Carpentry & Joinery

Birds mouth 2 thirds!

Anonymous user 16/03/2024 - 2.50 PM

Working on a loft conversion where joining two existing dormers, one shed is pitched and second is gabled and removing gabled roof to form one entire pitched shed dormer to do this a header plate runs over the two across 2 windows, the existing rafters are cut with birds mouths that are two thirds of the depth with a one thirds seat over and cut off at end of plate, so to extend I would have to follow this to maintain the same roof level and meet depth reqs for insulation. Issue that’s nagging me is the one third rule of thumb where trying to achieve this would mean raising the entire roof including existing or removing windows as they cannot go lower. Anyone else had this issue and how did you get passed it?