Background on thermal expansion

All metals expand and contract with changes in temperature. Although steel is by far the least affected of all the metals commonly used for roof and wall cladding, the changes in length experienced in very long runs of roofing are significant. On a clear hot summer day, with no wind, the steel temperature in roof cladding can reach approximately 50°C
in Colorbond ® Surfmist ®, 60°C in plain Zincalume ® and more than 80°C in Colorbond ® Night Sky®. Examples of the thermal changes in lengths of steel cladding that would result from various temperature changes in the steel are shown in adjacent table. The actual expansion or contraction between the end of a sheet and the last support would only be a fraction of the figures shown because the movement in the length of fixed cladding would normally take place from the centre towards each end of the sheet. The movement at each end is thus only half the total expansion or contraction. Transverse thermal expansion poses no problems in ribbed cladding because each rib absorbs some transverse movement.

Expansion joints

Thermal expansion effects are mitigated by slight bending of fastener shanks, thermal movement of the building structure, and slight flexing of the purlins (where they are
not restrained by cleats or bridging). However, for very long runs of roofing, you should include an expansion joint to overcome linear thermal expansion. the adjacebt table shows the maximum distance between the top and bottom rows of fasteners on a pierce-fixed sheet.
For LONGLINE in distances of greater than 35m, please contact your Lysaght branch for advice. If the total length of two sheets pierce-fixed through the lap, or a single sheet
exceeds this distance, then an expansion joint is needed. There should be no more than one pierce-fixed end-lap between expansion joints. An expansion joint involves overlapping the ends of the upper sheets over the ends of the lower sheets—but with a
clearance between them (about 15mm). A typical overlap is 250mm. The
clearance is usually created by having all the purlins for the roofing on the high side of the joint, higher than the roofing on the low side of the joint. An extra purlin is needed at
the joint. A baffle flashing provides weatherproofing. Where there is a risk of high winds, or the ribs result in a large opening, you may need protection, such as extra flashing or the inclusion of closed-cell foam infill strips.