Eastlake is a unique neighborhood demographically, presenting a complex
mix of economic and ethnic attributes in a compact area. Extrapolation of
census data suggests that the Study Area is home to approximately 7,000 residents in approximately 3,500 dwelling units. The median income of around
$31,000 is low relative to the City of Oakland as a whole, but the number of
people below the poverty line is lower than the city average.6 A wide range
of income levels exists in the area.
The neighborhood is widely perceived as one of the most ethnically diverse in
the region. Dozens of languages are spoken by immigrants from around the
world. Asians and African-Americans are the largest ethnic groups, comprising roughly one-quarter of the population each, with Hispanics accounting
for another 14 percent and the remainder White, or other races. Of residents
with Asian ancestry, many trace their roots back to Vietnam and other
Southeast Asian countries.
Pitched-roof houses.
Parking garage in study area. Source: Ingrid
Severson.
6
Estimates based on U.S. Census Bureau, U.S. Census 2000.
2-9
B. Architectural History
Known variously over the years as Rancho San Antonio, Clinton, Brooklyn,
and New Chinatown, Eastlake is a neighborhood with a complex and fascinating history that is reflected in the building stock found there today. While
the built remnants of the Ohlone communities that originally occupied this
area have disappeared, historic features dating back to Spanish settlement can
still be found. San Antonio Park, lying just southeast of the study area, once
served as the main plaza of Rancho San Antonio, the cattle ranch started by
the East Bay’s original Spanish settlers in the 1820s.
Apartment tower.
“Shops” building type.
Repair shop. Source: Ingrid Severson.
The area was incorporated into Oakland in 1872. At least one 19th century
home still stands in the neighborhood, along with several early 20th century
Victorian homes. Many multi-family residential and commercial buildings
found in the area today were constructed in the early 20th century, leading up
to World War II. After the War, Eastlake experienced the same pattern of
disinvestment that impacted many urban neighborhoods across the country.
Deteriorating buildings continue to impact quality of life in Eastlake today
and many structures in the study area are in fair or poor condition. Midcentury urban renewal projects also had an effect with hundreds of structures
demolished in these decades and replaced with apartment buildings—1,108
apartments in 57 buildings total.7 Many of these renewal-era buildings are
still found in the study area and factor significantly into the rooftop resource
assessment.
Since the era of urban renewal, the built conditions of the study area have not
changed as rapidly. Investment in the 1980s and 1990s was primarily directed
to areas near Lake Merritt where apartment towers, strip retail and big-box
retail were developed. However, it appears that Eastlake may be in the early
stages of economic transition. The neighborhood is located in the City of
Oakland’s Central City East Redevelopment Area, in which infrastructure
for development projects may be financed through tax-increment financing,
7
Urban Ecology, Clinton Park Plan, August 1999.
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providing a public incentive for private development. A multi-million dollar
streetscape improvement project was also initiated in 2002 for the business
district on 12th Street and International Boulevard between 5th Avenue and 8th
Avenue. Rehabilitation and new construction projects in Eastlake may provide an opportunity to incorporate rooftop features into the neighborhood.
IV.
BUILDING TYPOLOGY
Buildings in the Study Area are classified by type for the purposes of this assessment. The typology categorizes flat-roofed buildings into eleven types to
allow for generalized estimates of structural properties and roof loading capacities. Buildings with pitched roofs are not included in the typology since
they were assigned the rainwater harvesting and solar photovoltaic prototypes. Both systems are sufficiently lightweight to be installed on virtually all
pitched roof buildings, regardless of the building type. To account for the
potential of future development in the area, vacant lots were identified as opportunity sites in which new construction could plan for the inclusion of
rooftop systems. Table 2-1 describes the typical characteristics of each flatroofed building type and associated roof loading capacities.
“Big Box building. Source: Ingrid Severson.
Typical office building. Source: DC&E.
In addition to assigning a building type to each flat-roofed structure, a field
survey conducted by the consultants and Bay Localize volunteers recorded
discrete characteristics such as occupancy type, height, construction type and
era, and presence of a “soft story.” Estimates of loading capacity are refined
to account for differences in these factors when they do not match the typical
assumed characteristics of the building type.
A. Building Type Distribution
A wide variety of building types are found in the Study Area. Table 2-1 describes the building type split over the area. Table 2-2 illustrates the distribution of building types in the area.
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One of nine vacant lots identified as opportunity sites. Source: Ingrid Severson.
TABLE 2-1
BUILDING TYPOLOGY-TYPICAL CHARACTERISTICS
Building Type
Land Use
Occupied
Stories
House
Residential
Apartment Building
Additional
Loading
Capacity*
(psf)
Construction Material
Construction
Era
Size/Scale
1-2
Wood-framed
Any
Up to 4 units
20
Residential
1-4
Wood-framed
After 1950
4 to 10 units
15
Apartment Tower
Residential
5+
Concrete or Steel
After 1980
More than 10 units
5-7
Mixed Use
Retail/
Residential
2-5
Wood framed
Any
Varies
Varies
(8-12)
Shops
Commercial
1
Wood-framed
After 1970
Varies
17
Warehouse
Varies
1
Prior to 1960
Large, open floor plan
5
Big-Box
Retail,
Industrial
1
After 1960
Large, open floor plan
5
Repair shop
Commercial
1
Concrete block
Any
Smaller, open floor
plan, open storefront
7
Office Building
Office
2+
Varies
After 1960
Varies
17
Community Building
School, hospital, church,
auditorium, library, theater, police, fire, post office,
etc.
1+
Varies
Varies
Varies
Varies
(5-17)
Masonry or Concrete block
walls, riveted steel or largetimber columns
Concrete block or tilt-up concrete walls, interior steel posts
* A removal of pea-gravel/rock ballast (secured on the roofs of any of these buildings) can increase the “dead-load” capacity by an average of 4-5 psf for every inch of ballast removed.
2-12
TABLE 2-2
BUILDING TYPE DISTRIBUTION
Number
of Buildings
Percentage
of Total
20
2.7
160
21.3
8
1.1
Mixed Use
36
4.8
Shops
40
Warehouse
14
Building Type
House
Apartment Building
Apartment Tower
5.3
1.9
Big Box
7
0.9
Repair Shop
6
0.9
Office Building
7
0.9
30
4.0
1
0.1
Community Building
Parking Garage
Community building. Source: Ingrid Severson.
Pitched Roof Building
419
56.01
Total
748
99.91
Apartment building. Source: DC&E.
Warehouse building. Source: Ingrid
Severson.
2-13
BAY LOCALIZE
Pa
rk
Blv
d.
E. 19th St.
E. 18th St.
E. 17th St.
Lake Merritt
re
B
l vd
.
Foothill Blvd.
8th Ave.
7th Ave.
6th Ave.
5th Ave.
4th Ave.
3rd Ave.
2nd Ave.
Lak
e
1st Ave.
sh o
E. 15th St.
International Blvd.
Clinton
Square
Park
E. 12th St.
E. 11th St.
E. 10th St.
0
250
500 Feet
House
Big Box
Repair Shop
Apartment Building
Shops
Opportunity Site
Apartment Tower
Office Building
Unknown
Mixed Use
Parking Garage
Pitched Roof
Community Center
Warehouse
Study Area
Figure 2-2. Aerial view of Study area
indicating distribution of
building types.
3
ROOFTOP RESOURCE PROTOTYPES
For the purposes of this assessment, five rooftop resource prototypes were
developed. These prototypes provide assumed characteristics that can be applied to each of the rooftop resource models, including weight and productivity. This chapter describes the design concepts, architectural and maintenance
requirements, and potential benefits of the five prototypes. It also analyzes
the synergies and conflicts that could arise if the prototypes are implemented
in concert.
The rooftop resource prototypes are a set of design concepts that represent
various strategies for rooftop utilization. The design process was informed by
the objective of utilizing rooftops in a manner that is productive, sustainable,
and feasible. In addition, the goal of assessing rooftop capacity on existing
buildings in their current condition—rather than on new construction or on
structurally reinforced buildings—made loading a central consideration in the
prototype design. Table 3-1, found at the end of the chapter, describes the
components, cost ranges, and yields of the prototypes.
The prototypes are necessarily generalized to allow for variations in roof size
and type, roof slope, building type, wind variables, client budget, and other
conditions. They should be taken as examples of possible configurations and
should not be relied upon for specifications for any site. Before any specific
rooftop resource is developed, professional consultation should be obtained
to determine precise design loads and roof loading capacity.
I.
EXTENSIVE GREEN ROOF
A. Design Concept
Extensive green roofs are the most common type of green roof found today,
valued for their many environmental benefits. The prototype follows the
typical configuration of extensive green roofs in arid climates, in which lowgrowing, drought-tolerant ground cover is planted in 4 to 6 inches of growing
substrate and placed on an assembly of filter fabric, a drainage layer, root
3-1
Raised vegetable garden on rooftop. Source:
Resource Centres on Urban Agriculture and
Food Security.
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P P I N G T H E P O T E N T I A L O F U R B A N R O O F T O P S
O F T O P R E S O U R C E S N E I G H B O R H O O D A S S E S S M E N T
O F T O P R E S O U R C E P R O T O T Y P E S
TABLE 3-1 PROTOTYPE CHARACTERISTICS
Prototype
½" Drainage Mat
4" Mineral Substrate
Sedums
2¼" Drainage Board
Intensive Green
18" Organic/Mineral Substrate
Roof—Vegetables
Variety of Vegetable Crops
1 ¼" Drainage Board
Intensive Green
8" Organic/Mineral Substrate
Roof—Herbs
Herbaceous Plants
Growing Container
Hydroponic
Reservoir Container
Rooftop Garden 4" Inert Substrate
Variety of Vegetable Crops
Solar
Multicrystalline PV Panels
Photovoltaics
Mounting Hardware
Conveyance-Gutters/Leaders
Debris Screen
Rainwater
First-flush Diverter
Harvesting
Roof Washer
Storage Tank/Cistern
Extensive Green
Roof
Extensive green roof. Source: American
Hydrotech.
Extensive green roof assembly. Source:
American Hydrotech.
Major
Components
Maximum
Weight
Annual
Productive
Yield
22 psf
drainage and
energy benefits
108 psf
1.86 psf
vegetables
51 psf
perennial yield
16 psf
4 psf
vegetables
5 psf
N/A
1 kilowatt per
100 square feet
average
3,000 ga. per
structure
(1" rain on 100 sf
= 60 gal.)
barrier, and a waterproof roof membrane. This type of assembly can be installed directly on the roof or placed in trays that are installed as a modular
system. Extensive green roofs differ from intensive green roofs in the minimal depth of the substrate and more limited planting possibilities. Figure 3-1
illustrates the prototype assembly and vegetation.
