information/FAQs

Fig 1 Weathering protection

Weathering of timber

Weathering is the breakdown of the exposed surfaces of a piece of timber due to action of sunlight, rain, wind, dew, frost and fumes. These cause deterioration and erosion of the surface cells through degradation of the lignin that binds the wood cells together and sugars, starches and extractives are all washed away from exposed timber surfaces.

Initially dark colours become paler through leaching and bleaching and pale colours darken because of oxidation. Eventually all exposed timber surfaces become grey. The surface may appear dirty or blotchy in situations favouring fungal organisms, but in dry or coastal climates an attractive silvery grey usually results.

Weathering is a process distinct from decay, also called rot, which is caused by fungi.

Weathering is generally a slow process with studies suggesting an average erosion rate of about 6mm per 100 years. The effects of weathering do not significantly alter most properties of the timber, except for surface abrasion resistance.

Unprotected timber surfaces develop checks and cracks due to the repeated absorption and desorption of water by the surface. These cracks gradually increase in depth and can allow exposure of untreated interior sections of preservative treated timbers.

Weathering protection may be provided by the application of coatings such as paints, water repellents, water-repellent preservatives and pigmented penetrating stains. The main objective of these is to prevent or retard the uptake of moisture and the absorption of ultraviolet light. However, water-repellent preservatives and stains do not generally prevent timber surfaces from greying.

No clear coating can entirely block the weathering effect of the sun on the timber surface. The resultant degradation of the wood can quickly cause detachment of the coating from the timber surface. Clear coatings do not last as long as paints in fully exposed applications.

Figure 1 shows how architectural detailing can provide protection from weathering.

References:

Timber Datafile P1 – Timber Species and Properties of the NAFI Timber Manual.
Bootle, K.R. 1983. Wood in Australia. McGraw Hill.