Specifying timber exposed to weathering

Timber has been used for construction throughout history and the performance of timber in outdoor environments 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 by profiling wood components to shed water and dry down 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 that no two pieces are exactly the same (ie knots, grain and colour differences), which adds to its appeal.

When uncoated timber is exposed out of doors its colour and texture changes over time as part of a process termed ‘weathering’. Understanding the effects of natural weathering is an aid to making informed decisions in the timber design and specification phase. It will also help to manage expectations of timber behaviour and maintenance requirements.

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 of extractives in contact with other materials during the
  weathering process
• Removal of extractive stains on other materials
• Removal of staining resulting from wood–metal interactions
• Additional features often associated with weathering

This WIS was updated in May 2021 with some editorial rephrasing throughout, updated references and additional detail added on delignification, disfiguring surface yeasts and research to carry out when removing extractive stains.