1. Green Roof Assembly
The Extensive Green Roof prototype is intended to extend across the maximum feasible area of the building to confer the greatest storm water and energy benefits. Plants are established in four inches of substrate, considered a
minimum depth for plants to endure dry Bay Area summers. In this case a
3-2
Figure 3-1. Cross-section of Extensive Green Roof prototype.
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N e i g h b o r h o o d A s s e s s m e n t
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Extensive green roof modular tray system.
Source: Green Roof Blocks.
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blend with high mineral content and low organic content should be developed to provide moisture and air retention while minimizing the load imposed upon the roof. While the exact blend depends highly on the site, one
very appropriate medium is pumice, which is one of the lightest mineral materials mined within 500 miles of the Bay Area. Expanded shale is another
very lightweight mineral that exists in California; however, it is currently
shipped in from Colorado and other Western states, resulting in higher transportation costs and environmental impacts. Lava and scoria are available
from the Clearlake area in Northern California, but they are heavier than
pumice and expanded shale. Whichever mineral medium is selected should be
combined with a minimal amount of organic material, such as locallyavailable compost.
Beneath the substrate, a ½-inch thick recycled polyethylene drainage mat aerates and drains the media, and attached filter fabric prevents it from clogging
the drainage layer. Finally, a root barrier and waterproof membrane protect
the roof deck from the living layer above.
“Green Paks” modular tray system. Source:
Green Roof Blocks.
A popular alternative to the type of assembly described above (which is builtup directly on the roof) is the modular approach, in which the above components are assembled in container trays and installed on the roof. Modular
systems present a number of benefits. Perhaps their most notable benefit is
the flexibility they allow in installation and removal. They can often be installed without re-roofing, while the soil membrane system may require replacement or major repair to the roof membrane. In addition, building owners may be more open to experimenting with a green roof installation knowing that the trays can be easily removed if desired.
Whether the soil membrane approach or the modular system is chosen for a
particular roof, the described components of the extensive green roof assembly are almost identical in other respects.
3-4
R O O F T O P R E S O U R C E P R O T O T Y P E S
2. Plants
The Extensive Green Roof prototype is planted with of a mix of regionallyappropriate sedums, such as Sedum album, Sedum spathuifolium, Sedum spurium, and Sedum sexangulare. The specific species chosen for a particular
rooftop context depends on many factors, including:
♦ Initial budget and maintenance budget.
♦ Physical conditions, such as shading and wind.
♦ Roof slope.
♦ Retrofit schedule and seasonal variables.
Planting methods can vary, from direct seeding in the growing medium during installation to application of pre-planted container trays or vegetated
mats. Generally, materials for vegetated mats and modular tray systems are
more expensive but labor costs are reduced, while seeding or transplanting
plugs directly is more labor-intensive but reduces materials costs.
B. Architectural Requirements
Because extensive green roofs are a low-impact, low-maintenance rooftop resource, they can be useful on buildings with a wide range of characteristics.
Their implementation on existing buildings in the Bay Area is constrained,
however, by their weight. The lightweight Extensive Green Roof prototype
weighs approximately 22 pounds/square foot (psf),1 precluding installation on
many building types. This weight value is at the low end of the spectrum for
extensive green roofs.
Because the prototype is not intended to be used as occupiable space, egress
requirements are more lenient than for rooftop gardens. Older stairways
1
All green roof loading estimates are based on design loads from the German green
roof standard, “Guidelines for the Planning, Execution, and Upkeep of Green-roof
Sites,” published by FLL (Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau e.V.), 2002 Edition.
3-5
Sedums on extensive green roofs using modular
systems. Source: Green Roof Blocks.
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with steeper inclines to the roof, or common ladder and hatch access, would
both be sufficient to provide necessary maintenance.
C. Maintenance Requirements
Extensive green roofs are intended to be a low-maintenance technology. During the first year after installation, plants need to be irrigated as they establish
themselves. Planting hardy, drought-resistant, regionally-appropriate varieties such as those specified in this prototype will limit irrigation needs over
the long-term since these plants are accustomed to the arid conditions of Bay
Area summers. However, minimizing substrate depths to the level entailed in
this prototype would likely require that some irrigation occur on a regular
basis, depending on the conditions of the site. The extensive green roof needs
to be inspected only a few times a year to ensure that all components, including the membrane, are functioning as intended.
Extensive green roofs can reduce roof maintenance demands and as much as
double the life of the roof membrane by protecting it from extreme temperature changes, ultraviolet radiation, and accidental damage.2
D. Cost Range
The extensive green roof prototype is a relatively low-cost rooftop resource
strategy, though initial costs are higher than that of a conventional roof. Depending on the type of labor that is used, accessibility and size of the roof,
and the planting method, initial materials and labor costs are estimated at $15
2
City
of
Chicago,
Design
Guidelines
for
Green
Roofs,
http://egov.cityofchicago.org/webportal/COCWebPortal/COC_ATTACH/design_
guidelines_for_green_roofs.pdf (accessed April 8, 2007).
3-6
to $22 per square foot. This estimate assumes that the prototype is applied
when a new roof is needed and therefore excludes the cost of a new roof
membrane, which generally adds another $8 to $12 per square foot. Preplanted trays with irrigation in place cost approximately $25 to $30.3
In considering the financial viability of extensive green roofs, it is important
to note that they are more cost-competitive with conventional roofs when
using a life-cycle costing approach, which incorporates savings from reduced
maintenance and longer roof life, as well as ongoing energy efficiency savings.
Life-cycle costing is a valuable method for understanding the benefits of many
“green” technologies, which may cost more initially but which may result in
cost savings over the operating life of the product.
E. Benefits
While the extensive green roof prototype does not yield a harvest of food,
energy, or water, it does confer a number of environmental and aesthetic
benefits, including:
♦ Storm Water Retention. Extensive green roofs are valued for their
storm water retention capacity and a good deal of research is being conducted to quantify these benefits. By capturing and holding water in the
vegetation and substrate layers, the extensive prototype can mitigate
flooding and combined sewer system backup (where applicable) in heavy
rain events. This prototype can also improve the quality of runoff water
by capturing and holding pollutants in the substrate. Studies indicate
that an extensive green roof can absorb as much as 70 percent of rainfall
3
All costs are derived from Steven Peck and Monica Kuhn, “Design Guidelines for
Green Roofs,” Ontario Association of Architects and Gabrielle Fladd, “Green Roof
Matrix” (cost analysis paper presented in San Francisco Planning & Urban Research
Association, Green Roof Task Force meeting, San Francisco, CA, June 2007).
3-7
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from a storm event, releasing the remainder over an extended period of
time.4
♦ Energy Efficiency. The extensive prototype can improve building performance, effectively acting as a heat trap, while also shading the roof and
cooling the roof surface through evaporation and vegetative transpiration. A study in Canada found that when implemented broadly on a city
or regional scale, extensive green roofs can significantly reduce ambient
urban temperatures, thereby lowering energy demand for air conditioning. The living roof on Chicago’s city hall building stands as a popular
case study modeling the cooling effect of green roofs. It has been recorded as 7 degrees cooler than surrounding roofs on an annual average,
and up to 30 degrees cooler during the summer time.5
Intensive roof garden. Source: American
Hydrotech.
II.
INTENSIVE GREEN ROOF-VEGETABLE GARDEN
A. Design Concept
Intensive green roof—vegetable garden.
Source: The Rooftop Garden Project,
Alternatives, 2004.
The Intensive Green Roof—Vegetable Garden prototype is designed to supplement the environmental and aesthetic benefits of the extensive prototype
with food production and an open space amenity. Vegetables would be
grown in 18 inches of growing medium, the minimum depth to support a
large variety of vegetables. The assembly of the prototype is similar to that of
the extensive prototype, consisting of a water proof membrane, root barrier,
drainage layer, filter fabric, substrate layer, and plants. Figure 3-2 provides a
section of the green roof assembly.
4
ASLA et al., “Landscape Architects Release Green Roof Performance Report: Roof
Retained 27,000 Gallons of Stormwater in First Year,” http://asla.org/press/2007/
release091907.html.
5
Karen Liu, “A National Research Council Canada Study Evaluates Green Roof Sys-
tems’ Thermal Performances,” http://www.professionalroofing.net/article.aspx?
A_ID=130 (accessed April 8, 2007).
3-8
R o o f t o p
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F i g u r e 3 - 2 . C r o s s - s e c t i o n o f I n t e n s i v e G r e e n R o o f — Ve g e t a b l e s p r o t o t y p e .
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In this case, however, the vegetated roof is not intended to cover the entire
roof area. Instead, paths are created by closing in growing areas with retaining walls, constructed of lightweight materials such as wood or recycled plastic. A protective surface would be installed to protect the roof from foot traffic damage. This arrangement provides accessible spaces similar to groundlevel container gardens. While a multitude of different arrangements are possible, depending on the existence of fixed obstructions on the roof, it is estimated that this prototype would provide an average growing area of 60 percent of total roof area. This estimate takes into account space unavailable due
to fixed obstructions as well as space needed for paths and equipment storage.
Crop Variations
A wide variety of vegetables may
be suitable for rooftop environments in the East Bay, despite
such challenges as wind, heat, and
evaporation. Experimentation is
needed to better establish planting
possibilities. While the prototype
suggests several popular garden
vegetables, other appropriate
edible crops may also include
broccoli, celery, chard, collards,
eggplant, kale, mustard, green
onions, and peppers. In addition,
some of these crops may be
planted in less than 18 inches of
growing media, which was used
in this prototype as a conservative
value for growing a large variety
of edible plants in potentially
harsh conditions.
1. Green Roof Assembly
The prototype includes 18 inches of substrate, in this case a blend of about
equal parts mineral and organic content. Conventional topsoil and potting
soil are inappropriate media for rooftop environments, particularly on existing buildings, due to their weight. Instead, the medium used in this prototype would include approximately equal parts organic material, such as compost or bark humus, and mineral material such as pumice or scoria. The
drainage layer in this prototype is a 2¼ inch thick recycled polystyrene drainage board. The drainage course is filled with lava or similar mineral material
for structure. Other components are similar to the extensive green roof prototype.
