The learning resources ‘Timber as a material – Moisture content and durability’ describes how the moisture content of a piece of timber has an effect on its dimensions, in that it swells when it takes up moisture and shrinks as it loses moisture. It also describes how moisture content affects the resistance of a piece of timber to fungal decay and insect attack, with higher moisture content levels lowering this resistance significantly.

The moisture content of timber affects its strength properties (the measure of its ability to resist loads).

Timber can be dried by air drying and by kiln drying.

The lower the moisture content of a piece of timber, the stronger it is; the higher the moisture content, the weaker it is.

Not all strength values are affected to the same extent by changes in moisture content.

Moisture content is an important aspect of design consideration for timberusage. Service classes 1 to 3 define the permissible moisture content for use in various conditions.