Allowance often needs to be made for shrinkage and differential movement at connections and in construction. Failure to do so may lead to:
- Breakdown of the building envelope, allowing ingress of moisture;
- Unevenness in a finished surface, such as a floor; and
- Splits, cracks or gaps in flooring, door jambs, skirtings and cornices
Measures which can be taken to avoid this include:
- If timber is unseasoned (e.g. floor joists), use species with similar shrinkage values.
- Where timber is mixed with steel and/or concrete construction, use seasoned timber to minimise differential movement (refer to Figure 1).
- Joints and connections should be detailed to allow seasoned material to reach equilibrium inside the building. This is especially important in heated or air-conditioned buildings.
- Allow for vertical movement in unseasoned framing by leaving adequate clearance to the top of masonry veneer (refer Figure 2).
- Allow adequate clearance at the top of unseasoned members fixed to members that will not shrink (refer Figure 3 and 4).
Reference: Timber Datafile P4 – Timber – Design for Durability of the NAFI Timber Manual.