TRADA estimates that more than 80% of the in-service problems associated with wood are in some way related to its moisture content. The amount of water present in timber can affect its weight, strength, workability, susceptibility to biological attack and dimensional stability in a particular end use.
TRADA Technology’s Wood Information Sheet (WIS) Moisture in timber provides basic information for the specifier and user on the key facts and issues concerning moisture content. For example, timber below approximately 20% moisture content is not considered to be at risk from fungal decay.
Timber does not shrink or swell lengthwise along the grain, but as it loses water below the 25%-30% fibre saturation point it will shrink laterally. In constant conditions of temperature and relative humidity, timber will eventually reach a constant moisture content – the ‘equilibrium moisture content’ (emc). ‘Movement’ is the term for dimensional change that results from changes in emc that occur after the initial shrinkage has taken place. Movement occurs because, in practice, a stable emc is never reached.
Moisture in timber is available in PDF format free of charge from the TRADA website. The cost is £12.00 for non-members. To order visit www.trada.co.uk/Bookshop. The WIS has been updated for ease of downloading online.