Volume 2
9th Nordic Symposium on Building Physics - NSB 2011
Modelling of service life and durability of wooden structures
Hannu Viitanen, Ph.D. 1
Tomi Toratti, Dr.Tech. 1
Lasse Makkonen Dr.Tech. 1
Sven Thelandersson, Professor 2
Tord Isaksson, Dr 2
Eva Früwald, Dr 2
Jöran Jermer 3
Fin Englund, Dr 3
Ed Suttie, Dr 4
1
Technical Research Centre of Finland VTT, P.O. Box 1000, FI-02044 VTT, Finland
Lund University, Div. of Structural Engineering, Sweden
3
SP Technical Research Institute of Sweden, Wood Technology
4
BRE, Building Research Establishment, United Kingdom
2
KEYWORDS: Durability, modelling, service life, wood, structures
SUMMARY:
Basic factors and a draft model for evaluation of service life of wood cladding are presented. The
basic factors are based on research results on resistance and durability of wood products and on
practical experience on the factors affecting the durability of wood products in cladding. The input of
different aspects is integrated using an ISO factor method. The effect of different climate exposure is
introduced using a basic model on development of decay, which can be incorporated into a
hygrothermal model of building physics. This enables the assessment of the effects of the various
exposure conditions on the durability and service life of wood. It can also be used for the evaluation
of structural choices (e.g. protection to driving rain, coatings, etc.) and other affecting parameters
(e.g. geographical location and orientation) on the durability of wooden structures. The wood decay
model is here used in connection with the climatic databases which are based on weather
observations in Europe. These studies provide new tools to evaluate the durability and service life of
wooden parts and a preliminary European map of wood decay vulnerability is produced. The effects
of the projected climate change on wood decay may also be considered by this methodology.
1. Introduction
During their functional life, building and building components are exposed to several environment
conditions in numerous ways. For wood material, moisture stress and biological factors like mould
and decay fungi are often critical, especially in cladding and decking structures in exterior use
conditions. For mould and decay development, different mathematical models exist based on
laboratory and field studies. These can be used also for evaluating the different material properties for
durability and service life of wooden products.
In the future, the life time expectations and analyses of different building products will need more
data on the durability of products, service life and resistance against mould and decay, not only data
on wood material itself. Also the effect of other factors, like solar radiation, surface erosion, and
mechanical impact has a role for the durability of wood. The first step to evaluate the exposure
conditions is the macroclimate conditions. Driving rain, moisture, temperature and solar radiation are
the most important factors. The mould and decay models can be incorporated with climatic and
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building physic models to evaluate the effect of different exposure conditions on the durability and
service life of wooden products. The long term durability of building structures depends typically on
several factors, but the first stage for evaluation of durability and service life consist on evaluation of
the exposure conditions (ISO 15686-1), i.e. climate, type of local environment, building type and
orientation, design and details of the structure. For wood material, the microbes often play a key role
for the durability of material, especially in high humidity conditions.
2. Exposure conditions
2.1
Microclimate
The microclimate (conditions near the surface of the materials or buildings) is important for chemical
reactions and activity or organisms. The microclimate data is needed for critical states leading to
decay, mould growth or other undesirable effects. This can be a critical moisture threshold often
dependent on the duration of the moisture exposure, temperature, type of wood material considered
etc. This may be seen as a material property, whose variability also has to be taken into account in a
risk based design procedure (Isaksson 2008).
The time of critical conditions for the activity of mould or decay is most often important. The
moisture content is significant higher and critical time longer for decay to develop than that for mould
fungi. The risk of mould growth exists when ambient relative humidity RH is above 80 or 95 % at the
same time when temperature is above 0oC, but time is depending on the actual conditions. The time of
wetness in critical details may be high in the building structure even in cold and dry climates if the
moisture performance of the structure is poor.
Decay is the more severe result of high moisture exposure of wooden structures when the materials
are wet for long periods. According to laboratory studies, the growth of decay fungi and decay
development can start when the ambient humidity level in the microclimate remains for several weeks
above RH 95 – 100 % and moisture content of pine sapwood above 25 – 30 % (Viitanen and
Ritschkoff 1991, Morris et al 2006). According to experience, decay will develop when moisture
content of wood exceeds the fibre saturation point (RH above 99.9 % or wood moisture content 30 %,
but also the variation of conditions and temperature has an important effect.
