Finishes for exterior timber

TRADA Wood Information Sheet 2/3 – 1. Revised April 2005.

Timber exposed to the weather and not protected by some kind of finish will ‘weather’ and quite rapidly become bleached and grey. Whilst this effect may be desirable, often the result is dirty and patchy, giving the timber a neglected appearance. Prolonged exposure can lead to surface checking and splitting of the timber and loss of wood fibres from the surface, resulting in a woolly or corrugated appearance.

The splitting and checking is due to rapid changes in the moisture content of the timber surface caused by the combined actions of rain and sun which give rise to swelling and shrinking. The greying is caused by loss of natural colouring materials, accumulation of dirt from atmospheric pollution and often from mould growth on the timber surface. Since the severity of exposure on a building varies with aspect, overhangs, etc, weathering can be uneven, resulting in variations in colour and appearance.

For these reasons it is usually necessary to provide some form of protective treatment or finish to timber exposed to the weather.

The design of a building and its components can greatly influence both the performance of the timber and its finish. The main considerations are to ensure drainage and to minimise the absorption of water by wood, noting that end grain is particularly vulnerable. For example, end grain at the bottom of boards should be scarfed and sealed; sills should be adequately bevelled and throated; glued joints should be made with a durable adhesive. Further guidance is given in the Wood Information Sheet Durability by design.

Timber building components are sometimes stored on site for several months before they are installed into a weathertight building. During this time, they may be subject to damp or to contamination with dirt. It is sound practice therefore, to apply at least one coat of primer, paint or other finish in the factory before packaging for dispatch to the site. This minimises soiling or moisture uptake on lateral surfaces, and seals over end grain and joints.