Whole life costing - Timber cladding

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 in Use 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/m˛ and 650 kg/m˛ 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/m˛.

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

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 100mœ 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.blpinsurance.com provides durability information for building components.

Further information contact peter.mayer@buildinglifeplans.com or telephone: 020 7204 2441.