Know the Facts: External Timber Cladding

Top tips to prevent moisture and drainage problems

External timber cladding is increasingly being chosen for domestic and commercial buildings, which means more of your customers seeking advice on the right products to choose.

Most timber cladding is installed for aesthetics and also as a rainscreen to protect buildings from the British weather, but if that cladding is not installed correctly in accordance with British Standards, it can cause poor performance, drainage and ventilation issues. So, what do your staff need to know?

Cladding should be fitted over a drained and ventilated cavity, to allow the natural product to remain as dry as possible and give it a long service life. Ventilation provision is allowed for at the top and bottom, while the batten layer or layers should create a continuous drainage plane.

A waterproof breather membrane is then situated behind the support battens, which separates the external wet zone from the internal dry zone of the building. The fixings must also be chosen carefully to prevent staining and possible detachment of the cladding itself.

Timber also absorbs moisture from the air, which can cause it to expand and contract across the grain. This is fine as long as the building’s design allows for such movement. BS8605 External Timber Cladding Part 1: Method of specifying gives guidance on board width and thickness dimensions to accommodate such movement.

Durability
Clearly, outdoor cladding must be more durable than internal cladding as the wetter environment presents a greater risk of decay. There are many naturally durable cladding hardwood species and some softwood species offering a variety in grain and colour. Low durability species can be made more suitable for exterior cladding use by either treating with an appropriate wood preservative, applied under controlled factory conditions, or using a modified wood product.

The Timber Cladding Handbook, produced by TDUK in partnership with the Timber Decking & Cladding Association (TDCA), is available at www.timberdevelopment.uk

The TDCA provides guidance on timber cladding and is happy to help advise every part of the timber cladding supply sector.