Specifying timber exposed to weathering

Timber has been used for construction throughout history and the performance of timber out of doors is well understood. It has long been known that it is possible to extend the useful life of timber through good building/construction design and profiling wood components to shed water and thus dry down more quickly.

One of the reasons for selecting timber is its aesthetic appeal. As a natural material timber has characteristics that vary from piece to piece; this means no two pieces are the same, which adds to its appeal (ie knots, grain and colour differences).

When uncoated timber is exposed out of doors its colour changes over time as part of a process termed 'weathering'.

This Wood Information Sheet (WIS) explains the causes of weathering and its significance to timber performance. This is an overview of the subject with signposts to more detailed sources that are listed at the end.

Contents:

• What is weathering?
• The influence of moisture content on weathering
• Colour change
• Rate of wood surface removal
• Prevention of weathering by coating
• Effects on extractives leached on other materials during the weathering process
• Additional features often associated with weathering