Down to Earth Design

Down to Earth Design
design | educat ion | consult ing
with an exclusive focus on eco-sensible buildings
COB
Down to Earth Design
Sigi Koko. Principal
admin @ buildnaturally.com
202-302-3055 DC
215-540-2694 PA
www.buildnaturally.com
WWW .BUILDNATURALLY.COM
WHAT IS COB?
Clay soil offers a versatile building material used for
thousands of years to create beautiful and durable
structures. Clay can be formed into building blocks
(such as adobe) or monolithic walls (such as rammed
earth and cob).
Cob walls combine the same
ingredients as adobe sun-baked bricks – clay, sand, and
straw – but cob is sculpted in place when wet, thus is
familiarly referred to as “sculptural adobe”. Completed
walls are plastered with clay or lime to obtain a fine
surface finish. The ingredients for cob cost almost
nothing, especially if clay can be dug from the building
site.
T HE INGREDIENTS
1. sand: aggregate, provides strength and shrinkage
control; use course sand, such as “concrete sand”
2. straw: provides tensile strength and helps minimize
cracking; straw should not be chopped
3. clay: binder that holds all of the ingredients
together
Do not use top soil! Organic top soil adds inert fill into
the cob with no benefit. Top soil shrinks over time,
potentially creating voids in the finished wall. Remove
top soil to stockpile for gardening & look for clay below.
B ENEFITS
Completely non-toxic. All of the ingredients used to
make and finish cob are completely non-toxic. It would
not taste good, but technically you could eat it.
Local, Indigenous. Often the soil dug from foundation
excavation contains sandy clay and can be used to build
with. If you do not find clay soil locally, dry bagged
clay works as well, however it should hydrate for
several days to make the clay sticky enough.
High Thermal Mass. Cob walls have high thermal
mass that can store the sun's heat energy in passive
solar design and provide thermal storage for a masonry
heater.
Fully shaded cob provides free cooling in
summer by storing cool energy and absorbing humidity.
Low-Tech & Easy to Learn. Techniques for mixing
and building with cob are extremely easy to learn and
fun. Tools needed are few and inexpensive: shovels,
tarp, and buckets.
C HALLENGES
• building code compliance (if using cob structurally)
• extremely time & labor intensive
• only appropriate where high thermal mass, noninsulating wall provides the best energy efficiency
(difficult to heat otherwise)
TESTING SOILS FOR CLAY
RESOURCES
WEBSITES
W ORM T EST
Making a worm with your soil provides a
quick preliminary test to determine any
clay content in your soil.
It works
because clay is uniquely sticky when wet.
1. Take a small handful of soil to test and
remove any visible rocks.
2. Mix in a small amount of water, just
enough to make the soil malleable.
3. Roll the soil into the shape of a worm.
4. If the worm remains intact and
provides resistance to pulling apart,
the soil contains clay
S HAKE T EST
A simple shake test determines
relative percentages of clay
and sand contained in the soil.
It works because clay remains
suspended in water, whereas
sand and silt sink in water.
1. Fill approximately ¼ of a cylindrical-shaped glass jar
with crumbled soil (free of visible stones).
2. Fill to the top with water, close the lid, and shake
well, until all of the clay is dispersed.
3. Set the jar down on a level surface and watch for 10
seconds. All of the sandy solids will settle to the
bottom. Draw a line on the jar at the top of the
sand. The water remains cloudy with clay.
4. When the water becomes completely clear, draw
another line at the top of the settled clay. The ratio
between the height of the sand and the height of the
clay represents the ratio of sand to clay in the soil.
Note: it is difficult to differentiate silt in this test, as
silt is similar to sand, only smaller and spherical.
MAKE TEST BRICKS
Test bricks allow you to determine the best proportions
of clay and sand for your soil. The strongest cob
contains the maximum amount of sand, while still
having enough clay to provide excellent binding. Not all
clay is equally “sticky”, so different clays allow more or
less sand. Begin with a brick made with 100% clayey
soil, then make bricks with increasing amounts of sand
until it is clear that there is not enough clay in the
mixture (you can no longer keep the cob intact). Write
the proportions directly on each brick. Do not add
straw to the test bricks. Once the bricks are completely
dry, drop them from shoulder height, starting with the
sandiest brick. The first brick that does not break is
your ideal proportion of soil to sand.
www.cobcottage.com
www.cobprojects.info
en.wikipedia.org/wiki/Cob_(material)
www.buildnaturally.com/EDucate/Articles/Cob.htm
B OOKS
The Hand Sculpted House: A Practical and Philosophical
Guide to Building a Cob Cottage by Ianto Evans and
Linda Smiley
The Cobber’s Companion: How to Build Your Own
Earthen Home by Michael G. Smith
The Cob Builders Handbook: You Can Hand-Sculpt Your
Own Home by Becky Bee
Building with Cob: a step-by-step guide by Adam
Weismann and Katy Bryce
Building with Earth: A Handbook by John Norton
Build Your Own Earth Oven by Kiko Denzer
You Can Make The Best Hot Tub Ever by Becky Bee
HANDS-ON WORKSHOPS
Down to Earth Design teaches workshops on
building with cob, where we build walls, surround
masonry heaters, or construct a cob oven or hot tub.
WORKSHOPS INCLUDE:
• overview of cob construction, including ingredients &
properties of cob
• demonstrations of how to test soils for clay content
and mixing techniques
• guided hands-on experience mixing & building with
cob, including how to trim walls, prep for plaster,
create niches, and more
Check our website www.buildnaturally.com
additional information.
for