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Deterioration of building envelope of wooden apartment buildings
built before 1940 based on external survey
Paul Klõšeiko
Tõnis Agasild
Targo Kalamees, Ph.D.
Chair of Building Physics and Architecture, Tallinn University of Technology, Estonia
KEYWORDS: Wooden apartment buildings, facades, durability, service life, renovation
SUMMARY:
Several studies that have analysed the state of wooden apartment buildings have concentrated mainly
on heritage protection and architectural history. Furthermore, the scope has usually been restricted to
specific types of buildings. This study aims to discover patterns in the behaviours of homeowners in
maintaining their property and to specify key problems in the technical state of apartment houses on a
wider scale than before.
The study concentrates on the external side of building envelope (foundation walls, external walls,
roof, windows etc). A questionnaire to assess the state of different parts of the buildings was composed
and the data of 133 buildings was gathered. An analysis was carried out to connect probable causes
to consequences.
Severe shortcomings in the maintenance of houses were discovered. Vast majority of them are directly
or remotely connected to the rainwater drainage system – simple recommendations that should be
kept in mind by the homeowners can greatly contribute to the durability of the buildings. It was also
confirmed, that many of the houses will require major renovation in the near future.
1. Introduction
Urbanization in the beginning of the 20th century resulted in wooden apartment buildings becoming
mass consumption article in Estonia. Wood was the main structural building material until World War
II. Most of the old wooden apartment buildings are located in historic listed areas. Historic listed
residential areas are built-up areas of cultural and environmental value designated by a plan, the
integral social environment of which is to be preserved because of their historical street network,
greenery, building style, general characteristic architecture or other reasons of public interest. Often it
is not the individual building that is valuable, but rather the resulting ensemble or community. Today
there are about 40 historic listed areas in Estonian towns. Many of the historic listed areas are in
danger of destruction and need thorough and ongoing renovation. However, there is also a lack of
local guidelines aimed at increasing durability and energy-efficiency while preserving the milieu of
wooden districts. In 2004-2005 the Estonian Heritage Board prepared 15 sets of restoration guidelines.
Nevertheless, these stipulations mostly concentrate on architecture; less information is given about
structural durability, building physics and energy efficiency. This study, on the other hand, is
conducted on a wider scale and has the purpose of specifying the key problems in the technical state of
apartment buildings, and to discover patterns in the behaviours of homeowners in maintaining their
property. In this paper the technical conditions of the external side of the building envelope (facades,
windows, doors, roof etc.) are presented.
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2. Methods
2.1 The choice of studied buildings
Because of differences in building styles and materials, it was decided to cover four Estonian towns
with distinct areas of wooden apartment buildings – Tallinn (population: 412 000), Tartu (population:
103 000), Pärnu (population: 44 000), and Viljandi (population: 20 000) – within the study.
In some towns certain types of buildings can be distinguished. In Tallinn the “Lender house” (FIG 1
left) and “Tallinn house” (FIG 1 right) were two main types of apartment buildings before World War
II. The basic plan of the “Lender house” is very simple: centred staircase on the symmetrical axis and
four to six single-room apartments on every floor. The external door opened to a street level but
another entrance and the stairs were placed on the back because of fire restrictions. From late 1920s,
the three-storey wooden (log) buildings were permitted with a central stone staircase – this building
type is called a “Tallinn house”. The plan of the Tallinn-type was again very rational: on the first and
second floor there were two-room apartments and on the half-basement there were shops and small
businesses. The prominent or plain stone staircase with more than two floors was a typical accent for
the street facade. Similarly to Tallinn, other towns also had typical types of wooden apartment
buildings.
FIG 1. External view to a “Lender house” (left) and “Tallinn house” (right)
The majority of apartment buildings are private properties with almost none owned by the local
government or the state.
2.2 The survey
To comprehend the overall technical state of wooden apartment buildings and the most common areas
requiring improvement, the research is based on a statistical survey of 133 apartment buildings in four
towns. Due to the need for a large number of samples, it was deemed viable to conduct the study with
an external survey.
