Steelwork tolerances can be an issue when installing both pitched and curved roofs. Care must be taken to minimise deflection, sag and twist on purlins and rails.
Purlin Cleats can be as deep as 500mm, particularly with facetted rafters. Even a small misalignment can have a significant effect on the position of the top flange of the purlin where the roof cladding is fixed. To ensure this does not occur, the steelwork contractor must be aware of the cladding system requirements.
All built up metal roofing assemblies require a positive fall from apex to gutter, whether on a pitched or curved roof. The simplest way to achieve this is for the steelwork contractor to ensure a purlin is located close to the apex, as with all traditional portal frame designs.