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.
The most significant strength characteristic of metals used for profiled metal roofing is the tensile strength. To test for tensile strength the material is subjected to a longitudinal (stretching) load, and values are taken for yield strength (when it permanently deforms) and tensile strength (when it breaks). Elongation is also measured during this test.
The minimum tensile strength defines the grade of steel, eg, G550 for high strength light gauge steel, but to comply with this grade the yield strength and elongation must also fall within defined parameters. For G550 material, the minimum yield strength requirement is the same as the tensile strength, but for more ductile grades the yield strength requirement is lower.
Tensile strength is an important determinant of the strength of a profile, along with profile shape and material thickness. High tensile material will have more resistance to failure such as buckling around the fastener under wind uplift, pull through of the fastener head or buckling under foot traffic[MS1] load. However, tensile strength has a negligible effect on deflection under load.
Where 0.55 material is specified for straight corrugate or trapezoidal roofing, it is unacceptable to substitute G300 for G550 grade material as the resultant profile will have little strength advantage over 0.40 mm G550. When manufacturing aluminium roofs, aluminium trapezoidal and corrugate profiles normally manufactured from G550 steel should be manufactured from H36 aluminium rather than H34.
Aluminium is defined by a hardness grade ranging from H32 to H38. Typically, H34 is used for flashings, severe profiles such as trough sections and profiles that are to be curved. Most corrugated and Trapezoidal profiles are manufactured using H36.
It must be remembered that the alloy also affects strength. H36 aluminium in 5005 or 5025 alloys, which are typically used in New Zealand, will have considerably greater tensile strength than the same grade in a 3000-series alloy.
|Material||Grade||Typical End Use|
|Steel||G300||Flashings, ridging, spouting, curving, some trough sections.|
|G550||Corrugated and trapezoidal profiles, some trough sections.|
|Aluminium 5505/5025||H32||Lock seaming|
|H34||Flashings, curved roofing, trough sections, and tray roofing.|
|H 36||Flashings and profiled roofing, trapezoidal sections, and corrugate.|