In the present work within the WoodExter project, the decay growth model, expressed as mass loss of
pine sapwood, was applied in a temperature range of T= 0..30 0C and at relative humidities 95% and
above. It is noted that mass loss does not occur immediately when the wood is exposed to these
environments. Thus, there is a time lag, or an activation period, in the beginning. Based on the
experimental findings presented above, a model for variable conditions is proposed. This model is a
time stepping scheme.
For example, recorded temperatures and relative humidity are given for the Helsinki area. This
climate is shown in the figure 1 for a one year period. According to the model, this climate seems to
induce a low mass loss of 1.1 % in 4 years (Figures 2). During the first year, no decay development
will occur in untreated pine sapwood. After 3 and 4 years exposure, decay is expected to occur only to
a very limited extent in the surface of unprotected pine sapwood. Under normal use conditions, the
cladding is protected by paints or other coatings. The direct influence of water on the wood surface is
very small, and decay development will be significantly retarded or even negligible.
For advanced decay to develop, a significantly longer period is needed, and after a 10 years period,
severe decay in unprotected and uncovered pine sapwood can be expected in the Helsinki area. The
design of details has a strongly marked effect on the durability and service life of wood structure. If
there is a detail collecting the water, the moisture conditions are suitable for long time for decay to
develop. If the structure and details are well planned so that there is no water sink and the structure
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can be dried after occasionally wetting, the conditions for decay development will not be reached, and
there are actually no limits for the service life of wood.
100
Temp C or RH%
80
60
40
RH
temp
20
0
0
2000
4000
6000
8000
10000
-20
-40
Time [h]
FIG 1. Measured climate data (Helsinki) used in the decay model for one year (Viitanen et al 2010).
FIG 2. No activation of growth or decay development during the first and second years, an activation
of decay process after 4 years exposure may be expected (Viitanen et al 2010).
2.2
Macroclimate conditions for service life evaluation (SLE)
The macro-climate is one of the first steps to define the exposure conditions of the outdoor structure.
One example here is the Köppen climate classification, which is one possible way to evaluate the
macroclimate condition (Kottek et al 2006). Köppen climate classification will give on overall view
on the world climate classification and zones. This classification in intended for the botanical needs,
and will not give a detailed overview on the exposure conditions of buildings.
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For evaluation the climatic exposure conditions, the empirical wood decay model presented in the
previous chapter can be used for the ERA-40 data for air temperature, humidity and precipitation at 6
hour intervals. ERA-40 is a massive data archive produced by the European Centre of Medium-Range
Weather Forecasts (ECMWF). The reanalysis involves a comprehensive use of a wide range of
observational systems including, of course, the basic synoptic surface weather measurements. The
ERA-40 domain covers all of Europe and has a grid spacing of approximately 270 km. The nature of
the data and the reanalysis methods of ERA-40 are described in detail in Uppala et al. (2005).
The resulting modelled mass loss in 1961-1970 at the calculation points of the ERA-40 grid were
analyzed by a chart production software producing a maps of wood decay in Europe (Viitanen et al
2010). In these calculations, the α–factor of the empirical wood decay model was reduced during nondecay periods by the rate that corresponds to the recovery time of two years.
Nordic climate zone, North
Nordic climate zone, South
Continental Europe climate zone
Atlantic climate zones, North
Atlantic climate zones, Middle
Atlantic climate zones, South
Mediterranean climate zone 1 (wet)
Mediterranean climate zone, 2 (dry)
0,50
0,80
1,00
1,50
2,00
2,50
1,50
0,80
Nordic zones, North
Nordic zones, South
Middle Europe, North
Middle Europe, South
Southern Europe
Southern Europe, Southern parts
FIG 3. Modelled mass loss (in %) of small pieces of pine wood that exposed to rain in 10 years in
Europe (according to Viitanen et al 2010) and index of exposure evaluation for SLE. Left. Simulated
solar radiation in Europe and index of exposure evaluation for SLE. Right. The relative climate
exposure factors are calculated based on the area on map (e.g. Helsinki area has the number 1).
