Villa Tomati, a roof within the roof

Villa Tomati, a roof within the roof: a project to restore the past throughout knowledge.
Goffi, F.1, Bacigalupo, M.2, D’Alessandro, F3.
ABSTRACT
This paper aims at describing the diagnostic study on structural performance of the XV century wooden roof of Villa
Tomati, located in Genoa, Italy. Several wooden roofs were analyzed with the purpose to obtain fundamental data in order
to restore these ancient covers, which can be considered particularly valuable from the historic-artistic point of view.
General principles regarding both Restoration and Diagnosis of structural performance were framed in International
Charters (ICOMOS, 1995) (Venice Charter, 1964). In particular reference literature regarding diagnosis methods and
grading of timber is represented by the European one (Ceccotti, Uzielli, 1989), (Giordano, 1993), (Bonamini, 1996).
The structural performance analysis, which is based on the diagnostic study, is considered fundamental in order to define
a restoration project based on minimum interventions (ICOMOS 1995). Structural performance will be performed by
finite elements analysis using the packaging software STRAND 7. The final goal of the conservation project is to restore
the structure bringing it back to the original state, preserving structural conception and roof typology.
The work described shows the importance to perform a detailed diagnostic study prior to define the restoration
intervention in order to be conscious of Conservation State of the structures. This is fundamental in defining an adequate
restoration project, reaching the goal of operating minimum interventions and to maintain as much as possible of original
materials while preserving original structural conception and typology.
This paper describes the wooden roof typology found in this Villa and the diagnostic method applied for this study. A
detailed analysis of the results of the diagnostic process will be discussed. The structural analysis approach chosen for the
study of this complex carpentry will be presented and analyzed. Finally it will be possible to foresee possible directions
regarding restoration project strategies.
INTRODUCTION
This study is framed in a broader research work that has been developed during the last decade at the DIPARC4, focusing
on the analysis of Villa’s typologies from the late Gothic period to the late Renaissance period (XV-XVII century). These
studies regarded several aspects concerning typologies, structural conceptions, construction technologies, and materials
utilized in the construction of Villas during the period of time previously mentioned.
Villa Tomati has been dated to XV century based on typological analysis study. This Villa represents one of the first ones
built in Genoa, dating from the late Gothic period. Villas were commonly utilized for summer residence by the noble
families of the region (Fig. 1, 2, 7). Villa Tomati represents a unicum since it is the only one survived from late Gothic
period in this area, and it has been conserved without relevant alterations maintaining the characteristic L floor plan so as
most of its original features. The Villa typologies have been developed in the following century (XVI) with several
1
Currently pursuing a doctoral degree in Environmental Design & Planning, Virginia Polytechnic Institute and State
University, EDP program, 0113 Architecture Annex, Blacksburg, VA 24061, US. At the time of this study she was
associated with the DIPARC, Faculty of Architecture, Genoa, Italy.
2
Currently pursuing a graduate degree in Architecture, DIPARC, Faculty of Architecture, Stradone S.Agostino 37, 16123
Genoa, Italy. The dissertation will be discussed on the topic discussed in this paper.
3
Currently pursuing a graduate degree in Architecture, DIPARC, Faculty of Architecture, Stradone S.Agostino 37, 16123
Genoa, Italy. The dissertation will be discussed on the topic discussed in this paper.
4
Several research workers and students under Prof. Andrea Buti guidance have developed the work we refer to, within
the Faculty of Architecture, Genoa, Italy.
examples referring to a different typological model that has been started in Genoa by Galeazzo Alessi (1512-1572). The
roof object of this study has been built contemporary to the main building.
Fig. 1 Villa Tomati, Genoa, Italy, dating from XV Century.
Main prospect.
Fig. 2 Villa Tomati, main floor’s plan.
The wooden-roof typology defined as a “basket” roof refers to the local building construction tradition (Buti, A., et al.,
1985); which is characterized by the use of wood technology derived from naval carpentry. The roof structure, in fact, is
similar to the keel of a ship overturned upside down. It is a three dimensional structure in which each element contributes
to the general behavior of the wooden carpentry. Since the construction time the roof has not been modified significantly
and it has been preserved for most part; therefore integrity and authenticity remained unaltered so far.
The final goal of the preliminary study, conducted via diagnostic analysis and structural assessment5, is to help
understanding thoroughly the static behavior and the conservation state of this complex carpentry in order to define the
restoration project.
DESCRIPTION OF THE WOODEN COVERS TIPOLOGY
The wooden roof cover is quite complex (Fig. 3, 7, 10, 11): two series of roofs superimposed over each other constitute it.
