Abstract

All common building materials (concrete, metals, and wood) are susceptible to environmental degradation when they are used outdoors. The degradation that occurs when wood is used outdoors and above ground is termed weathering (Feist 1990a). Weathered wood is gray and its surface is often cracked and rough (Figure 7.1). However, under the weathered surface layer, which is no more than a few millimeters deep, the wood is essentially sound. The surface of weathered wood is often colonized by microorganisms and lichens, but conditions at exposed wood surfaces generally do not favor decay by basidiomycete fungi, which can extend deeply into wood and significantly reduce the strength of structural timber (Feist 1990a). Hence, weathering has little effect on the mechanical properties of structural timber and, accordingly, there are many examples of wooden buildings such as the stave (pole) churches in Norway that are still structurally sound despite having been exposed to the weather for over 1000 years (Borgin 1970).