In between rafters of 150mm there should be at least 125mm. Over rafters but under plasterboard should be at least 20mm.
If the dormer is made of timber then there should be at least 100mm in between timbers. 7 inch flat roof should have at least 125mm in between timbers and 30mm under plasterboards (for cold deck style).
Call your local Building Regulations Officer who will advise you if you are using different size timbers to the ones we have given.