The lower one is constituted by five decorated basket roofs (b.r.), which were originally meant to be seen. The
decorations on the intrados are/were visible from the major rooms (Fig. 3-4). Two of the five basket roofs (b.r. 1, 3, 5) are
now concealed because of the introduction, during XIX century, of false vaults6. The upper cover is constituted by a
single basket roof, which protects the five smaller ones lying underneath. The function of the upper cover is, in fact, to
offer a sacrifice-protection layer to preserve the lower roofs, which were considered more valuable because of the
decorated double saddle beams used for the construction (Goffi, Giannico 1999). The upper and the lower basket roofs
are structurally interrelated by means of rafters and by sharing some pitches. Two of the lower basket roofs are
structurally independent (b.r. 2, 3) from the upper basket roof.
In general diagonal beams, horizontal beams, small beams and small-rafters constitute the main structure of basket roofs.
The small-rafters were used to reduce the unbraced length of diagonals and horizontal beams; those elements, in fact,
carry most of the dead and live loads. The structural system is completed by the superimposition of the wood plank layer,
over which lays allurement mortar and the final layer constituted by slate slabs. The whole roof is allowed to deform
slightly under any type of load pressure because of the elasticity of the timber structure realized, meaning that the
structure can deform slightly without collapsing under major wind loads effects for example. The wood plank acts like a
5
This part of the study is currently being developed.
The restoration project foresees to remove those false volts in order to make again visible the basket roofs, as they were
originally meant to be.
6
connecting slab that can help redistributing the forces homogeneously over the main wood elements of the structure.
Regarding this it is not totally correct to distinguish a main structure (bearing structure) from a secondary structure
(finishing layer constituted by wood plank + allurement mortar + slate) since all the elements cooperate together to the
global behavior of the structure. In the realization of this kind of structural typology there is no need to use beams of great
size. Due to the fact that each element contributes to the general behavior, as explained before, beams have limited
dimensions7. This facilitates also the finding and preparing of material for construction.
The small beams constituent the lower basket roofs are carved to realize the characteristic transversal section form of the
double saddle beams (Goffi, Giannico 1999). The double saddle beams were subsequently decorated underneath with
geometrical (b.r. 3) and floral drawings (b.r. 1, 2, 4). Regarding the basket roof 1 those decorations were located in
correspondence of the diagonals and horizontal beams on broad wooden boards that served to hide these structural
elements.
Fig. 3 Details of the roof interiors:
upper basket roof and lower b.r. 2.
Fig. 4 Detail of the decorations on the intrados of basket roof 2.
These wooden boards were decorated with representations of
the sky dome; gold and lapis lazuli stone were used to create
the color pattern (Fig.5). Because of the severe damage
suffered the most part of these boards have been lost; a few
have been preserved and need to be restored and repositioned
in their original location.
The roof beams lay their hedge on sleepers positioned along
the perimeter of the building. Both the iron rods and the
sleepers, which constitute a ring around the building, help
containing the horizontal forces generated by the roof
structure. The horizontal forces produced by this structural
typology are minor respect to the ones produced by traditional
wooden trusses roofs (Galliani, 1984).
Fig. 5 Detail of the sky dome decorations in gold and
lapis lazuli color pattern on basket roof 1.
REPORT ON THE DIAGNOSTIC STUDY OF STRUCTURAL PERFORMANCE
The diagnostic study consists of a 1st phase of visual analysis and a 2nd phase of Non Destructive Testing, both performed
in situ in order to help verifying the structural reliability of the wooden roof. Therefore it is essential to determine
7
Average dimensions: base equal to 7 cm and height equal to 14 cm, inter axis between elements equal to 38 cm.
physical and mechanical characteristics of materials with adequate accuracy, furthermore it is fundamental to quantify
and qualify decay suffered by the structure while in service. Visual grading of timber has been performed following the
method of Prof. G. Giordano (Giordano, 1993). A few N.D.T. helped determining moisture content (Hydromette HTR
300®) and quantifying decay (resistance to dynamic penetration, Pylodin 6J®).
The results of the different analysis performed over more or less 850 beams were schematized in thematic maps reporting
the results regarding wood species surveyed, grading of timber, moisture content evaluation and quantification of decay
extension.
Wood species found in this structure are Silver Fir (Abies alba Mill.), Chestnut (Castanea sativa Mill.) and Oak (Quercus
sp.). Conifer wood has been used for most of the roof structures (80% of material analyzed); Chestnut has been used
mainly for b.r.5 and for a small part of the elements constituent the upper basket roof, finally Oak has been used just for
few small rafters (Table 2).
Wood specie
TABLE 1. Grading of wood according to Giordano, 1993.