A special questionnaire was created to cover all parts of the building that could be judged externally –
thus generating an impression of the specific neighbourhood. Buildings that had recently been
completely renovated would have provided too little data, thus they were excluded from the study. The
survey was carried out mainly by two researchers. To ensure the conformity of the results, the
questionnaire was filled in by them together, while later on other people also helped under
supervision.
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To assess the technical condition of building components, six-category classification for technical
condition of the building component was used:
• 0 – failure/dangerous situation, very severe damage: needs immediate renovation;
• 1 – bad, severe damage: needs renovation during one year;
• 2 – satisfactory, moderate damage: renovation recommended within three to five years;
• 3 – good, slight damage: maintenance recommended within five years;
• 4 – very good, very slight and small damage: may need maintenance within ten years;
• 5 – excellent, no visible damage, new structure in good condition, correct performance.
FIG 2 shows an example of the use of the categories in the case of cladding’s paint layer.
FIG 2. Example of the assessment categories of cladding’s paint layer
The score was given according to the state of the worst example of the component. These assessments
were accompanied by a catalogue of probable defects and their causes. Taking into account the
essence of the building (decorations, roof, windows, thickness of the foundation wall) an estimate of
maximum thickness of reasonably installable heat insulation was also noted. To illustrate the report
and to answer questions arising later on, photos of the buildings and their details were also taken in the
process of the survey.
Sections of the questionnaire:
• Foundation wall: dimensions, material, condition of the rendering, windows, causes of defects;
• Facade: material, decorations, condition, windows, doors, maximum thickness of additional
thermal insulation;
• Rainwater drainage systems: existence of the components, condition;
• Roof: material, leaks, condition;
• Chimney: existence of the components, condition;
• Recommendations: required tasks to be attended and timeframe to do so (1, 3, 10 years).
3. Results
3.1 Roofs – defects and their probable causes
Originally, after the construction of the building, a standing seam metal roofing (service life with
maintenance ≈60 years) was used to cover the apartment buildings of the observed era. During the
second half of the 20th century, asbestos-cement boards (service life with maintenance ≈40 years) were
widely used as the only available replacement (often mounted atop of sheet metal roofing). However,
by now, the service life of both materials is nearing the end or already past their service life, which is
visible both from observation and from statistics.
Category 1 Category 2 Category 3
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Nowadays, availability of building materials is no longer a problem. Corrugated metal sheeting is a
cheaper alternative to the stone or standing seam roofing and has begun appearing on the buildings of
the observed era. Nevertheless, according to the requirements of the historic listed areas, it is
necessary to use as much original or similar to original building solutions as possible.
TABLE 1. State of the roofs distributed by roofing materials
Type of roofing material Standing seam
sheet metal
Asbestos-cement
board
Corrugated
sheet metal
Stone roofing
Percentage of roofing materials
(note: several materials could be
present on a roof
simultaneously)
54 % 35% 15% 9%
Average category (FIG 2) of the
technical condition
Good
(cat. 3)
Satisfactory
(cat. 2)
Very good
(cat. 4)
Good
(cat. 3)
Roofs in bad or dangerous
condition, (cat. 0 and 1) 3 % 4 % 0 % 0 %
Roofs requiring replacement
within 3 years (cat. 2) 13 % 35 % 0 % 25 %
Roofs requiring replacement
within 10 years 33% 48% 0% 17%
Typical defects of roofs are the following:
• Flora on the roof that prevents water drainage: 50 % of stone and asbestos-cement roofs
suffered from growth of moss or plants;
• Loose or missing roofing tiles: most of them are still on the roof or around the building;
• Corrosion of metal: 63 % of metal roofs required repainting within 3 years – in these cases the
protective layer on the metal sheets has degraded, while painting would be a relatively cheap
and simple way to postpone complete replacement of the roofing;
• Cracking of the asbestos-cement boards – caused by heavy loads, bending of the framework and
temperature deformations due to too tightly fastened boards;
• Missing or defective water flashings
• Rotting soffits
The causes of most common defects can be traced to a lack of maintenance and/or the end of roofing’s
operating life. The first point is supported by the fact that many of the defects (e.g. moss, loose tiles)
could be eliminated with minimal costs by the residents themselves.
