Various methods are suggested in the available codes to design roof detailing in cold formed screwed-down roof systems made of cold-formed purlins covered with metal sheeting. The evaluation of the global anchorage force of the roof and the local forces in fasteners is pursued aiming at designing support detailing and purlin-sheeting connection, respectively. Two factors, “R” and “r”, are here employed to synthetically describe the stabilization forces that are required at the supports and along the span. It is shown that these two factors are implicitly provided by some codes for steel structures, but they can be even directly calculated by a simplified finite element model of the whole roof system, here presented. A comparison with the other available approaches is carried out, revealing that the proposed model is an extremely useful tool both for the design and the verification of the structural elements. The described procedure for calculation is shown to be extremely general and easily adaptable to new roof systems, with structural details even not foreseen in the actual codes. For example it can be used to solve the lack of Eurocode 3, Part 1-3, Chapter 10, because, as opposed to the assumption of Eurocode, elastic lateral support can here be envisaged, instead of full lateral support.