Grade
σadm
σadm
σadm
Compression
st
Spruce
Abies alba Mill
Chestnut
Castanea sativa Mill
1
2nd
3rd
1st
2nd
3rd
(N/mm2)
11,0
9,0
7,0
11,0
9,0
7,0
Bending
(N/mm2)
11,5
10,0
7,5
12,0
10,0
8,0
Traction
(N/mm2)
11,0
9,0
6,0
11,0
9,0
6,0
σadm
Shear
(N/mm2)
0,9
0,8
0,7
0,8
0,7
0,6
Modulus of
Elasticity
(N/mm2)
13.000
12.000
11.000
10.000
9.000
8.000
Grading of timber revealed that mostly material of third grade has been used (60% of 850 beams), regarding each species
found (Table 1, 2). The remaining 40% was subdivided between second (30%) and thirst grade (10%).
In general, regarding the upper cover, moisture content varied in the range 13-16 %, therefore material performs in best
conditions (Fig. 6). The bad maintenance conditions of some regions of the covering layer (slate + allurement mortar +
wood plank) determined higher moisture content for 50% of the beams at the extrados side8. Regarding the lower basket
roofs they do not reveal particular problems regarding moisture content, since the upper cover protects those. The
moisture content values registered in fact are normal (13-15%). The lower basket roof 4 represents an exception, here in
fact high moisture content values have been observed; the average value exceeds in fact 20%. This is due to very bad
maintenance of the upper roof cover and to the fact that this basket roof surmounts an open lodge. Here fungus attacks
combined with high M.C. determined decay of sapwood. A second monitoring phase regarding the moisture content will
be required after restoration, in order to evaluate the reduction of it. If M.C. will not decrease a preservative treatment will
be advised in order to stop fungus attacks (Table 2).
Regarding decay pathologies several kind of attacks have been observed, due mainly to Anodiidae and Ceramyicidae;
depth of decay was not severe and varied in the range 0,25-0,50 cm. More or less 45% of the 850 elements examined
presented a decay depth equal to 0,25 cm, which corresponds in average to a reduction of the section area of 9%. Just
10% of the elements presented a decay depth of 0,5 cm, which corresponds in average to a reduction of the section area of
18%. Regarding the remaining 45% of material, 35% of the elements were intact, no decay was observed on them, while
another 5% showed a decay depth equal to 0,75 cm and the remaining 5% had a decay depth exceeding 1,00 cm (table 2).
Few beams were missing due to fire destruction (b. r. 1, 2) and need to be replaced with material of the same wood
species.
8
Values reached 20-30% of moisture content.
Fig. 6 Upper basket roof. Thematic map schematizing moisture content survey.
Fig. 7 Villa Tomati, Genoa, Italy, dating from XV Century. Transversal section.
TAABLE 2. Summary report on the diagnostic study results regarding wood species,
grading of timber, moisture content and decay.
ROOF
COVER
Total
beams
N°
Wood species
Grade
Beams % / Total
Moisture
content (%)
Beams% / Total Beams % / Total
Silver Fir
Abies alba Mill.
1st
2nd
3rd
10%
40%
50%
13-15%
92
Silver Fir
Abies alba Mill.
1st
2nd
45%
55%
13-15%
BASKET
ROOF 3
94
Conifer
1st
2nd
3rd
80%
10%
10%
BASKET
ROOF 4
88
1st
2nd
3rd
BASKET
ROOF 5
120
Silver Fir 50%
Abies alba Mill
Chestnut 50 %
Castanea sativa Mill.
UPPER
BASKET
ROOF
305
Silver Fir 75%
Abies alba Mill
Conifer 25%
TOTAL
850
BASKET
ROOF 1
151
BASKET
ROOF 2
Conifer
Silver Fir 60%
Abies alba Mill
Conifer 30%
Chestnut 10%
Castanea sativa Mill.
Decay (cm)
Original
Average size
bxh (cm)
Section area
% reduction
0.00
0.25
0.50
30%
50%
20%
12x16
100%
0.25
0.50
80%
20%
10x15
13-15%
15-18%
60%
40%
0.00
0.25
0.50
>1.00
40%
40%
10%
10%
6x12
15%
40%
45%
> 25%
100%
1st
2nd
3rd
10%
20%
70%
13-15%
100%
1st
2nd
3rd
20%
40%
40%
1st
2nd
3rd
25%
35%
40%
13-15%
15-18%
>25 %
100%
N.A.
7%
14%
8%
16%
9%
23%
45%
7x11
0.00
0.25
0.50
0.75
15%
50%
20%
15%
7x14
Intrados
13-15% 70%
15-18% 30%
Extrados
13-15% 50%
15-18% 25%
21-25% 15%
> 25% 10%
0.00
0.25
0.50
45%
40%
15%
7x13
80%
15%
5%
0.00
0.25
0.50
0.75
>1.00
35%
45%
10%
5%
5%
11%
21%
30%
11%
21%
STRUCTURAL PERFORMANCE ANALYSIS METHOD
Structural analysis, currently on process, will be performed by finite elements numerical analysis using the software
package Strand7. A detailed structural analysis, based on the results obtained by the previous diagnostic study, is in fact
necessary in order to comprehend the general behavior and the role and contribution given by each structural element.
