The NZ Metal Roof and Wall Cladding Code of Practice is a comprehensive design & installation guide, and a recognised related document for Acceptable Solution E2/AS1 of the NZ Building Code.
Before any transverse flashings are fixed, the framing timber must have a maximum moisture content of 18%.
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.
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 18% before any transverse flashings are fixed.
Alternatively, use steel top hat purlins.