All uncoated timber irrespective of species will eventually weather to various shades of grey, when exposed outdoors. If the effect is uniform it may be described as silver-grey. leaving a timber façade to weather naturally can minimise maintenance costs, but in many cases the resultant effect can have unexpected and inevitable characteristics. Unfortunately this uneven look can be viewed as looking cheap or drab and reflects badly on the timber. Therefore it is extremely important that designers and architects take this into consideration when drawing building designs.
When wood is exposed out of doors without a protective coating, the surface undergoes rapid changes to its appearance and texture; these effects are mainly due to exposure of the timber surface to a combination of sunlight and moisture. The first effect to be observed is often a temporary brown staining as extractives in the timber rises to the surface where they oxidise.
Sunlight degrades the wood’s surface and in combination with moisture, enables satin fungi to flourish and cause the timber surface to turn grey. The degree of staining varies according to the duration and intensity of wetting. Roughening of the timber surface also occurs and the timber may split and eventually erode. These effects are driven by a combination of sunlight, water, mechanical surface and heat-weathering.
Five Stages of Wood Weathering
As timber dries, extractives accumulate on the surface where they oxidise to a brown stain. The effect is usually short lived, as rainwater will reach the extractives, although it can persist where the timber is protected from leaching.
Timber is composed of fibres held together with a glue like substance call lignin. Sunlight – Sunlight degrades the surface lignin, which is then eroded away as organic acids and other compounds leaving the fibres largely intact. Weathered timber surfaces tend to roughen as the fibres are exposed. The colour also changes; dark timbers bleach whilst light timbers darken.
The greying of damp wood is generally due to the presence of stain fungi; on weathered timber fungi live off lignin breakdown leached down the surface. It can colonise timber surfaces after less than one year of weathering though the rate depends upon water availability. The effects are most pronounced on upward facing surfaces and those exposed to wind channelling, splashing or high resistive humidity.
Repeated movement of timber due to moisture fluctuation may lead to surface cracks. These will vary according to the timber’s characteristics and how the board is fixed.
The surface of the external timber will slowly be worn away due to a combination of photo-degradation, mechanical abrasion by windblown particles and biodegradation. The rate varies depending upon site conditions.