Several structural analyses will be performed; in a first one the original conception will be analyzed (in this stage of
analysis decay in not taken in account), in a second one the structure will be modeled considering also decay factors. In
the second analysis, in fact, the resistant sections will be reduced proportionally to decay depth evaluated. In a third
numerical analysis the new elements introduced during the past few years as provisional elements (Fig. 8, 9) will also be
taken in account: this will allow to understand current structural behavior of the wood carpentry. Comparing the results of
the three stages will help to understand structural changes and the role of each element at present time and in the past.
Finally, several structural schemes will be investigated, in order to evaluate possible restoration interventions.
Fig. 8-9 Interior view of basket roofs before and after the introduction, for safety reasons, of provisional elements.
CONSIDERATIONS ON THE RESTORATION PROJECT
The diagnostic study of timber elements enlighten the fact that ordinary maintenance has been neglected during the recent
years, in fact the covering layer is in bad conservation state. In particular the slate and wood plank need to be totally
replaced. Neglecting maintenance during the last decades accelerated decay exponentially; if maintenance of the covering
layer would have been performed during the last years this structure could have been in excellent conservation state. In
fact, moisture content is significantly higher in those regions were the slate and wood plank layers were in worst
conditions.
One of the main objectives of the restoration project is to eliminate the numerous new elements introduced in the structure
during last years as provisional elements, which were needed to prevent risky situations from occurring. In some cases the
new elements introduced created new problems weakening the structure that needed support (Fig. 8-9).
New elements could eventually be introduced if final assessment will revel it is needed. Several solutions will then be
evaluated in order to select the most convenient in term of minimizing the introduction of new materials.
The final goal of this overall study is to advice proper actions in order to strengthen the timber structure maintaining the
original structural logic. Few original elements will be replaced with new material of the same wood kind, because of the
severe damage suffered, and a limited number of new elements could be introduced to upgrade the structure if needed.
What seems likely to happen is that some small-rafter will need to be added. The introduction of wood rafters will allow
upgrading the structure in line with the original structural conception.
Fig. 10-11. Two views of the model of the roof carpentry of Villa Tomati. It is visible the upper basket roof which
constitute the protection-sacrifice layer for the five basket roofs lying underneath.
FINAL WORDS
The final goal of each intervention on buildings of proven historical value is currently recognized by several international
agreements as conservation. Nevertheless our actions aimed at conservation have to be defined by considering the
numerous constraints and issues arising from the problem itself. Our goal should be to reach a possible solution, which
can be defined as a subtle compromise between what should be conserved and what could be sacrificed.
Accurate diagnosis is the key tool to define minimum interventions and to preserve integrity and authenticity; therefore it
should be considered a mandatory step to be undertaken before any intervention on buildings of proven historical value
may occur (Buti, A., 1994).
In general it is not sufficient to evaluate the Conservation State of structures and upgrade them, but it is essential to
determine the causes that produced decay since the non-identification of these will determine the recurrence of decay
pathologies.
The wood cover analyzed survived for more then four hundred years with little maintenance. Most part of the decay
observed recently occurred due to complete absence of maintenance. The bad conservation state of the cover layer (slate +
allurement mortar + wood plank) is the main reason for the decay occurred in the last years and for the high values of
moisture content registered. This enlighten the fundamental importance of maintenance, which could allows these
structures to survive in excellent conservation state for centuries, without needing important restoration interventions.
This suggests that the restoration intervention should not conclude with the intervention itself but should foresee the
planning for future maintenance over permanent bases.
AKNOWLEDGEMENTS
A special acknowledgement goes to Prof. Andrea Buti (DIPARC, Faculty of Architecture, Genoa, Italy) for his precious
guidance throughout every stage of the work. The fundamental contribution of Arch. Claudio Montagni is also recognized
for the historic research and for the typological study conducted on the monument, which leaded to the dating of the Villa
Tomati to the XV century.
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Buti, A., 1994. La diagnostica strutturale negli interventi di recupero. In Acts of the Conference Recupero e
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Proceedings of the 1st RILEM Symposium on Timber Engineering, Stockholm, Sweden.
ICOMOS International Wood Committee. November 1995. Principles for the Preservation of Historic Timber Structures.
The Venice Charter, Venice 1964. International Charter for the Conservation and Restoration of Monuments and Sites.
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