I have a single storey rear extension on my detached house with a mon-pitched roof - standard construction with slates on roof. I want to remove the ceiling joists and rebuild so the ceiling is to the rafters. I presume that to do this I will need to remove the current roof - joist/ rafter construction and redesign a new roof construction of just rafters (to include some velux type openings for additional light). Can anyone advise as to design/ size of timbers, etc. The basic dimensions of the extention is 2.36m across the pitch x 5m long (internal measurements). Also, will I need to submit loading calcs to Building Control or is this pretty basic stuff?