2. Plants
The prototype is designed to provide year-round vegetables that could thrive
in the Bay Area. A selection of vegetables was developed based on several
criteria, including:
♦ Regional suitability
♦ Growing medium depth requirements
♦ Plant weight at maturity (structural consideration)
♦ Height at maturity (wind loading considerations)
♦ Full sun tolerance
♦ Normal to low water needs
♦ Growing season
3-10
Based on these criteria, plants were selected that could be rotated to grow on
a year-round basis. Cool-season crops in the prototype are spinach, mustard,
carrots, and beets. Tomatoes, cucumbers, and winter squash are included as
warm-season crops, along with leaf lettuce, which in many cases can be grown
year round. Variations on this arrangement should provide an opportunity
to grow cool-season crops twice yearly—planted in the early Spring and Fall—
while planting warm-season crops in late Spring. Additional crops that could
be grown on this prototype are listed in the side box.
The vegetable garden prototype would introduce a new roof load of approximately 108 psf to the structure.6 Providing this type of structural support would require a uniquely-tailored structural design incorporated into
new construction. As an occupiable space, the vegetable garden prototype
would require a code-compliant stairway or elevator, as well as guardrails or
fencing around the roof edge.
C. Maintenance Requirements
The prototype would require substantial maintenance, much of which is associated with normal vegetable gardening activities. Maintenance demands
would include regular irrigation, pruning, weeding, fertilizing, and pest control. Water needs could be increased relative to ground-level gardening due to
higher rates of heat- and wind-induced evaporation. Depending on budget,
irrigation could take place through hand-watering or sub-surface drip irrigation, the latter of which would reduce water use and labor requirements. In
6
All green roof loading estimates are based on material design loads in the German
green roof standard, “Guidelines for the Planning, Execution, and Upkeep of Greenroof Sites,” published by FLL (Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau e.V.), 2002 edition.
3-11
Intensive green roof assembly.
American Hydrotech.
Source:
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addition to regular maintenance of the growing area, inspection and repair of
the roof membrane would be required on an occasional basis.
D. Cost Range
Initial costs depend greatly on the type of labor that is used, accessibility of
the roof, and planting method, as well as the size of the roof. Given these
variables, it is estimated that materials and labor costs to install this prototype
would range from $30 to $45 per square foot of growing area, plus an additional $20 to $40 per linear foot for guardrails and an optional $2 to $4 per
square foot if irrigation is installed.7 This estimate assumes that the prototype
is applied when re-roofing is needed and therefore excludes the cost of a new
roof membrane. It also assumes that structural and architectural requirements of the Building Code are already satisfied.
E. Benefits
In addition to providing storm water retention and treatment, energy efficiency and aesthetic benefits, this prototype provides an open-space amenity
to building residents. Food production on the roof also results in a number
of valuable outcomes, such as new wildlife habitat, recreational and educational opportunities, and readily accessible fresh, healthy produce. Adequately maintained year-round, this prototype would yield approximately
1.86 pounds of vegetables per square foot annually.8
7
Stephen Peck and Monica Kuhn, “Design Guidelines for Green Roofs,”
http://egov.cityofchicago.org/webportal/COCWebPortal/COC_ATTACH/design_
guidelines_for_green_roofs.pdf (accessed March 24, 2007).
8
Nancy Garrison, “Home Vegetable Gardening,” University of California Coopera-
tive Extension, http://vric.ucdavis.edu/veginfo/veginfor.htm. This figure is an average of vegetables’ approximate yields, multiplied by three seasons.
3-12
Implemented on a broader scale, this type of green roof can improve local
food security and reduce “food miles traveled,” thereby reducing the energy
and climate impacts of food transportation. A study conducted in 2001 at
Iowa State found that in the conventional American agriculture system, food
travels an average of 1,500 miles from its origin to its point of consumption.9
III.
INTENSIVE GREEN ROOF—HERB GARDEN
A. Design Concept
The concept of this prototype is largely similar to the Vegetable Garden prototype, except that herbs are grown instead of the wider variety of vegetables.
Plants included are rosemary, thyme, and cilantro. The growing medium
remains the same—a lightweight, soil-free mix of approximately equal parts
organic and mineral material—but substrate depth is reduced to 8 inches, a
minimum for many herbs that would be suitable for a rooftop environment.
The drainage layer is also similar to the Vegetable Garden prototype, but its
thickness is reduced to 1¼ inches. Figure 3-3 illustrates the green roof assembly and vegetation.
An approximate load of 51 psf would result from installation of this prototype.10 As an accessible garden, this prototype would be considered an occupiable space, subject to code requirements for stairways and guardrails.
9
Leopold Center for Sustainable Agriculture, Food, Fuel and Freeways, An Iowa Per-
spective on How Far Food Travels, Fuel Usage, and Greenhouse Gas Emissions,
2001.
10
June
All green roof loading estimates are based on the German green roof standard,
“Guidelines for the Planning, Execution, and Upkeep of Green-roof Sites,” published
by FLL (Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau e.V.), 2002
edition.
3-13
Intensive roof garden atop the Fairmont Royal
York Hotel in Toronto grows vegetables,
herbs, and edible flowers. Source: Lorraine
Flanigan.
Figure 3-3. Cross-section of Intensive Green Roof—Herbs prototype.
R o o f t o p
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C.
Maintenance Requirements
Maintenance needs would be significant and would include regular irrigation,
pruning, weeding, fertilizing, and pest control. Irrigation needs would be
reduced compared to the Vegetable Garden prototype, but a sub-surface drip
irrigation system would still be desirable. As with all green roofs, regular
inspection of the roof membrane would be required, and occasional repair of
the membrane could prove necessary.
D. Cost Range
Given the cost variability factors that apply to all green roofs, it is estimated
that materials and labor costs for this prototype would range between $28 to
$40 per square foot, plus an additional $20 to $40 per linear foot for guardrails
and an optional $2 to $4 per square foot if irrigation is installed.11
E. Benefits
In addition to the environmental and social benefits previously noted, this
prototype would provide an ongoing supply of culinary herbs.
11
Stephen Peck and Monica Kuhn, “Design Guidelines for Green Roofs,”
http://egov.cityofchicago.org/webportal/COCWebPortal/COC_ATTACH/
design_guidelines_for_green_roofs.pdf (accessed March 24, 2007). This estimate assumes that the prototype is applied when re-roofing is needed and therefore excludes
the cost of a new roof membrane. It also assumes that structural and architectural
requirements of the Building Code are already satisfied.
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IV.
HYDROPONIC VEGETABLE GARDEN
Hydroponics, also known as ‘organoponics’ in Latin America, is a horticultural method that supplies plant roots with liquid nutrients, eliminating the
need for organic material that provides nutrients under conventional methods. Plants are provided with nutrient solution, and are either grown in an
inert mineral substrate or are suspended above the solution without substrate.
The hydroponic model substantially reduces the weight of vegetable cropping
systems by eliminating the growing medium.
Commercial hydroponics. Source: Aaron
Lehmer.
No fewer than six different techniques can be used to operate the system,
some of which involve such equipment as water pumps, air pumps, computerized monitors, timers, and lighting, when used indoors. This model of hydroponics does not require artificial lighting as there is adequate sunlight for
growing plants on the roof. The water supply is plumbed from the building
to feed into the hydroponic system. A unique design component of the rooftop hydroponic prototype is a shade-cloth or light screen meshing as a ‘lid’ to
protect the growing medium from being blown away in the wind. This retaining cloth may be stapled along the edges of the container trays.
A. Design Concept
Rooftop hydroponics. Source: The
Rooftop Garden Project, Alternatives,
2004.
The Hydroponic Rooftop Vegetable Garden prototype utilizes a low-tech,
low-cost method that minimizes weight while maximizing vegetable productivity, variously called Simplified Hydroponics or Popular Hydroponic Gardens (PHG). The design is based on concepts developed and implemented
around the world by the UN Food and Agriculture Organization (FAO), the
UN Development Program (UNDP), and the Institute for Simplified Hydroponics.
In this prototype, containers are filled with lightweight mineral substrate to a
depth of 4 inches. Perlite is used as a base, and combined with inert organic
material such as rice hulls, peanut hulls, grain chaff or coconut coir. The ad3-16
dition of these organic materials provides a more balanced substrate, thus fostering a healthier medium for the plants. This growing medium is ideal for its
extremely low weight, but it should be noted that other lightweight bases
could be used, such as pumice. Appropriate plants include cooking and salad
greens, most summer and winter vegetables and herbs. Root vegetables can
be grown hydroponically but require greater substrate depths than specified
in the prototype. Planting methods may vary among vegetable types, but
generally seedlings are transplanted into the substrate, where they have regular access to the nutrient solution.
The prototype utilizes the “flood and drain” method of hydroponics, in
which the nutrient solution is circulated back and forth between the growing
container and a reservoir container. The growing container is constructed
with a drain approximately one inch above the base, allowing for some accumulation of solution in the bottom of the container but draining the remainder. If the growing container is elevated and placed at a minimal slope, drainage can be gravity-fed. Alternatively, a pump can be used to continually flood
and drain the growing container, recirculating the nutrient solution.
Accounting for roof obstructions, pathways and storage areas, it is assumed
that growing area would constitute 60 percent of the total roof area. Figure
3-4 illustrates the components of the prototype.
Under normal conditions, the prototype would add approximately 9 psf to
the roof load. However, in a heavy rain event, the substrate may be saturated
and an inch of water could accumulate in the bottom of the growing container, resulting in a maximum load of approximately 16 psf.
This prototype would be considered an occupiable space, subject to code requirements for access and guardrails.
3-17
Figure 3-4. Cross-section of Rooftop Hydroponic Garden prototype.
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Source: Based on designs in Bradley and Marulanda 2000
C. Benefits
The hydroponic prototype combines many of the benefits of the Intensive
Green Roof-Vegetable Garden prototype with those of the Rainwater Harvesting prototype. The prototype would appeal to residents as an attractive
open space amenity. In terms of energy efficiency, the hydroponic containers
would shade the roof and vegetation would provide ambient cooling through
evapotranspiration, but the thermal mass benefits would be less than a conventional green roof. Generally, the prototype would not deliver energy efficiency gains comparable to green roofs.
Instead of the storm water retention associated with green roofs or the
ground-level water storage of a rainwater harvesting prototype, the hydroponic system would capture rainwater in the growing containers and drain it
to the reservoir containers for reuse. While centralized water storage would
be prohibitive due to excessive loading, this decentralized water storage across
the roof would distribute and minimize the load. Depending on the design,
the Hydroponic prototype could capture and reuse all of the 14.3 gallons per
square foot of rainwater that falls in an average year. Additional irrigation
water would be conserved in dry months by recycling the water back and
forth between the growing container and the reservoir container.
