Durability - Timber cladding
Timber
cladding is an attractive and sustainable option for external walls.
Peter Mayer of Building LifePlans assesses commonly
available options and their whole life costs.
Introduction
Increasingly timber cladding is specified as the weathering envelope for walls. Timber is renewable, reusable, biodegradable and it has minimal embodied energy. This should be balanced against transport costs, the need to use chemical preservatives for some timbers and a shorter service life than material such as masonry.
There are many reasons why timber deteriorates, but the main cause of
failure is fungal decay where moisture content within timbers is in excess
of 20%.
BS EN 335 classifies the in-service environment for timber into five hazard
classes to enable specifiers to assess how likely timber is to deteriorate
in a given situation. External timber cladding is inUse Class 3 because
its moisture levels frequently exceed 20%. This means some timber species
will require preservative treatment. BS 8417, which covers the preservation
of timbers, draws on a suite of European standards to enable selection
of timbers and the type of preservative treatment necessary, for design
service lives of 60, 30 or 15 years in different hazard classes.
• Timber's natural durability is determined to BS EN 350
• The need for treatment for different Use classes is determined
to BS EN 460.
• BS EN 351-1 defines the penetration and retention needed to give
a desired service life with a preservative treatment to BS EN 599.
The heartwood of timbers which are classed as ‘durable’, and ‘moderately durable’ do not need preservative treatment for external wall cladding. Timbers that are classified as ‘slightly durable’ do require preservative treatment. All sapwood require treatment.
Timbers not requiring preservative treatment
Western red cedar is a ‘durable’ timber
from North America. It may be coated with oil to maintain its appearance,
with the associated regular costs of recoating.
European Larch is ‘moderately durable’. UK-grown
larch, larch from Siberia or regions where trees are slow-growing and
older than 60 years have a similar expected service life with a similar
age and growth pattern. The density of larch is between 470 kg/m³ and
650 kg/m³ at 12% moisture content. This gives larch a better resistance
to impact compared with western red cedar, which has a density of 330-390
kg/m³.
Douglas fir from North America is ‘moderately durable’.
However. BRE Digest 494 recommends that UK-grown Douglas fir be classified
‘slightly durable’ and given preservative treatment.
European oak is a ‘durable’ hardwood. Oak
is more expensive and needs to be specified carefully to minimise its
tendency to warp. Shorter lengths are recommended as well as fixing shortly
after being cut at the sawmill. Hardwoods may require fixing with screws
through pre-drilled holes, which increases installation costs.
Timbers requiring preservative treatment
Spruce, Fir and Pine: These non-durable species require preservative treatment. Typically this will be by pressure impregnation of chemicals with preservative properties such as boron or organic solvents.
Thermally modified timber is an alternative to chemically
treated timber. A controlled heat treatment to temperatures over 200°C
confers improved durability and stability to timber, typically European
redwood (Pinus sylvestris).Expected service life for thermally modified,
uncoated timber cladding is 30 years.
Lodgepole pine from Canada is a ‘moderately’
to ‘slightly durable’ softwood, which is treated with preservative
and coated in a high-performance paint system at the manufacturing plant.
Expected period to first maintenance is 15 years and there after every
3 – 5 years.
European whitewood is a generic term that includes spruce
and fir. These are ‘slightly durable’. Preservative treatment
is required to give a service life 5 to 10 years. With a well-maintained
paint system expected lives can be extended to up to 30 years.
Durability issues
Design strategies can enhance durability by reducing the risk of wetting
and removing moisture promptly:
• Protect from rainwater with large eaves overhangs
• Terminate the cladding at least 150mm above ground levels, preferably
250mm
• Ensure cladding is not directly in contact with porous materials
• Include a well ventilated and drained cavity behind the cladding.
This should be at least 19mm wide
• Moisture content of the timbers should be between 13 – 19%
, less for heat-treated timber
• Seal end grain
• Allow sufficient gap between cladding sections to prevent capillary
paths; 5mm is recommended
• Use stainless steel fixings
The Timber Research and Development Association and BRE provide detailed
good practice guidance.
Specification options
| Capital cost £/mú | Net present value for 60 years £/mú | Service life years | |
| Timbers not requiring treatment, heartwood only | |||
| Western red cedar Thuja plicanta finished size 18mm thick and 125mm exposed face | 38 | 50 | 40 - 60 |
| European Larch Larix decidua finished size 21mm thick and 135mm exposed face | 32 | 47 | 30 - 40 |
| Douglas fir Pseudotsuga menziesii finished size 19mm thick and 125mm exposed face | 35 | 58 | 25 - 35 |
| European oak Quercus robur finished size 19mm thick and 125mm exposed face, screw fixed. | 46 | 59 | 40 - 60 |
| Treated timbers | |||
| European redwood Pinus sylvestris heat treated finished size 25mm thick and 125mm exposed face. | 28 | 48 | 25 - 35 |
| Lodgepole pine Pinus contorta finished size 16mm thick and 125mm exposed face. Preservative treated and precoated high performance paint system | 32 | 70 | 25 - 35 |
| European whitewood Picea and Abies species finished size 19mm thick and 120mm exposed face. Preservative treated. Paint or stain finish. | 23 | 190 | 20 - 30 |
Table notes
• A discount rate of 3.5% is used to calculate net present values.
Service lives are average.
• All finished naturally and fixed using stainless steel nails,
unless otherwise stated.
• Costs are based on 100mú external wall, vertical cladding and
include replacement, maintenance, allowance for minor repairs and repainting.
• Recoating cycles depend on the rate of deterioration of the protective
film. Typically repainting is carried out every 2 to 5 years, longer for
high performance paint systems.
• A cost analysis based on project specific information is essential
for a realistic best value appraisal.
First published in Building 2006
Update notes
2008 edits. Updating terminology: Hazard Classes are now referred to as Use classes. Heat treated timber is referred to as Thermally Modified Timber following the publication of a draft european standard: DD CEN/TS 15679:2007 Thermal modified timber. Definitions and characteristics.
Further information
Building LifePlans provides latent defects warranty
for all buildings.
BLP Construction Durability Database at www.componentlife.com
provides durability information for building components.
Further information contact peter.mayer@buildinglifeplans.com
or telephone: 020 7204 2441.
