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.
All metal roof and wall 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 causing damage to the cladding, fixings, or structure and without producing undue noise.
Metals have a high thermal expansion coefficient and generate more movement than many other building materials during heating and cooling. Plastic roof lighting and non-ferrous metal cladding move even more than steel.
The ribs of metal trapezoidal, or corrugated roof or wall cladding take up the expansion across the width of the sheets, but special provisions are needed over the sheets' length.
Where end laps are through fixed, or where the roof and wall cladding are connected by curved sheets, they must be considered as one length.
Changes of the direction of the cladding also require expansion provisions.
The amount of metal expansion given in various building related literature, N.Z Standards, and other publications can be misleading because it is calculated from theory and not real conditions.
Non-ferrous metals have a higher rate of expansion than steel. To prevent the material from being over-stressed the length of copper, aluminium, and zinc cladding or flashings are restricted. Because these materials are not as strong as steel, expansion provisions such as slip joints, sleeves, and welts are necessary.
Steel cladding does not need the same degree of expansion provisions as non-ferrous metals, and vertical wall cladding does not require the same provisions as roof cladding, because of solar radiation angle.
Horizontal wall cladding is restricted to the same length requirements and fixing provisions as roof cladding. (See 6.2.2 Roof Cladding Expansion Provisions for the indicative figures for maximum fixed lengths of roof and wall cladding.)Oversized holes and washers an give some room for expansion and contraction, but over long spans, it is not enough to allow movement without stress or distortion. In such cases, a slip joint or expansion joint should be used. The recommended method is to form a step in the roof structure, which allows the cladding to move independently (see 126.96.36.199A Change of Pitch Junction Flashing) , or if the pitch is greater than the minimum by 2°, the roof can be 'sprung' (see 8.3.8B Watershed (b)).
To assure independent movement no fixing should go through both sides of an expansion joint.
With warm roof construction or composite panel systems, thermal movement should be considered in relation to the top sheet, insulating core, and the lining sheet. (See 14.5 Insulated Panels) Temperature changes from one surface to another produces different expansion rates. The resulting tendency of bowing increases the tensile and shear forces on fasteners.
Site assembled semi-composite panel systems do not pose the same problem, as each element can be dealt with independently and no shear forces are transmitted to the insulation.
Where profiled metal sheets are fixed horizontally in long lengths or to curved corners, expansion provisions should be made as in 6.2.2 Roof Cladding Expansion Provisions
The amount of linear expansion — especially in relatively short sheets — may appear insignificant, but considerable forces develop if the sheet is fully restrained at both ends and with no deflection.
Sheet lengths of polycarbonate (PC) and rigid PVC should be more restricted than other plastic roof lights because they expand far more per degree of temperature change than metal or GRP. Roof lights constructed from PC and PVC should have fixing holes larger in diameter than the shank of the primary fastener, and the fasteners should be centred in the fixing holes to allow for expansion or contraction of the roof lights.
When the outdoor temperature is significantly higher than indoors, double skin roof lights fabricated from PC and PVC expands more than a single skin roof light, because of the different expansion of the skins. That may result in ponding on roof lights installed on low pitched roofs. (see rooflights section 9.0)