In general, hydroponic systems represent a more productive growing method
than conventional gardening. Because sufficient nutrients are supplied close
to the base of the plant, roots do not spread horizontally to satisfy their nutritional requirements. This growth pattern allows for closer spacing of
plants. In addition, hydroponically grown plants have an advantage over soil
based plants in that energy that would be expended in root growth is utilized
instead for leaf, flower, and fruit growth. Based on estimates of the FAO, the
Hydroponic Vegetable Garden prototype would yield approximately
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4 pounds of vegetables per square foot.12 However, as this volume level is
based on figures from professional hydroponic technicians, the actual volume
may vary as much as two pounds less than the figures in this report. Typical
field conditions can average a yield of 2.5 pounds per square foot.13
V.
Ufafabrik factory in Berlin—Templehof green
roof research. Source: www.ufafabrik.de.
SOLAR PHOTOVOLTAIC ELECTRICITY
A. Technology Concept
Solar photovoltaic (PV) technology offers an opportunity to produce clean,
renewable electricity on a wide range of residential and non-residential rooftops. While the technology has existed for many years, recent advances are
improving the efficiency of PV cells, which are the individual units that produce electricity. These cells are combined into modules that are available in
single crystal, multi-crystalline, and amorphous silicon (thin-film) varieties,
differing in their efficiency and cost.
Today, single crystal and multicrystalline systems are the most common and
cost-effective types of installations. Single crystal and multicrystalline cells
are comparable in most ways, though the former is generally the more efficient and the latter generally the more affordable. Both are commonly installed in the Bay Area and both are considered “high-efficiency” systems.
For the purposes of the PV prototype, the characteristics of the more com-
12
Charles Schultz, “Soilless in Singapore,” Growing Edge Magazine,
http://www.growingedge.com/magazine/back_issues/view_article.php3?AID=17032
4 (accessed April 1, 2007).
Juan Izquierdo, FAO. Personal communication with Brian Holland, DC&E, May
2007.
13
Willow Rosenthal, City Slicker Farms.
Severson, October 2007.
3-20
Personal correspondence with Ingrid
mon multicrystalline type will be assumed. These panels have a typical generating capacity of 1 kilowatt (kW) per 100 square feet.14
Over time, other types of photovoltaic technology may become more attractive options. Thin-film photovoltaics may become more cost-effective as
manufacturing methods improve, and advances in building-integrated photovoltaics may one day make it possible to generate electricity affordably
through building materials themselves. PV dealers and contractors should be
consulted to determine the most appropriate type of installation for a specific
application and point in time.
In addition to consideration of solar cell type, any PV system must be installed with an acceptable tilt and orientation to be effective. Ideally the array
should face south, but southeastern and southwestern orientations are also
acceptable. In applying the prototype on pitched roofs, assumed orientation
is either southeast (135 degrees) or southwest (225 degrees). For flat roofs, the
prototype has a southern orientation. Currently, public financial incentives
are directed toward electricity production during times of peak demand,
which results in greater impetus for southwestern-facing installations. Once
again, this condition may change over time as the structure of financial incentives will be a key determinant in the arrangement of any installation.
With regard to tilt, an ideal tilt angle is that which sets the panel perpendicular to the sun, which is the latitude of the location. The latitude of the Study
Area is 37.7 degrees north. While installing a PV system at an ideal tilt sometimes involves additional cost, this prototype assumes that all arrays are set at
the area’s ideal tilt.
14
Liz Merry, “Solar Electric System Basics,” Solar Energy Resource Guide, NorCal
Solar, 2007.
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Roofs fitted with photovoltaics must be able to support the weight of a typical multicrystalline PV panel and mounting hardware, which is approximately 5 psf. The location should be mostly unshaded by trees or adjacent
buildings and panels should be set back from roof obstructions that cast shadows, such as enclosed stairway landings or mechanical equipment.
C. Cost Estimate
Installation costs for a residential PV system in Oakland averaged $8.68/watt
between 2006 and 2007 with rebates of about $2.50 per watt.15 While most
panels have a power warranty of 25 years (meaning at year 25 they are guaranteed to produce 80 percent of their original output), initial costs are often
paid back in only ten years, providing at least 15 years of no-cost electricity.16
Inverters typically have a warranty of 10 years, and need to be replaced between year 15 and 20. The cost of inverters has been decreasing significantly
quicker than the cost of the PV panels.
D. Benefits
Solar photovoltaics can play an important role in a clean energy future, reducing the region’s dependence on fossil fuel imports and mitigating greenhouse
gas emissions associated with natural gas power plants. In the near term,
rooftop photovoltaics can also protect residents from rate spikes and reduce
peak loads on the electrical grid, diminishing the need for costly and underutilized infrastructure to accommodate peak demand. Depending on the orien15
NorCal Solar.
“September 2007 Update and City Solar data spreadsheet.”
http://www.norcalsolar.org/local-activism/bay-area-solar-installs-2007-6.html
cessed October 10, 2007).
16
(ac-
Andy Black, “Payback and other Financial Tests for Solar Electric Systems,”
http://www.ongrid.net/papers/PaybackOnSolarSERG.pdf (accessed April 1, 2007).
3-22
tation, a 1kW module of the prototype is expected to produce between 1,278
and 1,415 kilowatt hours (kWh) annually.17
VI.
RAINWATER HARVESTING AND REUSE
A. Technology Concept
Rainwater collection has been practiced for centuries as a way of stretching a
scarce resource. Harvesting water from the roof is a simple and elegant solution, powered not by electricity or sunlight, but by gravity. Since drainage is
a standard component for most roofs, much of the infrastructure needed to
harvest rainwater is already in place on existing buildings. In the U.S., rainwater is most often used for irrigation, less often for fire protection or toilet
flushing, and rarely for potable water supply. This prototype will be designed for irrigation purposes as the most feasible use for the Study Area.
Figure 3-5 provides a diagram of one potential arrangement of rainwater harvesting components. The actual design of the system will depend on sitespecific conditions.
1. Catchment and Conveyance
The first step in rainwater harvesting and reuse is to capture the precipitation.
Typically, pitched roofs are fitted with external gutters and downspouts to
carry water off the roof and away from the exterior walls of the house. In
this case, the prototype rainwater harvesting system entails installation of a
cistern connected to the downspout, intercepting water that would otherwise
17
All calculations of PV capacity and production were generated in the “PV Watts”
online modeling tool, developed by the National Renewable Energy Laboratory. It is
available at http://rredc.nrel.gov/solar/codes_algs/PVWATTS/.
3-23
Cistern at residence of Robert van de
Walle. Source: Robert van de Walle.
R o o f t o p
R e s o u r c e s
Leaf Screen
Gutter
Basket Strainer
Down spout
Screen
Overflow
Inlet
Outlet
1.5" ball valve
Figure 3-5. Diagram of assembly of rainwater catchment system
using 50 gallon drum.
Source: Courtesy of Southface (Atlanta, Georgia)
R e s o u r c e s
Figure 3-6. Diagram of integrated Rainwater Harvesting and Solar Photovoltaics prototypes.
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reach the ground and run away from the foundation of the building.18 Leaf
screens are installed at the gutter/downspout connection and at the downspout/cistern connection to keep debris from clogging the system. A roofwash system should be fitted to the gutter to divert the first flush of storm
water, which may be laden with pollutants from the roof surface.
While some flat or low-slope roofs have external drainage systems like the
one described, others do not have drainage or use internal downspouts that
drain directly to the storm sewer or to a ground-level discharge spout. In
these cases, capturing rainwater is a more expensive proposition as the drainage system needs to be modified with external conveyance equipment.
2. Filtration
After collection and conveyance, several features should be incorporated to
filter out roof debris and pollution before storage. A simple debris screen
should be fitted to the gutter or downspout to catch leaves and other large
particles. In addition, a first-flush diverter is commonly used to capture the
first few gallons of rainwater during a storm, which is usually more laden
with pollutants that have accumulated on the roof between rain events, such
as dust and bird droppings. The first flush of storm water is then drained
separately from the rainwater harvesting system. The recommended capacity
of the diverter varies by roof type, regular presence of pollutants, and regular
duration between rain events, but a general rule of thumb suggests one gallon
of diversion capacity is a minimum for each 1,000 square feet of catchment
area.19
18
Rainwater captured for irrigation
at Stopwaste.org headquarters in
Oakland. Source: Sarah Sutton.
Not all rain that falls on the roof will be drained, due to such factors absorption and
evaporation. A runoff coefficient of 0.85 is assumed for pitched roofs in this prototype, meaning that 85 percent of fallen precipitation will be conveyed into the rainwater harvesting system. The assumed runoff coefficient for flat roofs is 0.50, accounting for pooling conditions or gravel ballasted roofs.
19
Texas Water Development Board, Texas Manual on Rainwater Harvesting, Third
Edition, 2005.
3-26
Another potentially necessary filtration component is the roof washer. This
feature is usually installed to filter smaller debris and organic materials. Many
roof washers take the form of a container with one or two canister filters inside, installed directly before the cistern. The necessity of this component
will depend on the type of irrigation system used; drip irrigation systems in
particular may become clogged if this type of micro-filtration is not employed.
3. Storage
After catching and filtering the rainwater, it must be stored for later use.
Storage is the limiting factor in this prototype, since space is often limited for
installation of cisterns. Below-ground cisterns have been used for individual
buildings or community systems, particularly when they can be planned into
new construction, but the expense of excavation and pumping water back up
into the distribution system will prove cost-prohibitive for many residents
and building owners.
Instead, this prototype assumes that above-ground cisterns are the most feasible option in the Study Area. Cisterns vary in type and size. Plastic or steel
cisterns are commonly available and are durable and movable. Reclaimed
containers such as trash cans or steel/plastic drums could also be used. Cistern size would be based on the size of the roof, rainfall patterns, the amount
of space available for siting and the proportion of rainwater versus municipal
water use that is desired. While some lots could accommodate a 1,500-gallon
cistern, others would be confined to a 500-gallon or even 110-gallon tank.
Based on observations of density and open space in the Study Area, an average storage capacity of 1,000 gallons is incorporated into the prototype.
4. Distribution
Water can be distributed to meet landscaping needs either through drip irrigation or watering by hand. Ideally, the cistern is sited at the highest elevation
on the lot, enabling gravity-fed irrigation. This is often not the case in the
Study Area. If drip irrigation is used, a pump would be needed to pressurize
the system in some circumstances.
3-27
Residential cistern. Source: Marc Richmond.
Residential underground cistern in
Sausalito by 450 Architects.
Source: Richard Parker.
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B. Architectural and Site Requirements
As described previously, buildings should have an external drainage conveyance system, most commonly found on pitched roof residential buildings.
The site requirements for storage will depend on the size of the cistern. Dimensions of storage tanks vary, but a circular 1,100-gallon cistern can measure
as little as 6 feet in diameter by 6 feet in height, while a 55-gallon barrel can fit
in spaces of just a few feet wide.
