Compression Timber

Buckled Ridge Flashing

Buckling of Flashings

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Before any transverse flashings are fixed, the framing timber must have a maximum moisture content of 12%.

Transverse flashings, such as ridging, are sometimes prone to compressive buckling which is blamed on metal expansion but is usually due to timber shrinkage, and a phenomenon known as compression timber.

Compressed Timber

Compressed Timber

Compression wood has a darker appearance in the early wood (summer) growth rings; to visually distinguish such severity levels in sawn timber accurately requires microscopic identification. (Source: Rooflink)

Abnormal growth causes this defect in timber, which can cause shrinkage ten times more shrinkage than normal.

It is not easy to recognise compression timber visually and roofers are advised to measure the moisture content, particularly of ridge purlins, before fixing. Framing timber must have a maximum moisture content of 12% before any transverse flashings are fixed.

Alternatively, use steel top hat purlins.