What I mean is a roof where the purlins are large and the primary load supporters. In American framing, and I assume in English as well, this usually means 1 set of purlins. In German framing there might be 2 or 3 sets of purlins supporting common rafters.
It is 'Principal purlin' because the purlins and their supports, and not trusses, support common rafters. As opposed the principal rafters which are generally upper cords of trusses supporting common purlins.
My experience with German and Swiss framing leads me to believe that the principal purlin is superior for many reasons -easier to frame, forces more straight-forward, joinery simple, and overhangs are a piece of cake. But I know not everyone agrees.
DLB