C. Cost Estimate
Costs vary widely depending on the need for new conveyance gutters and
pipes, the size and type of the storage unit, the type of distribution, and the
type of labor used for installation. If reclaimed 55-gallon barrels are installed
at existing downspouts and residents water by hand, costs can be as little as
$100. On the other hand, storage tanks alone can cost between $0.45 and
$1.00 per gallon, or $225 to $500 for a 500-gallon cistern.20 California-based
vendors sell 1,000-gallon, polyethylene tanks for an average of $550 with costs
for shipping averaging around $200. Plastic tank prices will fluctuate based
on the current petroleum market.21
D. Benefits
This prototype assumes a storage capacity of 1,000 gallons per structure,
which would be stored for summer irrigation. Rainwater harvesting can create numerous benefits to water quality and water supply. These include:
20
Ag
Extension
Communications,
Montana
State
University,
"http://www.montana.edu/wwwpb/pubs/mt9707.html (accessed April 1, 2007).
21
Ingrid Severson. Phone calls to vendors, October 2007.
3-28
♦ Conservation of potable water.
♦ Conservation of energy required to deliver and treat potable water.
♦ Minimizing need for upgrades to water treatment facilities.
♦ Minimizing the stress on water delivery systems.
♦ Reduction of storm water runoff.
♦ Reduction of pollutants entering the watershed through runoff.
♦ Reduction of the thermal impact of runoff to wildlife and vegetation due
to heat gain from the roof.
Rainwater harvesting and associated water conservation are increasingly
viewed as critical responses to potential climate change impacts. Models suggest that the Sierra snowpack could decrease by as much as 70 to 90 percent
and that early Spring flow from this source could decrease by 30 percent under a medium-warming scenario.22 Combined with growing demand, particularly in Southern California regions impacted by supply constraints of the
Colorado River Basin, these trends could stretch the region’s water supply in
dramatic ways. In response, rainwater harvesting offers a feasible and effective means for conserving water and protecting water quality.
VII.
MULTIPLE PROTOTYPE INTERACTIONS
This section identifies opportunities and constraints for implementing multiple rooftop resource strategies in the same roof space. While there are many
unknown variables that influence these possibilities, research is beginning to
look at what type of synergies may come about through interaction of these
systems. This section considers a few of the most likely interactions.
22
California Energy Commission, “Our Changing Climate: Assessing the Risks to
California,” July 2006.
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A. Extensive Green Roof and Photovoltaics
This combination presents both positive and negative interactions, and existing research has not adequately tested its feasibility. Installation of solar panels above the green roof vegetation would create a good deal of shade and
would keep precipitation from falling evenly over the vegetation. Nevertheless, some Sedum varieties have demonstrated shade tolerance, including Sedum ternatum and Sedum telephium. Shading would reduce evaporation as
well, potentially allowing for reduction of substrate depths beyond what is
otherwise feasible in the seasonally arid Bay Area climate or the elimination
of installed irrigation.
Ufafabrik factory in Berlin—Templehof green
roof research. Source: Ufafabrik.de.
A research plot maintained by University of Applied Sciences Neubrandenburg and the Technical University of Berlin has had success not only in green
roof plant growth under a photovoltaic installation, but also in demonstrating
increased PV output in this scenario. Their research indicates that green roofs
can improve the efficiency of photovoltaics mounted above them. Ambient
temperatures on the test plot were reduced 16 degrees Celsius compared to an
adjacent conventional roof, which improved the efficiency of the PV and resulted in an average 6 percent increase in energy yields.23
The combined load of the Extensive Green Roof prototype and the Solar
Photovoltaic prototype would be approximately 27 psf, precluding the possibility of combining these prototypes on existing buildings without structural
retrofit.
B. Green Roofs and Rainwater Harvesting
Installation of these technologies in concert is technically feasible. Precipitation that is not taken up by the green roof vegetation is sometimes drained to
23
Manfred Kohler, et. al., “Positive Interaction Between PV Systems and Extensive
Green Roofs,” Green Roof Infrastructure Monitor, Green Roofs for Healthy Cities.
April 2007.
3-30
downspouts as in the conventional scenario. However, the cost-effectiveness
of this strategy is questionable due to the retention and absorption capabilities
of the green roof. Runoff coefficients of green roofs can range from 0.50 to
0.80, allowing as little as 20 percent of the precipitation to drain into the water storage system.
C. Photovoltaics and Rainwater Harvesting
Photovoltaic systems do not intercept or impede the flow of water from the
roof, so these prototypes can usually be implemented together. Because
photovoltaic panels do not absorb any water, the runoff coefficient of a roof
fitted with an installation may be improved relative to that of normal asphalt
roofing material, resulting in a marginally higher catchment capacity. In addition, photovoltaic systems require periodic washing to remove dust and dirt
buildup and the wash water could be harvested under this scenario.
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4
FINDINGS
This chapter presents the conclusions of the analysis, including discussion of
how prototypes were assigned to each building and what considerations were
not taken into account in the assignment process. The chapter also describes
the benefits of rooftop resource development in terms of increased productivity of energy, food, and water. Finally, the chapter concludes with a summary of incentives for and barriers to future rooftop utilization.
I.
PROTOTYPE ASSIGNMENT
Each building in the Study Area is assigned one or more rooftop resource
prototypes. The following criteria are incorporated into the assignment process:
♦ Prototype Load and Roof Loading Capacity. Loading is a primary
consideration in matching prototypes with suitable buildings. The analysis shows that the type of rooftop resource development that can take
place on existing buildings is heavily dependent on the building type and
associated loading capacity.
♦ Roof Access and Building Code Access Requirements. Access is another primary consideration in the assignment. The Intensive Green
Roof and Hydroponic prototypes would be highly difficult to install and
maintain without stair or elevator access. As occupiable spaces, these
prototypes are also required by the Building Code to have stair or elevator access. Therefore, the assignment of these prototypes depends on the
existence or potential construction of a code-compliant stairway or elevator.
♦ Occupancy Type. In some cases, more than one prototype would meet
the above primary criteria for a building. Occupancy type is a secondary
criterion that allows for consideration of what prototype the building occupant would more likely choose based on its costs and benefits.
4-1
F I N D I N G S
The following sections describe how buildings were designated with each prototype. Figure 4-1 illustrates the pattern of potential rooftop resource development in the Study Area.
A. Intensive Green Roof
Because of the loads associated with the Intensive Green Roof prototype, it
could not be developed on any occupied buildings in the Study Area. However, the prototype was assigned to the one parking garage in the area. In
addition, nine vacant lots in the study area were identified that will likely be
developed over the next several years. As reflected on Figure 2-2, these lots
were classified as “Opportunity Sites” and were fitted with the Intensive
Green Roof prototype. Because new construction developments can plan to
accommodate the load of a living roof, these buildings will carry this feature
more readily than existing buildings with a retrofit of a green roof.
Advances in growing media may soon lead to the production of extremely
light-weight materials that can further reduce the weight of intensive green
roofs and allow limited vegetable production on existing buildings. At this
time, however, these products are not readily available in the San Francisco
Bay Area.
B. Hydroponic Vegetable Garden
Post-war residential and institutional buildings are estimated to have the highest loading capacities in the Study Area, ranging between 15 and 20 pounds
per square foot (psf). The prototype adds a maximum of approximately 16
psf to the roof load when applied across the entire roof area. However, the
load is less than 15 psf when applied to 60 percent of the area, as specified in
the prototype.
4-2
BAY LOCALIZE
Pa
rk
Blv
d.
E. 19th St.
E. 18th St.
E. 1
8 th
St .
E. 17th St.
Lake Merritt
La
ke
sh
or
e
Bl
vd
.
Foothill Blvd.
8th Ave.
7th Ave.
6th Ave.
5th Ave.
4th Ave.
3rd Ave.
2nd Ave.
1st Ave.
E. 15th St.
International Blvd.
Clinton
Square
Park
E. 12th St.
E. 11th St.
E. 10th St.
0
250
500 Feet
Intensive Green Roof-Vegetable Garden
Solar Photovoltaic with Rainwater Harvesting
Rainwater Harvesting
Hydroponic Rooftop Garden
Solar Photovoltaic
No Resource
Study Area
Figure 4-1. Aerial view of Study area with buildings
assigned rooftop resources pototypes.
F I N D I N G S
The limiting factor in this case is roof access. Buildings under four stories are
not required by Building Code to provide stairway access to the roof, and
most do not. At the same time, buildings with occupiable space on the rooftop are required to provide stairway access to the roof. The implication is
that many one-, two-, and three-story buildings are not currently equipped to
accommodate this prototype.
For the purposes of the assessment, the following assumptions were made for
flat-roofed buildings with adequate loading capacities:
♦ Buildings four stories and higher were assumed to have code-compliant
stairway access since it is currently required, and are assigned the Hydroponic prototype.
♦ Two- and three-story buildings were assumed to have only ladder and
hatch access and are not assigned this prototype.
♦ One-story buildings with adjacent open space were assigned the prototype, with the expectation that a code-compliant external stairway could
be installed at relatively little cost. The existence of adjacent open space
was recorded during the field survey.
These assumptions are necessary because roof access could not be determined
on a building-specific basis. However, a limited number of two- and threestory buildings may have stairway access to the roof and should not be excluded from consideration in the future on that basis only.
C. Extensive Green Roof
Loading is the main constraint in retrofitting buildings with extensive green
roofs. The prototype, which was designed to minimize the green roof load,
weighs approximately 22 psf. The strongest roofs in the Study Area have
loading capacities of between approximately 17 psf and 20 psf, found in postWar residential and institutional buildings. Therefore, it is likely that the
only cases in which extensive green roofs can be applied are those where addi-
4-4
F I N D I N G S
tional dead load capacity can be obtained by removing pea gravel or rock ballast from the roofs of these building types.
Gravel has sometimes been used to surface conventional built-up roofs, where
layers of felt and asphalt are built up from the roof deck. Just a few inches of
pea gravel are laid on top of the roof, weighing as little as 4 or 5 psf. However, by reconfiguring this assembly with a green roof, that additional capacity may be obtained, potentially allowing for retrofit with the 22 psf extensive green roof. In other cases, heavier rock ballast has been used to secure
loose laid single-ply membrane roofs, a roof type that has gained in prominence since the early 1980s. The commonly referenced standard for ballasted
single-ply roofs calls for a minimum of 10 psf of rock ballast.1 If the ballast is
removed, additional dead load capacity is again generated, and the extensive
green roof may become a feasible roofing option.
