Flashing Buckling Due to Timber Shrinkage

Transverse flashings such as head barges, ridges, and change-of-pitch flashings are particularly exposed to stress from timber shrinking as it dries. The worst can be avoided by not fixing flashings until timber moisture content is below 12%, but that is not always practical. The longitudinal shrinkage of normal radiata timber is approximately up to 3 mm per metre.

Compression timber periodically makes an appearance. Compression timber can occur in a short section of timber, which can shrink on drying up to 10 mm per metre and cause buckling of flashing and broken joints. Compression timber is hard to recognise by eye. The solutions is to wait for the timber to dry before installing flashings or, failing that, the flashings will need to be replaced once the timber moisture content has stabilised. (see Compression Timber)

Flashing Buckling Due to Timber Shrinkage

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An alternative is to use steel top hat purlins, or to fix flashings using clips that allow for expansion and contraction.