Dada Joe
Songster
I am sorry, after 30 some odd years of playing building contractor, I assume that everyone knows how to build. I am learning, not only in this site, that this is not the case. By convention, a rafter runs from the ridge to the roof edge. Typical building codes require them at no more than 24" on center. A purlin crosses the rafters at whatever center the roof panel will span. To minimize sway in plywood, rafter spacing is set to no more than 24" on center. With plywood or OSB as decking, purlins are not required. The roofing material I intend to use has a span rating of 7'-0", but I will add purlins at 4'-0 on center to hold a straighter roof line. Typical building supply store materials have a span rating of 24", so purlins have to be installed to keep the panels from doing what is shown in Nanny's pics