The applicability of this approach will vary depending on location. It is possible that very few gravel surfaced or rock ballasted roofs exist in the Study
Area, partly because of the age of the building stock (rock ballast was not
commonly used before the 1980s) and partly because of windy conditions that
may have restricted their use in the past. More investigation and collaboration will be needed between engineers, roofing contracts, and code officials to
determine the feasibility of gravel or ballast replacement for green roof retrofits.
For the purposes of this study, the Extensive Green Roof prototype was not
assigned to any buildings in the Study Area. In addition to the uncertainty
surrounding the cost and technical feasibility of gravel and ballast replacement and the prevalence of these roof types in the Study Area, it was determined that the Hydroponics prototype was a more productive and low-cost
strategy for rooftop greening on the buildings in question. Though extensive
green roofs can also be installed on pitched roof structures of a limited slope,
American National Standards Institute/Single Ply Roofing Industry, Wind Design
Standard for Ballasted Single Ply Roofs, November 19, 2002.
1
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F I N D I N G S
these buildings were not designated with green roofs because they are also
more suitable for other prototypes, such as solar photovoltaics and rainwater
harvesting.
D. Solar Photovoltaics
The Photovoltaic prototype requires buildings with 5 psf of roof loading capacity. The southern-oriented prototype was assigned to all flat-roofed buildings with between 5 psf and 15 psf of loading capacity.
All pitched roof buildings were designated with the Photovoltaic prototype.2
With regard to orientation, the street grid and buildings in the study area are
oriented 45 degrees off of north, so the prototype is for southeastern and
southwestern-facing installations. The tilt was assumed to be ideal for the
area, at approximately 37 degrees, though installation costs can be reduced
without substantially sacrificing performance by installing panels flush with
the pitched roof.
The Study Area has high solar insulation with minimal shading, due to sunny
conditions, generally uniform building heights and an absence of mature
trees.
E. Rainwater Harvesting and Reuse
The Rainwater Harvesting prototype was assigned to all pitched roof buildings, including those with the Photovoltaic prototype, and to selected flat
roof buildings. Pitched roofs shed water more efficiently than most flat roofs.
It is assumed that all pitched roof structures in the study area will have adequate loading capacities for photovoltaics, and that usable roof space will average 40 percent for
pitched roofs buildings and 40 percent for flat roofs, accounting for shading and roof
obstructions.
2
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F I N D I N G S
In addition, most pitched roof buildings in the study area are detached residential units, which are assumed to have some open space to accommodate
water storage. Parcels with flat roof buildings were assessed for the existence
of adequate open space and only these buildings were designated with the
Rainwater Harvesting prototype. No buildings with green roofs or hydroponic gardens received the prototype.3
II.
OTHER CONSIDERATIONS
A. Cost-Benefit Analysis
The assignment methodology took into account the technical and regulatory
constraints and opportunities for rooftop resource development and only
used cost-benefit considerations as a secondary criteria. Clearly, however, a
number of financial factors will play a major role in determining how rooftops are utilized. Initial costs bar many residents from taking advantage of
rooftop resources, despite cost savings that accrue on an ongoing basis for
each technology. Also, higher initial costs often correspond with higher impact technologies, such as solar photovoltaics or intensive green roofs.
The cost-benefit calculus will inevitably change over time to reflect evolving
economic conditions and social values. For example, as government moves to
reduce greenhouse gas emissions, it is possible that electricity will become
more expensive due to market-based mitigation mechanisms like carbon taxes
or cap-and-trade systems. Similarly, government may institute new incentives
for rooftop resource development in response to citizen concerns about
community livability and sustainability. These incentives will change the
direction of rooftop utilization depending on their objectives.
For pitched roof buildings, a runoff coefficient of 0.85 is assumed. Flat roof buildings may have inadequate slope to drain most of their water, or could have gravel ballast that would absorb water, so the runoff coefficient is reduced to 0.50 to reflect
these possibilities.
3
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F I N D I N G S
While it is outside the scope of this study, analyzing the costs and benefits of
each rooftop resource prototype is an important task that will assist building
owners in making tough decisions about how to use their building to its fullest potential.
B. Seismicity
The most prominent earthquake-related factor relating to additional roof
loading is the existence of a soft story, as described in Chapter 2. This factor
was incorporated into the loading capacity assumptions used in the assignment process. However, for some individual applications of rooftop resources, additional seismic effects should also be explicitly considered in
greater detail.
For instance, ground accelerations experienced during earthquakes cause
building damage in proportion to the building’s mass. In general, additional
load that is less than 5 percent of the total building mass does not significantly
affect the earthquake safety of that building. Many buildings weigh 50 psf
for each level above grade. In this case, if 10 psf is added to the roof, the total
weight is increased by 20 percent for a one-story building and 5 percent for a
four-story building. Thus, the seismic resistance of one- and two-story specialty buildings (those which are not conventional wood construction) should
be explicitly considered by a qualified professional during an overall assessment of the roof loading capacity.
III.
PRODUCTIVITY OF ROOFTOP RESOURCES
Based on the prototype characteristics and the assignment schema above, the
total output of rooftop resources can be estimated for the Study Area. The
outcomes of the exercise tell a story about what is feasibly achievable, in just
one neighborhood of many, if a commitment is made to utilizing rooftops.
4-8
F I N D I N G S
Table 4-1 on the following page illustrates the application of prototypes and
their benefits.
A. Electricity from Photovoltaics
In the ¼-square mile Eastlake neighborhood, 8.5 megawatts (MW) of renewable electricity capacity could be installed without sacrificing other rooftop
uses that may be more appropriate. This total includes 4.6 MW on pitched
roofs and 3.9 MW on flat roofs. These photovoltaic installations would generate over 11 million kilowatt hours (kWh) of electricity per year. With annual per capita electricity consumption of around 6,700 kWh,4 photovoltaics
would satisfy approximately 25 percent of the Study Area’s electricity demand under this scenario. Please refer to Appendix A, Assumptions and
Methodology, for an explanation of the solar calculations.
B. Vegetables from Intensive Green Roofs and Hydroponic Gardens
Under this study’s scenario of rooftop utilization, 18 structures would be developed with the Hydroponic Rooftop Garden prototype, and ten structures
with Intensive Green Roof-Vegetable Garden prototype, providing approximately two acres of growing area. These gardens would yield approximately
273,373 pounds, or 124 metric tons, of vegetables annually. This higher-thanaverage yield is the effect of year-round growing methods as well as hydroponic productivity greater than that of conventional methods.
California Energy Commission, “Per Capita Energy Use by State in 2003,”
http://www.energy.ca.gov/electricity/us_percapita_electricity_2003.html
(accessed
April 18, 2007).
4
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TABLE 4-1
PROTOTYPE ASSIGNMENT AND PRODUCTIVITY
Number
of
Structures
Total
Annual
Yield
Extensive Green Roof
0
None
Intensive Green Roof –Vegetables
10
34.1 metric tons*
of vegetables
Intensive Green Roof – Herbs
0
0
Hydroponic Rooftop Garden
18
Solar Photovoltaics
668
Rainwater Harvesting
623
Prototype
90 metric tons
of vegetables
11,609,024 kWh/year of
electricity; 8.5 megawatts
of capacity
1,869,000 gallons of
irrigation water
* Productivity for the Intensive Green Roof. Vegetables prototype was derived from one existing parking garage and nine existing, vacant lots (“Opportunity Sites”) that were projected as
being built up with new structures integrating this prototype.
Current annual consumption of the nutritional “dark-green leafy” and “deep
yellow” vegetables included in the prototype is about seven pounds per capita.5 Based on current consumption, the buildings with this prototype could
meet this type of vegetable demand for approximately 38,127 Oakland residents. However, current consumption of these vegetables is significantly
lower than that recommended by the USDA, partly because of limited access
to affordable fresh produce. USDA recommendations for leafy greens and
deep yellows translate into about 31.6 pounds per capita. Using this figure,
the garden prototypes could produce enough of these vegetables to satisfy the
recommended consumption for approximately 8,500 residents, which is more
than the population of the Study Area itself.
USDA Economic Research Service, Moving Toward the Food Guide Pyramid: Implications for U.S. Agriculture, 1999, http://www.ers.usda.gov/publications/aer779/
(accessed on April 1, 2007).
5
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F I N D I N G S
C. Irrigation Water from Rainwater Harvesting
According to a report by the Public Policy Institute of California, outdoor
water use in the Bay Area averages approximately 0.19 acre-feet, or 8,276 gallons, per household annually.6 Approximately 26.9 million gallons, or 82.5
acre-feet of rainwater falls on the roofs of the buildings assigned to the rainwater catchment prototype in the study area each year. If all of this rainwater
could be captured, stored and reused, outdoor water use needs would be met
for over 3,000 households, assuming a consumption rate at the state average.
However, storing this volume of water would present an enormous technical
challenge within an urban setting in that most of the storage would need to be
installed underground, much of it in the public right-of-way. Large volumes
of water storage are particularly necessary because 83 percent of Oakland’s
average precipitation falls between the months of November and March,
while almost all of the irrigation demand occurs between the months of April
and October.
The quantity of water that can be stored and used as irrigation water using
this prototype is more than just the 1,000 gallons of storage capacity, since the
cistern will empty during irrigation and re-fill with the next rain. A common
method in calculating rainwater harvesting and reuse is the “water balance
method,” in which monthly storage and demand are compared to determine
the balance of water that remains in the cistern at the end of the month. For
example, a system may start with 1,000 gallons of water in the cistern, use
1,400 gallons through the month, capture another 1,000 gallons during the
month, and end the month with 600 gallons in storage.
However, use of this method requires an understanding of irrigation demand
for the specific site, while the purpose of this study is to estimate the potential
Ellen Hanak and Matthew Davis, “Lawns and Water Demand in California,” PPIC,
http://www.ppic.org/content/pubs/cep/EP_706EHEP.pdf (accessed April 15, 2007).
6
4-11
F I N D I N G S
for harvested water at the neighborhood scale. Without an understanding of
water demand (that is, how large an area is being irrigated, what types of
plants and soils exist on the site, etc.), it is exceedingly difficult to quantify the
amount of rainwater that can be stored and used. The best estimate, therefore, relies on an assumption: that the harvested water is being used for a
landscape that will require intermittent irrigation during the year, and that
each structures’ cisterns are filled and emptied three times in the course of a
year.
This is a conservative estimate that assumes limited irrigation from October
through April, allowing the cisterns to fill up a total of three times per structure, thus processing a total of 3,000 gallons of water in a year. If applied to
all buildings designated with the rainwater harvesting prototype, the Study
Area could capture and use 1,869,000 gallons, or 250,446 cubic feet, of rainwater annually.7 This amount of water will be captured from 83 percent of
the buildings in the study area, providing for a portion of the irrigation needs
of each building.
IV.