It is possible that besides indicating the homeowners’ wealth, the state of the roofing can also be used
to judge the overall care for the building, as it is a crucial part of the building and an unwillingness to
mend the deficiencies shows irresponsibility.
3.2 Facades – defects and their probable causes
Originally, the analysed building types have had wooden cladding. Apart from being repainted, the
claddings have, in most cases, remained unmodified.
Due to the need and desire to decrease the thermal transmittance of the external walls and to make the
building appear as plastered building, the rigid wood-cement chip boards with lime plaster were added
during the second half of the 20th century. The share of such buildings was about 27 % of plaster-
covered facades. Today, they usually suffer from cracked and broken off grout (along the edges of the
boards) which leads rainwater directly to the bearing wall.
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Wooden cladding may have a lifespan of 100 years and more when maintained properly; however, on
31% of all the buildings, the state of paint was under satisfactory levels. On many occasions it was
apparent, that such situation had lasted for several years already. In such cases it is increasingly
difficult to apply a new layer of paint that would have maximum potential durability and will, as
observations confirmed, peel off in several years after being repainted. Another common defect was
failing to clean the wood before painting, which means that the newly applied layer would peel off
with the old one.
TABLE 2. Distribution and state of buildings by cladding material
Wooden facade Plaster facade
Share of all buildings (note: several cladding types
were present on same houses simultaneously) 93 % 8 %
Buildings requiring repainting within 3 years 63 % 46 %
Buildings requiring new cladding within 3 years 8 % 46 %
Buildings requiring new cladding within 10 years 34 % 73 %
TABLE 3 gives the most frequent areas with a deteriorated state of cladding. While some of them (e.g.
water flashings, corners and windows) have a naturally higher moisture load, and are expected to wear
out quicker, it is clear that other damage (leaking drainpipes, plants and trees against the walls, cables
tilted toward the façade) can be minimized by proper maintenance.
TABLE 3. Main areas on the facade showing signs of damage
Main areas
showing signs
of damage
Water
flashings /
planks
above
found. wall
Around
eaves
Near
drainage
system
Near
corners
Around
canopies
Near trees
and
vegetation
Around
windows
Near cables
with wrong
inclination
Occurrences
on buildings 48 % 30 % 26 % 21 % 11 % 9 % 8 % 8 %
Canopy roofs were present on about 2/3 of the observed buildings and their condition was good on the
average. Nevertheless on 11% of all cases, they did not have enough inclination away from the
building or were tilted the wrong way altogether, and thus, posed a threat to the building.
3.3 Windows, doors – defects and their probable causes
As windows are the “eyes of the building” they are important architectural components of historic
listed buildings. Originally, the buildings were installed with double pane windows in separate
wooden frames. The state of the windows is particular to the individual one and on average rated at
“satisfactory”, with the U-value of 2,7 W/(m2 K) at best. Until the beginning of the 1990-s, windows
might have been replaced with the same type of windows. Nowadays, several other types of windows
are becoming prevalent. On an average house, 38 % of the windows have been replaced. On the whole
selection, occurrences of windows by frame material are given in TABLE 4. There are a variety of
ways to improve the attributes of windows ranging from restoration to the replacement with new
windows.
Water flashings were present on every window on 54 % of the buildings. On the rest of the cases, on
average 62 % of windows were equipped with flashings. The shortcomings usually originate from the
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replacement of the windows, when the price is of the utmost priority and minimal equipment is
purchased. It is not uncommon to see even the polyurethane mounting foam left uncovered.
The condition of the external doors is rated “good” on average.
