All kinds of timber will take up or lose moisture, and adjust dimensionally, in response to changes in the humidity of its environment.
The actual equilibrium moisture content (EMC) will vary from place to place and from season to season, 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 10-15%. The main exception to this is some sections of western red cedar, cypress pine and hardwood weatherboards which may be supplied unseasoned.
Unseasoned cladding will shrink as it dries, and allowance must be made for shrinkage by increasing the overlap of boards. Wider unseasoned boards may also develop slight cupping across the face unless restrained. Unseasoned hardwood boards should be fixed as soon as possible after delivery.
When machined profiles are produced from seasoned timber the design of the edge - joint will allow for movement due to seasonal changes in equilibrium moisture content. Other than exercising proper care and good workmanship during installation, no special precautions need 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 equilibrium moisture content during different seasons, or the drying of unseasoned boards, does not create stresses sufficient to cause splitting of the timber. Individual boards must not be nailed together at the lap.
Reference should also be made to the Timber Utilisation and Marketing Act of Queensland, and the Timber Marketing Act of N.S.W. which govern the moisture content of timber products, including cladding.
Storage and Handling
It is important to keep timber cladding dry and clean from the time it is received on the building site until it is fixed and finished. Dirt and stains or water damage will not necessarily be hidden by the finishing system and may be expensive to remove-if they are removable.
On-site storage of timber cladding, prior to fixing, should be under cover on bearers spaced at no more than 900 mm centres. If outside storage cannot be avoided timber stacks should be at least 150 mm clear of the ground and covered with waterproof sheet arranged to prevent rain being driven into the stack. Covers should be kept just clear of the ground to allow ventilation under the stack.
Pre-finishing the boards
If the cladding is to have a natural or stained finish, it should be liberally coated with water repellent preservative by either dipping or brushing on all surfaces. This practice is just as important for CCA treated pine as it is for non-preservative treated timbers.
Water repellents should meet the requirements of Australian Standard AS 1607, and should be used according to the manufacturers recommendations.
Some pigmented stain finishes cannot be applied over water repellent preservatives and compatibility of the systems should be checked before
All freshly cut surfaces should be given supplementary treatment with water repellent before fixing.
Warning: Linseed oil or a mixture of mineral turpentine and linseed oil should never be used on timber cladding as a natural finish. Refer "Finishing and Maintenance".
Before fixing, all vertical joins, window sills, heads and other openings should be adequately flashed in accordance with good building practice. Flashing should extend beyond the ends and edges of openings and be securely fixed at least 25 mm under the cladding when covered by it.
Sarking and Vapour Barriers
Waterproof, vapour permeable sarking such as fire retardant breather paper should always be installed in walls subject to high wind conditions and wind updraughts. Sarking should be fixed on the outside of studs, and directly under the timber cladding. Vapour barriers, where considered necessary, should be installed on the inside of studs.
Frames should comply with the requirements of the local building authority. Preferred spacing of studs or fixing battens is 450 mm and 600 mm. When cladding is fixed diagonally, it should be fixed at not more than 650 mm centres measured along the board, i.e. vertical stud spacing of not more than 450mm centres for 45 fixed boards.