STRUCTURAL IMPROVEMENT OPPORTUNITIES
In individual applications of rooftop resources, there will be some buildings
that can support rooftop resources with additional productive capacity if
structural improvements are conducted. This section describes a few of the
most feasible retrofit options for various prototypes.
Build it Green, Advanced Training Manual for Certified Green Building Professionals,
February 24, 2007 e-mail from Marc Richmond, Practica Consulting, October 25,
2007.
7
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F I N D I N G S
A. Extensive Green Roof Prototype
Buildings were identified that could potentially support an extensive roof
after conducting relatively minor improvements to or verifications of the roof
structure. Improvements were considered minor if they did not significantly
disrupt building operations and could be conducted for an overall cost on the
order of $10 per square foot of the total building area (2007 dollars).
For wood buildings, such improvements could consist of sistering additional
rafters to the existing rafters. Buildings constructed of concrete masonry
units may only require installation or verification of steel beams. However,
any proposed improvements to a particular building to increase its roof loading capacity should be designed by a qualified professional.
B. Intensive Green Roof-Herb Garden Prototype
For some buildings, if major improvements are planned, a few additional
structural improvements could provide sufficient loading capacity for the
weight of the Intensive Green Roof-Herb Garden prototype.
One opportunity is to add to roof framing during a roof replacement. Many
buildings could feasibly support the weight of a shallow substrate intensive
roof if the structural roof framing is significantly supplemented. In particular, a simple case is a steel building with bare metal decking at the roof but
concrete topping over metal deck at floor levels. Adding concrete topping to
the roof deck could increase the roof loading capacity substantially. This effort may also require adding supplemental roof framing beams.
When planning a seismic upgrade or other major renovation for any building
type, roof framing could be added as required to accommodate a heavier roof
system. Many building types are natural candidates for seismic upgrades, either because of local mandates or because of documented poor response in
prior earthquakes. These include:
4-13
F I N D I N G S
♦ Unreinforced masonry (“brick”) buildings, for which California Senate
Bill 1633 (introduced 2/2006) mandates safety upgrades.
♦ Concrete frame buildings constructed before 1975, which was prior to
more stringent, ductile detailing requirements for reinforcing bars.
♦ Tilt-up (“big box”) buildings constructed before about 1990, which was
prior to more stringent requirements for anchoring heavy walls to lightweight roofs.
♦ Steel moment frame buildings constructed between 1978 and 1995, which
typically used heavy framing with welds that have exhibited brittle response.
♦ Soft-story or open-front buildings.
Strengthening costs increase disproportionately if strengthening is required
for columns and foundations. In buildings of five stories or more, strengthening existing columns and foundations may not be required if the additional
weight of a heavier rooftop system may be small relative to the total building
weight. Likewise, buildings of two stories or less may not require column
and foundation strengthening if column and wall sizes were chosen for their
conventional size and are thus stronger than necessary. These conditions
must be verified by a qualified professional before any additional load is introduced.
Buildings on hard or stiff soils, which are not prone to liquefaction in an
earthquake, are less likely to require foundation strengthening. Buildings
with little deterioration, including more modern buildings, will require less
strengthening than a building with visible deterioration.
V.
POLICY OPPORTUNITIES
The results of the Eastlake neighborhood assessment demonstrate a profound
opportunity to utilize Oakland rooftops for economic development, envi-
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F I N D I N G S
ronmental sustainability, and food and energy security. However, a number
of financial and regulatory barriers lie in the way of rooftop resources development. This section describes incentive programs and code revisions that
could spur a transformation in the way Oakland’s built environment contributes to its quality of life.
A. Incentive Programs
As described previously, high initial costs limit the degree of implementation
that is possible for many residents. Local governments and service providers
have a role to play in offering incentives for rooftop resource development
when such strategies make good sense from a policy or financial perspective.
1. Green Roofs
Currently there are few incentives offered for green roof installation. Owners that install green roofs may qualify for a federal tax credit under the Energy Policy Act of 2005, which provides a credit equal to 10 percent of the
cost of insulation materials. However, State government has not instituted
incentives for green roofs, nor does the City of Oakland offer programs or
incentives for the technology. The City of Chicago, however, is a national
leader in green roof implementation as the City offers direct incentives in the
form of $5,000 grants for green roof installation on residential and small
commercial buildings.
Green roofs can be an asset to service providers, who may benefit by encouraging wider implementation of the technology. For example, a green roof’s
storm water retention qualities reduce the volume of runoff that must be captured, conveyed, treated, and discharged, thereby alleviating demand for new
infrastructure. In Germany, where approximately 10 percent of all buildings
are fitted with green roofs, storm water taxes are levied in many cities based
on the runoff generated by an individual property. While this type of tax
may not be feasible in the Bay Area, water districts like the East Bay Municipal Utilities District do benefit from green roofs and may find it possible to
offer storm water rebates to “low-impact” customers. Similarly, PG&E cur-
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F I N D I N G S
rently offers rebates for a variety of high-efficiency appliances, insulation,
high-performance windows, and cool roofs, but does not have a rebate for
energy-saving green roofs.
Green roofs also can contribute to several points in the Leadership in Energy
and Environmental Design (LEED) green building certification program.
LEED is widely recognized for improving the performance of commercial
buildings and providing a marketing incentive to developers and building
owners through its respected “green” brand. Depending on their design,
green roofs can assist in obtaining points relating to improving energy efficiency, reducing storm water runoff, mitigating the urban heat island effect,
restoring open space, and providing water efficient landscaping.
Finally, developers in Portland, Oregon are able to increase their profitability
by installing green roofs in exchange for bonuses in floor-to-area ratio (FAR).8
This approach, sometimes called “amenity zoning,” utilizes zoning code provisions as additional incentives.
2. Photovoltaics
Incentives are available for solar photovoltaic installation at the State and
Federal levels of government. California has been a leader in encouraging the
adoption of solar technology, and most recently stepped forward to enhance
solar incentives through the $3.3 billion California Solar Initiative. The California Solar Initiative provides a rebate based on PV system performance.
Currently, residential users receive $2.20 per watt of photovoltaic power installed; in the Bay Area, this rebate is administered through Pacific Gas and
Electric. Incentives for builders and developers installing photovoltaics on
new residential construction are also available through the California Energy
Commission. The CSI legislation also increased the number of customers
that are permitted sell their renewable electricity into the power grid. These
State incentives are making residential PV applications, in particular, increasingly affordable. At the Federal level, businesses can receive a tax credit of 30
8
FAR is a standard measure of commercial density.
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F I N D I N G S
percent of the PV system cost, while homeowners can receive a credit of up
to $2,000.
No incentives for rainwater harvesting are available from local, State, or Federal government. However, commercial buildings may be eligible for a rebate
through EBMUD’s water conservation program. EBMUD’s Water Smart
Landscape Rebate Program offers $1,000 for installation of high-efficiency
irrigation, but no rebate for rainwater harvesting systems on residential buildings.
Other cities in the U.S. are finding it desirable to offer incentives to encourage water conservation and water quality improvements through rainwater
harvesting. For example, the City of Austin provides rebates of between $45
and $500 for rainwater harvesting systems that provide irrigation water. The
Portland Bureau of Environmental Services provides $5,000 grants for
“downspout disconnection,” which includes rainwater harvesting systems.
B. Regulatory Barriers
There are few regulatory barriers to implementation of photovoltaic or rainwater harvesting strategies. As a newer technology, green roofs are not as
well understood, and in some cases regulations have yet to adapt to the design
and construction industry’s enthusiasm for the technology.
One barrier to adoption of green roofs is a lack of recognition of their benefits in current zoning codes. All open space is not equal, but very few codes
specifically reward installation of environmentally beneficial features such as
green roofs. One exception is the City of Seattle’s new Green Factor program, which allows developers to meet open space requirements by choosing
among a suite of landscaping components, each with a corresponding point
value, instead of merely setting a minimum area requirement. Each component is assigned a value based on its benefits. Because green roofs are consid-
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F I N D I N G S
ered a highly-beneficial approach, they satisfy a larger portion of the total
point requirement and enable an overall reduction in open space area. This
reduction can maximize the amount of area that can be built upon and improve the financial performance of a project.
In a broader sense, green roofs are a relatively new technology in the Bay
Area and code officials may need time to inform themselves of some of the
technical considerations. Building codes do not necessarily discourage green
roofs, but for some municipal staff, questions may remain about code implications. Part of this uncertainty results from the absence of a credible green
roof standard in the U.S.; the German FLL-Greenroof Guidelines, (ForschungsgesellschaftLandschaftsentwicklung Landschaftsbau e.V., Guideline
for the Planning, Execution and Upkeep of Green Roof Sites, Release 2002) is
widely used but lacks regional applicability to the US and the Bay Area, and
in some cases may not reflect the concerns of code officials. Fortunately, an
ASTM (American Society of Testing and Materials) green roof standard is in
development that will soon provide additional guidance to professionals in
designing, approving, and installing green roof systems.
VI.
CONCLUSION
This neighborhood assessment shows conclusively that rooftop resources can
be developed on existing buildings in the Bay Area, without structural improvements. In addition, new construction can be designed with increased
loading capacity, allowing for living roofs that are able to provide high yields
of fresh, organic produce and underground cisterns with a generous capacity
for rainwater storage. Hydroponic rooftop gardens and solar photovoltaics
show the most promise for existing buildings, while extensive and intensive
green roofs and rainwater harvesting present additional challenges, some of
which may be overcome in time as greater investment is warranted. Today,
building owners can install rooftop technologies and improve water quality,
save energy, grow fresh produce, generate clean electricity, and contribute to
greater community resilience and livability.
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F I N D I N G S
Education and leadership can bring about the kinds of benefits that so many
cities have successfully demonstrated. Policy and government support are
essential keys to fostering the implementation of these systems. Rooftops are
currently untapped resources, and a package of appropriate design, development incentives, and public support is crucial to fulfilling their great potential.
4-19
F I N D I N G S
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A
P P E N D I X
A
ASSUMPTIONS AND
METHODOLOGY
........................................................................................................................
........................................................................................................................
APPENDIX A
ASSUMPTIONS AND METHODOLOGY
In order to produce meaningful estimates of rooftop productive capacity,
assumptions were made at each step of the study process. This appendix
provides background information on what assumptions were included and
describes the methodology by which information was analyzed and
conclusions drawn.
A. Existing Conditions Analysis
Existing conditions were documented through a combination of aerial
photograph analysis and a field survey of the study area. A GIS (Geographic
Information Systems) base map of building “roofprints” was created based on
the aerial photograph and information about roof slope and existing rooftop
resources was obtained from the photograph.
