All timber will take up or lose moisture in response to changes in environmental conditions, and will undergo corresponding dimensional changes.
The equilibrium moisture content (EMC) of timber will vary between locations, and between seasons in each location, so that timber in service may have a moisture content ranging from about 6% to 18%. Timber cladding will usually be supplied as "dry", "kiln dried" or "seasoned" and will have a moisture content in the range of 10-18%. The main exception to this is some western red cedar, cypress pine and hardwood weatherboards which may be supplied unseasoned.
Unseasoned cladding and "wet after" CCA treated cladding will shrink as it dries, and allowance must be made for shrinkage by increasing the overlap of the boards. Wider unseasoned boards may also develop slight cupping across the face unless restrained. Unseasoned hardwood boards should be fixed in place as soon after delivery as possible.
When machined profiles are produced from seasoned timber, the design of the edge-joint allows movement due to seasonal changes in EMC. Other than exercising proper care and good workmanship during installation, no special precautions need to be taken to compensate for slight expansion and contraction in service.
When fixing, it is important that nails are placed so that movement of boards caused by changes to EMC or the drying of unseasoned boards, does not create stresses high enough to cause splitting of the timber.
Individual boards must not be nailed together where they overlap.
Reference: Timber Datafile FP1 – Timber External Cladding of the NAFI Timber Manual