TABLE 4. Distribution and state of buildings by window frame material
Old wooden framed
windows
New plastic
framed
windows
New wooden
frame
windows
Restored
wooden framed
windows
Occurrences of windows by
frame material 89 % 57 % 34 % 13 %
Average technical condition Satisfactory (cat. 2) Excellent (cat. 5)
3.4 Rainwater systems – defects and their probable causes
While the material of the drainage downpipes and gutters was in a bad state on 26 % of the buildings,
the completeness and proper functioning, on the other hand, proved to be a problem on a much larger
scale – 72% of rainwater systems were defective.
44% of the buildings had a completely renovated rainwater drainage system. Nevertheless, alarmingly,
50% of them had deficiencies ranging from slight to severe. TABLE 5 gives an overview of these.
The main problems were caused by design defects (non vandal-proof lower sections of the pipes, ends
of the drainage pipes devised so that the water reached foundation wall), construction errors (pipes
connected in the wrong order) and by low maintenance (from smaller clogs to trees growing in the
gutters).
On many buildings in Tallinn, the worn-out drainage pipes were disconnected from the gutters – whilst
used temporarily, it can be considered sensible as the solution avoids water pouring directly onto the
façade, but when becoming permanent, such actions should be discouraged.
TABLE 5. Deficiencies with proper functioning of rainwater drainage systems
Rainwater flow to the
external wall or
foundation wall
Defects and
discontinuance of
rainwater systems
Clogging of
the rainwater
systems
Of all rainwater drainage systems 62 % 44 % 29 %
Of renovated rainwater drainage systems 40 % 21 % 14 %
3.5 Foundation walls – defects and their probable causes
The bearing material of the foundation walls of the surveyed buildings is usually brick, granite or
limestone, depending on the specific town. The main sources of risks are given in TABLE 6.
TABLE 6. Occurrences of main deficiencies of foundation walls (% of all buildings)
Moisture damages Different settlement Ground level over the top of foundation wall
58 % 32 % 8 %
Differential settlements were most emphasized in Tartu (soil contains a layer of peat) and Viljandi,
with 61% and 40% of buildings affected respectively. On some occasions the building was partially
situated on an old town wall and thus suffered from this phenomenon.
Excess moisture was the most common cause for damage, as 58% of buildings had signs of chipping
paint to completely destroyed mortar, bricks and limestone. The most frequently suspected reasons for
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the wetting of the foundation wall are found in TABLE 7. Algae can be seen as an indicator of
constantly moist surface – its growth was detected on 57% of foundation walls.
TABLE 7. Causes of excess moisture on foundation walls
Rainwater
downpipe
Vegetation Missing or
broken gutter
Sprays from
the street
Flows down
the wall
Wrong street
inclination
Gutter
outlet
35 % 22 % 20 % 14 % 14 % 13 % 11 %
The constantly added layers of asphalt have raised the street level so much, that on 8% of the
buildings it even exceeded the top of foundation wall. Instead of the masonry wall, the water from the
street then reaches the cladding and most probably the wooden bearing wall as well. Wrong
inclinations of the street were to blame on 13% of the cases.
3.6 Potential to add thermal insulation
When there is desire to further insulate the exterior walls at least the following should be considered:
depth of the eaves, placement of the windows, thickness of the foundation wall, firewalls and
neighbouring buildings. Field assessments of reasonably installable (i.e. without reconstruction of the
roof or foundation wall) additional thermal insulation thicknesses from technical point of view are 6
cm for facades with carvings and 8 cm with them.
On buildings already insulated with wood-chip boards, the previous insulation can be removed and a
more efficient material may be installed.
Naturally, to retain a coherent cityscape, the visual side of the result is also important and often
requirements by the heritage and historic listed protection office are in effect. Apart from technically
risky internal insulation, moving windows to the outer surface of the wall could be considered.
4. Discussion and Conclusion
In this study most severe damages were related to water which is flowing through roof, onto the
facades or foundation walls due to failure of rainwater system. Similarly, studies of Ekstedt (2002)
showed that the water protection efficiency plays important role on the service life prediction of
exterior wood coatings. In addition to humidity, the degradation of wood or the degree of mould
growth depends on exposure time, coating typology, wood substrate, temperature (Gobakken et. al
2010). Lauter and Time (2005) showed, that local location, surface treatment, material quality, and
micro location are significant parameters influencing the moisture content of wooden facade.
