Concave Roofs
Roofing can be spring-curved into concave shapes; however, designers should be aware of the limitations on the minimum pitch where the curve is terminated. The extra uplift load that will be taken by the fasteners at the centre of the curve, and take into account the catchment area of the roof.
The pitch for concave roofs must not be less than 8° for corrugated profiles, 4° for symmetrical trapezoidal profiles, and 3° for other profiles. Screws and load spreading washers must be used for fixing cladding on all sprung concave curved roofs. The purlins must have additional fixing to the structure to resist the extra uplift load on sprung curved sheets.
The additional load produced by draping concave and convex metal roof cladding depends on the radius of curvature and the thickness of the metal. The induced load has two forces:
- additional load on the fastener; and
- additional load on the purlin/rafter or truss connection.
Although the former is the responsibility of the roofer, the COP recommends that the purlin connection is inspected for adequacy.Using a proprietary purlin strap can be an economical solution for the increased connection load.
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