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 excessive 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, and it can shrink up to 10 times more than normal. It is not easy to recognise compression timber and roofers are advised to measure the moisture content, particularly of ridge purlins, before fixing.
An alternative is to use steel top hat purlins.
Fixing roof cladding should be treated in the same way as internal linings, i.e., do not fix transverse flashings when the moisture content of any timber is more than 18%. The thickness of flashings should always comply as specified in 8.1 Flashing Materials
If flashings are positively fixed, framing timber that does not meet this requirement can cause failure of ridging and flashings due to timber shrinkage when drying.
The metal expansion allowances quoted in many publications can be misleading because the information is based on theoretical metal expansion values and is not related to real-world conditions.
Figures published for metal expansion rates are given linearly per degree, but it does not take into account the many other factors that mitigate the theoretical figure. (See 7.3 Thermal Expansion And Contraction.)
It is necessary to make provision for cladding and flashing movement; when long lengths are used and positively screwed or riveted together, they should be regarded as one length.
The maximum length before expansion provision should be made for either cladding or flashings will vary according to colour, micro-climate, ventilation and fixing spacings. It is , however, possible to provide indicative figures based on a study of empirical data over time. The maximum recommended flashing length without any expansion provision is similar to that of roof cladding, i.e., every 12 m for coated steel flashings.
Aluminium rivets, which have a low shear value, will fail when there is no provision made for expansion in flashing lengths of over 12 m. Using aluminium joints is only acceptable if they are used at the prescribed distances, and are not used to replace expansion joints.
Lengths of coated steel ridging, cappings, and apron flashings over 12 m should have a slip joint as described in 8.9 Flashing Expansion Details.
Inadequate provision for expansion can also cause Roofnoise.