In the simulation, the calculation was based on the relative humidity and temperature in air so that the
humidity of air was set to 100% during precipitation, at non-freezing temperatures. This modification
was needed for the modelling of the potential activity of decay. The present maps on Europe of decay
development are based on evaluation of decay activity studied in laboratory. However, they will give
theoretical evaluation on the effect of climatic conditions on the decay development in different
geographical area. In the second stage, the exposure mapping presented in the figure 3 was simplified
to be used for service life evaluation.
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0,70
0,85
1,00
1,14
1,30
1,60
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The risk of decay activity in different part of Europe can be evaluated on the map. If we evaluate the
decay activity rate in Helsinki to be 1, then the decay risk in north-western part of Portugal and in
West Ireland is 2 times and in Atlantic part of France and Belgium it will be between 2 and 2.5 times
higher than that in Helsinki. In North Scandinavia it would be between 0.5 and 0.25, which will point
out, the effect if climate on risk of decay development in outdoor structure varied vide within Europe.
These coefficients can be used as one step to evaluate the effect of macroclimate conditions on
service life of cladding and decking.
Another way to evaluate the macroclimate conditions is presented by Thelandersson et al (2011)
using Meteonorm climate data. By calculating the daily dose and accumulating the dose for one year a
measure of the risk of decay is obtained. This is made for several sites, and the result in terms of
dosedays can be compared between the different sites. To be able to compare different sites, the dose
was transferred to a relative dose by dividing it by the dose for the “base-station” Helsinki. Due to the
variation of climate across Europe, relative doses between 0.6 (northern Scandinavia) and 2.1
(Atlantic coast in Southern Europe) were obtained.
The effect of local conditions is also remarkable considering the situation and the local environment
of the building. The effect of local conditions or meso-climate conditions can be evaluated e.g.
according the classification shown in the table 1.
Table 1. Definition of local conditions (Factor E2, see the table 2)
Rating
Description
Light
Local conditions have little impact on performance as the three
features all offer sheltering (i) land topography (ii) local buildings
(iii) >5km from the sea (so no maritime effect).*
Medium Local conditions have some impact on performance as one of the
three features does not offer sheltering (i) land topography (ii) local
buildings (iii) >5km from the sea (so no maritime effect).
Heavy
Local conditions have an impact on performance as two of the three
features do not offer sheltering (i) land topography (ii) local
buildings (iii) >5km from the sea (so no maritime effect).
Severe
Local conditions have a significant impact on performance as the
three features do not offer sheltering (i) land topography (ii) local
buildings (iii) >5km from the sea (so no maritime effect).**
* e.g. Building is sheltered by hills and neighbouring buildings and is inland.
** e.g. Building is on a flat plain, with no nearby buildings and is 1km from the sea.
The local conditions include: type of the environment (protected by other buildings or vegetation or
exposure of the building on weathering especially near the open sea). In the project, a total of 74
completed proformas for cladding on buildings have been gathered from across Europe. This has
brought together data on materials, design (water shedding, ventilation), exposure (climate,
orientation, height above ground), installation and details of the project and images of the products.
3. Effect of design on the service life evaluations
The long term durability of building structures depend typically on several factors, but the important
stage for evaluation of durability and service life consist on evaluation of the exposure conditions and
their effect on the performance degree, which actually is very significantly depending on the structure
and design of the building and the components.
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A basis for a quantitative probability based design is the definition of a so called limit state or
performance degree. This is a more or less precise definition of the limit between acceptable
performance and non-acceptable performance. An example is onset of mould growth or discolouring
in materials in the building envelope, which can be regarded as non-acceptable since it may create
aesthetic and health problems in a building. Another example is attack from decay fungi, which will
reduce the capacity of a load bearing structure which is comparable to a serviceability limit state for
structures. The start of attack of decay fungi has been used as limit state in this work.
The ISO 15686 identifies a wide range of parameters important to Service Life Prediction and the
most important factors for wooden structure are shown in the table 2.
Table 2. Factors for service life of wooden facades (modified based on Vesikari et al 2001).