A field study about damages of wooden buildings conducted in 1995 in Finland revealed that 82% of
wooden houses built after 1950 have serious moisture damage (Heikkilä 2005, Partanen et al 1995).
The worst damage has been leaking of gently sloping roofs, floor damage caused by building too close
to the ground, damage of wet areas and damage caused by pipe leaks. However, the damage has
resulted more from experiments and errors in ways of building than from the use of wood as a main
building material. Wood is just the material that is not very moisture-tolerant and brings easily all
mistakes out. The data collected from 349 residential building surveys in seven European countries
(Balaras 2005) showed that the elements in the worst condition and requiring replacement are roofing
and facade. The information from previous studies is coherent with the current one.
As the most critical factor in degradation of wooden facades was free water, this should be taken into
account in design of buildings. Nevertheless, in many cases designers still concentrate mainly on
vapour diffusion alone. Designers should also pay attention to plain design such as directing the
rainwater away from house, keeping the rainwater system simple and durable. If a wooden building
has wide eaves and inclinations around house that direct the water away from house the damages are
less prone to occur. Building user should pay attention to any kind of signs informing about possible
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problem which will lead to damages since any kind of problem is much easier and less expensive to
fix in its early state. The preceding should be taken into account during renovation as well – wrong
solutions can seriously shorten the operating life of a building.
Based on current survey, we may conclude that on 23% of the studied buildings extensive construction
works ought to be carried out in the next 10 years (see TABLE 8). In those cases, it is clear that
substantial investments are needed. It might be in the interests of such buildings, if they had a full
refurbishment along with new room planning done, thus preserving the building and making them
more attractive in real-estate market. It is important to understand that as the buildings were largely
located in historic listed areas and need protecting from unsuitable renovation, renovation is also
needed to keep the milieu in existence and sustainable.
TABLE 8. Required renovation within 10 years (percentage of all the studied buildings)
New
cladding
New
roofing
Foundation wall
renovation
New
windows
At least 3 of the previous
combined
37% 52% 71% 17% 23%
It is recommended that universally approved renovation solutions should be worked out for different
levels of refurbishment (from single components to reconstructing everything except the bearing wall).
However, they should also incorporate energy performance and profitability analysis – these areas
have so far not been looked into in case of wooden apartment houses.
As relatively simple mistakes in managing the buildings were numerous, it might be necessary to
conduct a research on the sociological side of the matter and organize campaigns to raise the
awareness of proper maintenance and consequences of inaction.
5. Acknowledgements
Data used were derived from the national research project “Technical condition and service life of
Estonian wooden apartment buildings”. The financial support of the Credit and Export Guarantee
Fund KredEx and Tallinn University of Technology is gratefully acknowledged.
References
Balaras C.A., Droutsa K., Dascalaki E. & Kontoyiannidis S. 2005. Deterioration of European
apartment buildings, Energy and Buildings 37 (5). pp. 515–527.
Ekstedt, J. 2002. Studies on the barrier properties of exterior wood coatings. Doctoral thesis. Royal
Institute of Technology, Stockholm.
Gobakken, L.R. 2009. Surface Mould growth on painted and unpainted wood; Influencing factors,
modelling and aestethic service life. Doctoral Thesis, Institutt for Naturforvaltning, Universitetet
for miljø- og biovitenskap.
Heikkilä, J. Wood as a Housing material. 2005. Finnish Experiences of the Durability of Wooden
Houses. XXXIII IAHS World Congress on Housing Transforming Housing Environments through
Design, Pretoria, South Africa.
Lauter, P. and B. Time. 2005. A method to assess the effects of climate and material parameters on the
moisture conditions of a wooden facade. 7th Nordic Symposium on Building Physics, Reykjavik.
Partanen, P., Jääskeläinen, E., Nevalainen, A. et al. 1995. Moisture Damage of Single-Family Houses
– Clarification of Prelevance and Repair Costs. KTL, Kuopio (in Finnish).
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