Thermal Expansion And Contraction

All metal cladding and flashings are subject to expansion and contraction caused by changes in temperature, and their design should allow for this movement. The energy produced should be absorbed without damage to the cladding, fixings or structure. The recommendations in this section are specific to preventing damage and leaks through thermal movement. Thermal movement can also cause disturbing noise levels in dwellings with shorter member lengths than those recommended in this section. (See Roof Noise.)

The ribs of metal trapezoidal or corrugated roof and wall cladding absorb expansion across the width of the sheets, but special provisions are needed over the sheets' length.

Much of the longitudinal expansion is taken up by the bowing of the sheet between fastened supports. The extent to which this happens depends on the profile strength and support spacings.

Failure by thermal expansion normally results in shearing of the fastener. Fasteners into lightweight steel purlins up to 3 mm in thickness are less vulnerable as they tend to rotate rather than be subjected to repeated bending resulting in fatigue failure. Fasteners into hot rolled steel sections or timber are far more vulnerable to this mode of failure and in all run lengths over 20 metres provision for expansion must be made when fastening into such supports.

Where overlapping sheets are fastened through the ends, they must be considered as one length to calculate thermal movement. Unfastened end laps are not recommended.

Wall cladding does not require the same provisions as roof cladding, because of solar radiation angle.

Oversized holes and washers give some room for expansion and contraction, but it is not enough to allow movement without stress or distortion over long spans. In such cases, a step joint should be used. (See Step Joint)