Code
Factor
Parameters / factors for estimated service life
A1
Wood material
Wood species, decay and weather resistance, water
permeability, board quality, dimension, wood modification,
preservation
A2
Coating
Coating type and properties (thickness, opacity, color), needs
for repainting (maintenance)
B1
Structure, design,
Structure of the houses: eaves, height of the wall and
foundations (B1). Structure of the façade; board type, bonds
B2
especially details
and joints, ventilation, protection of joints and end grains,
fixing (B2)
C
Work execution
Achievements and treatments details, fixing, wood moisture
content, storage condition
D
Indoor environment
Temperature, RH, condensation (not so important for exterior
structure)
E1
Exposure conditions Point of compass, type of environment (protective – exposed)
macroclimate (E1) and local conditions, exposure to driving
E2
rains (E2)  microclimate conditions
F
Use conditions
Indoor environment, moisture stress, mechanical injuries
G
Maintenance
Care of accidental damage, serviceable, repainting (opaque –
stains) time of repaint
Obviously it’s difficult to include the all relevant factors in to models of service life evaluation and
the models are often descriptive and not exacts. The factors, however, may have conflict and may
affect different ways to other factors. During the life time of a building, different level of maintenance
and repair will be needed for acceptable performance. For wood material in exterior use, the
maintenance is normally involved for the service life evaluation depending on the exposure conditions
and structure.
In the service life evaluation of cladding and decking, the onset of decay is the most important limit
state, and in the “Woodexter” project, different aspects of service life evaluation was studied: survey
on the effect of different methods to evaluate the development of decay and effects on the service life
of wood, evaluate the different factors to be important to the service life of a building. Several factors
could be found, but the defining of their exact effect can be very complicated. An evaluation method
is based on the factor method and the evaluation of their function is based on a mathematic function.
In this paper, a service life evaluation based on factor method of ISO 15686 is presented.
The protective effect of eave is the first evaluated design factor. The sheltering from eaves is
described by a factor which is a function of the ratio of eave overhang relative the positions of the
detail under consideration. The sheltering effect can be used for both decking and cladding, and
similar the distance from ground (Figure 4).
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Distance
from
ground
d=Position of
design detail
Wall height
Eave length e
FIG. 4. Illustration of effect of eave overhang and definition of distance from ground and the index
for SLE (factor B1, see table 2)..
Several other design factors can be detected, like joint of the boards, protection of the end grain of the
boards, performance of the coating (Table 3).
Table 3. Rating of design details (Factor B2)
Rating
Description
1. Excellent Excellent design with features to maximize water shedding
and ability to dry when wet. The end grains are well protected.
2. Good
Good design with features to provide water shedding and
ability to dry when wet (corresponds to the reference of a
horizontal board without possibility of moisture trapping)
3. Medium Design with a limited probability of water trapping. and with
some ability to dry when wet
4. Fair
Design with medium probability of water trapping and limited
ability to dry when wet
5. Poor
Design with high risk of water trapping and very limited
ability to dry when wet. The end grains are not protected.
(1
The index is for well coated cladding
The maintenance of the defects and coated surface is very important. If the detail of structure is poor,
the coating may even give worse results even in the case of repaint or paint maintenance. The
maintenance of the coating, however, is very important for the aesthetic condition of the cladding, but
it will also secure the protection of the cladding against rain and ambient humidity.
The combine of the factors can be performed using different methods. Thelandersson et al (2011)
used relation of exposure and resistance index for engineering modeling of service life. In the present
paper, the evaluation is based on the ISO 15686 factor method, and the effect of single factors on the
accepted and expected service life of exterior wood structure can be evaluated using the model:
Service Life Factor = A1 x A2 x B1 x B2 x C x E1 x E2 x G (see table 2).
The service life is accepted when the product of calculation is lower than 1. The factor A1 includes the
properties of wood material and the factor A2 includes the type and properties of the coatings. Norway
spruce has index 1. The additional protection index can be added (e.g. impregnation or modification).
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The index of coating varies depending of the opacity and protection capacity of the paint. The factors
C, F and G have not been explained in this paper. The wanted service life can be separately added to
the evaluation system.
4. Conclusions
The durability and service life of wood structure is based on several factors which should taken care
in manufacture of the wood products, design, execution, maintenance and for evaluation of the
exposure conditions. The evaluation can be used by design and performance evaluation of structures
in different exposure conditions.
5. Acknowledgements
The authors gratefully acknowledge the financial support of WoodWisdom-Net and the wood industry
partnership Building with Wood for funding the research work within project “WoodExter”. The
“WoodExter” research partners are thanked for their cooperation and collaboration in this project.
ECMWF ERA-40 data used in this study have been provided by ECMWF.
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