Temperature Range

Aluminium and zinc, which have twice the expansion rate of steel, do not necessarily expand to this degree because of the different characteristics of mass, emittance, and radiance which affects their temperature range. Copper expands one and a half times as much as steel, and stainless steel can expand up to 1.5 times as much as steel depending on composition.

The theoretical expansion of steel roof cladding in mm is 12 x temperature change x length in metres/1000.

Steel expansion rates can be calculated as follows:

Given a length (e.g., 30 m) and that the material (e.g., a light-coloured uninsulated roof) moves through a 60°C range (e.g., + 50°C -10°C), the theoretical increase in length is 12 x 60 x 30/1000 = 21.6 mm.

This amount of movement of roof cladding and components does not have to be provided for in practice, because:

• The building also expands with the ambient temperature, although to a lesser degree.
• Fasteners into light gauge purlins will roll rather than bend. The purlin flange may also roll to a degree.
• The roof cladding bows between purlins when it is constrained. Sighting down a corrugated steel roof on a warm sunny day will show an undulating line compared to a straight line when the roof is cool. The forces created by expansion and contraction are self-levelling, i.e., each component moves under load until the resisting force is more than the expansion force.
• When a length of sheeting is solid fastened at the centre and unconstrained at either end, the movement is towards the ends of the sheeting; meaning the actual expansion or contraction movement is only half that of a full length of roof or wall cladding fastened at one end. Special design of the ridge or head barge flashing is required in these cases to allow free movement. Alternatively, sheets can be solidly fixed at the upper region, so all expansion takes place in the lower part of the sheet towards the eaves.