A field survey was conducted by volunteers to document building types and
construction characteristics.
The volunteers were trained in simple
techniques for identifying building characteristics and ten groups were each
assigned a portion of the study area to walk and analyze. Collected
information was entered into the GIS database and correlated with estimates
of roof loading capacity.
B. Estimating Roof Loading Capacity
Roof loading capacity for each building type was calculated using the
following procedure:
1.
Unobservable structural properties, such as rafter size and spacing, were
assumed for each type based on the building’s observed features and on
the professional experience of the project team’s structural engineer.
2.
For each building type, a total roof loading capacity was calculated,
corresponding to its assumed properties.
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3.
The code-level applied roof loading of 20 pounds per square foot was
subtracted to obtain the remaining roof loading capacity.
4.
On a case-by-case basis, each building’s loading capacity was adjusted
based on any special circumstances where an observable feature differed
from the typical features of that type. Adjustments included:
a. Age Penalty
Buildings constructed in an area other than that assumed for calculating
typical properties were penalized to account for greater possible
deterioration. For example, if an apartment building matched all typical
features listed in Table 2-1, except its construction era was pre-War, we
subtracted 5 pounds per square foot from the building’s loading capacity.
This is because the material properties assumed for an apartment building are
based on mid-century construction.
b. Height Penalty
Buildings with more stories than the typical range specified for that type were
penalized to account for the additional loading on the vertical load-bearing
members, i.e. walls and posts.
c. Construction Material Adjustment
For a building whose primary construction material was weaker or stronger
than the typical construction material for that building type, the loading
capacity was either reduced, in the case of weaker materials, or adjusted
upward, in the case of stronger materials. Brick buildings that were assumed
to be wood under their building type were particularly penalized.
d. Soft-Story Penalty
A soft-story is a story level (usually the first story above grade) which has
significantly less earthquake resistance than the adjacent stories. In an
earthquake, the soft story will “sway” or “lean” much more prominently than
the stiffer stories, and the earthquake damage will be concentrated in that
story. Buildings with open fronts were penalized in order not to overload a
building with potentially low earthquake resistance.
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e. Cumulative Adjustments
If a building had more than one adjustment, the adjustments were added
(cumulative).
Special consideration was given to buildings whose roof loading capacity
equaled or exceeded 10 pounds per square foot, but did not reach the load of
the Hydroponic prototype (16 pounds per square foot). For these buildings,
a general average of 60 percent of rooftop area was designated for the
hydroponic systems, assuming that the remaining area would require a
protective surface for human traffic. The surfacing for this prototype was
assumed to weigh five pounds per square foot. The consideration for this
design scheme would average less than 15 psf, given an averaging for the
distribution of weight between the hydroponic system and the pathways; this
enabled the Hydroponic prototype to be applied to many buildings with
loading capacities of 15 psf.
C. Productivity Calculations
A primary goal of the study was to estimate the quantity of food, energy, and
water that could be harvested from the Eastlake study area, and by
extrapolation, East Bay neighborhoods in general. While some explanatory
text regarding these calculations is included in Chapter 4, and the Findings
section, the following provides a more thorough description of the
assumptions and the calculations process.
1. Extensive Green Roof
Despite presenting a variety of other benefits, this prototype was not assigned
to any buildings or analyzed for productivity because food, energy and water
are not produced on the extensive green roof.
A-3
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2. Intensive Green Roof-Vegetable Garden
Neighborhood-wide productivity in this prototype is a function of plant
selection, growing area, and a “productivity coefficient” for each crop. The
criteria used for plant selection is described in Chapter 3 and Prototypes. The
growing area was assumed to average 60 percent of the area of any roof,
which is intended to account for mechanical equipment, vents, stairway
landings and other roof obstructions, as well as space for paths to maintain
the roof and storage of maintenance equipment. This proportion was applied
to the total area of roofs designated with the prototype (61,232 square feet, or
1.4 acres), to determine the total growing area of 36,739 square feet, or 0.84
acres. Note that this calculation assumes development of all nine opportunity
site with intensive green roof vegetable gardens.
Productivity coefficients for this prototype describe the amount of food that
can be grown in a space of a certain size—for the purposes of this study,
pounds of vegetables per square foot. Coefficients were derived from
University of California Cooperative Extension data.1 These figures clustered
between 0.30 to 1.0 pounds per square foot for each harvest. To account for
variation in site-specific plant selection, and in recognition of the fact that
there were no outliers to skew the outcome, the coefficients were averaged to
obtain a typical coefficient of 0.62 pounds per square foot per harvest.
Because a year-round gardening approach is assumed for the East Bay, this
coefficient was multiplied by three growing seasons, resulting in an annual
coefficient of 1.86 pounds per square foot of growing area. By multiplying
this figure by the total growing area above, an annual yield of 68,334 pounds,
or approximately 34 tons, was determined.
Nancy Garrison, Urban Horticulture Advisor, UC Cooperative Extension--Santa
Clara,
“Home
Vegetable
Gardening,”
http://vric.ucdavis.edu/
veginfo/commodity/garden/tables/table4.pdf (accessed April 1, 2007).
1
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3. Intensive Green Roof-Herb Garden
This prototype was not assigned to any buildings or analyzed for
productivity, but was included for the sake of testing options for lighterweight edible garden prototypes, and could yield a perennial harvest of
culinary or medicinal herbs.
4. Rooftop Hydroponic Garden
Yields for the hydroponic prototype were calculated on a similar basis as the
Intensive Green Roof-Vegetable Garden prototype. Growing area was again
assumed to comprise an average 60 percent of any roof designated with the
prototype. Total roof area assigned the hydroponic prototype was 82,731
square feet, or 1.9 acres. This resulted in growing area of 49,638 square feet,
or 1.1 acres.
The productivity coefficient was obtained from estimates by the UN Food
and Agriculture Organization, which has stimulated the use of Popular
Hydroponic Gardens (PHG) like the one described in the prototype in many
nations, and was confirmed with the Institute for Simplified Hydroponics,
which pioneered the use of PHGs in the developing world.2 Their initial
estimate of 40 kilograms per square meter (8.2 psf) annually,3 however, was
based on six annual harvests of lettuce as opposed to 3 annual harvests of leafy
greens.4 The FAO figure was divided into a per harvest yield, then multiplied
times three growing seasons to obtain an estimate annual yield of 4 psf. This
was multiplied by total growing area to determine the total annual yield of
approximately 198,554 pounds, or 90 metric tons, of vegetables.
Peggy Bradley, Institute for Simplified Hydroponics, personal email communication
with Ingrid Severson, May 31, 2007.
2
Charles Shultz, “Soilless in Singapore,” Growing Edge Magazine, retrieved from
http://www.growingedge.com/magazine/back_issues/view_article.php3?AID
=170324 (accessed April 1, 2007).
3
Juan Izquierdo, FAO, personal communication with Brian Holland, DC&E, in May
2007.
4
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5. Solar Photovoltaic Electricity
Assumptions regarding electricity output were based on regional sources and
on the PVWATTS calculator produced by the National Renewable Energy
Laboratory. The first step in these calculations was to determine the roof
surface area that would be suitable for a photovoltaic installation. Because
buildings were oriented 45 degrees off of north-south, it was assumed that half
of the area of pitched roofs (southeast and southwest faces) would have
suitable solar access. The total “plan-view” area of southeast or southwestfacing pitched roofs was 861,225 square feet. Total surface area of 1,148,212
square feet was determined using a typical roof pitch angle in a trigonometric
function. Based on aerial analysis and field observation, it was estimated that
40 percent of this space, or 459,285 square feet, would be suitable for panel
installation, due to constraints relating to shading and architectural detail.
Finally, the area of flat roofs assigned the prototype was determined
separately and multiplied by 40 percent to determine the suitable flat roof
area of 392,463 square feet.
The next step was to determine an annual “productivity coefficient” for
electricity generation. The PVWATTS Version 2 calculator was used to
calculate annual kilowatt hours (kWh) of electricity produced per square foot,
specific to the Eastlake area weather conditions. It was determined that
approximately 12.78 kWh/square foot could be produced on southeast faces
and approximately 13.59 kWh/square foot on southwest faces, assuming an
ideal tilt of 37.7 degrees.
Southern-oriented panels could produce
approximately 14.15 kWh/square foot on flat roofs with panels oriented
south. Of course, these estimates will vary depending on environmental
conditions at the site and on the specific equipment in use. However, the
PVWATTS calculator is a widely-used tool that incorporates a variety of
factors, including a DC to AC derate factor of 0.77 that considers differences
between Standard Testing Conditions rating and actual field performance, as
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well as losses at the inverter, transformer, diodes, connections, AC and DC
wiring, and losses due to soiling.5
Using the productivity coefficients generated by the PVWATTS calculator,
total annual electricity yield of 11,609,024 kWh was calculated for the
Eastlake area, produced by 8.5 MW of photovoltaic panels.
Description of the assumptions and methodology for this calculation was
included in Chapter 4, Findings. To reiterate, total “plan-view” area of roofs
designated with this prototype was determined using GIS. Total catchment
capacity was calculated by multiplying roof area by annual precipitation and
applying a runoff coefficient of 0.85 for pitched roofs and 0.50 for flat roofs
to account for absorption, pooling and evaporation, and for spillage in the
conveyance of water.6
Catchment capacity is somewhat irrelevant, however, if cisterns and other
storage infrastructure are not available to hold the captured water. An
average storage capacity of 1,000 gallons was assumed based on aerial
photograph analysis and field observation of available open space. This is the
capacity available at any one time, but over the course of a year more than
1,000 gallons can be stored as water is used for irrigation, freeing up
additional storage capacity.
As most rainfall in the Bay Area occurs from late fall through early spring
and the highest demand for irrigation water occurs in the summer, it is
assumed that the harvested water will account for only a portion of the
irrigation needs for individual properties. The water demands will vary upon
a number of factors including plant selection, solar exposure and soil type.
Seasonal variations in rainfall and drought will also impact water need
For more information on the PVWATTS Version 2 parameters, see
http://rredc.nrel.gov/solar/calculators/PVWATTS/system.html.
5
6
Email from Marc Richmond, June 24, 2007.
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throughout the rainy months. Therefore, a conservative estimate assumes
that the cisterns will be filled and emptied an average of three times per year,
resulting in 3,000 gallons harvested per structure or 1,869,000 gallons
harvested annually and used for irrigation in the study area. To account for
these flows, a basic monthly water balance calculation was performed that
assumed all water captured could be usefully applied to the landscape between
rains in the fall, winter, and spring, and the remainder of the captured water
would be used in the dry season during the summer.
A-8

Tapping the Potential of Urban Rooftops

Transcription

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