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Increasingly, timber cladding is specified as the external 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, which occurs if the moisture content within the timber 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 in hazard class three 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 a design service life 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 hazard classes is determined to BS EN 460
• BS EN 351-1 defines the penetration and retention needed to give a service life with a preservative treatment to BS EN 599.

The heartwood of timber, which is classed as "durable", and "moderately durable" need not be treated for external wall cladding. Timbers that are classified as "slightly durable" and all sapwood require treatment.

Timber 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 470kg/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 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

Heat treatment is an alternative to chemical treatments. Heating timber to temperatures of more than 200°C improves durability and stability.

European redwood is a species that is frequently treated. Expected service life for uncoated claddings is 30 years.

Lodgepole pine from Canada is a "moderately" to "slightly durable" softwood, which is treated with a preservative and coated with a high-performance paint system at the manufacturing plant. Expected the period to first maintenance to be 15 years and thereafter every three to five 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 of five 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 150 mm above ground levels - preferably 250 mm
• Ensure cladding is not in contact with porous materials
• Include a well-ventilated and drained cavity behind cladding. This should be at least 19 mm wide
• Moisture content of timbers should be 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.