Timber is unusual as many of its engineering and other properties are influenced by its moisture content. These include weight, strength, stiffness, workability and performance of fixings. Changing moisture content also influences its dimensions and durability (i.e. its susceptibility to biological attack). It is estimated that over 80% of the in-service problems with timber are in some way related to moisture content.

This module has considered the importance of in-service environmental conditions when designing in timber. The module then discussed the wood moisture content resulting from its use in differing environmental conditions and outlined the use of Service classes for structural design. Two examples were presented to show how the current structural timber design standards calculate the strengths and modulus of elasticity under Service class 3 conditions.