I am about to build a lean-to extension to the rear of my 1950's brick bulit house, 7.5m long x 4.5m wide. My plan is to have a 15 degree pitched roof to feature a single pitch vaulted ceiling. Will C24 250mm rafters @ 400mm centres span this 4.5m alone? Covering will be a Decra Oberon Metal tile. Will a 200mm Crown Fibre glass roll between rafters with an insulation backed plasterboard to underside meet Part L1 building regs?