Oh, sorry for that ;-) Go and have a beer !
Roof ridge to eaves about 5 metres, vaulted roof, on sideways across the back of the house. Basically, a low pitch roof (about 11 degrees) or anything less than
15 degrees should be designed like a flat roof, since most of the load is downward, also meaning that thicker rafters are needed. That was according to tables in the older part A. The design is 225mm purlins every 730mm, and the "rafters" are really counterbattens (50mm), so the primary structural component is the purlins. The direction of the purlins is the shorter dimension, and at this pitch with usual rafters, they would have to be about nearly 225mm also, and follow the roof pitch all the way down, thus cutting off more headroom at the lowest part, despite losing the 50mm. WIth the purlins, the lowest purlin is 730mm from the bottom of the roof, and beyond this the ceiling can be flattened out, thus increaseing head height at the lowest point. The pitch is constrained by the required head height, and originally the ridge of next door, but now I am building higher, by a window over the side abutment. Roof designed passed by Struct Eng. Think of metal framed buildings with metal roofs for the idea of serveral purlins and thinner members resting on them. It does make sense and does improve lower head height for constrainted pitch, whilst importantly also enabling most of the insulation to be lost in the thickness of the roof. I should have posted a diagram.Why am I putting the roof on "sideways" across the back of my house and not the usual way round ? Well partly to mirror the next door (which has a much smaller extension, this steeper pitch etc), but also to put a lovely high ceiling and arched window in the end wall of the kitchen where it will look cool !
Or maybe I should just put on a flat roof and felt.
Now you can finish your jug of foaming ale !
Simon.