pitch roof

Hi all, We are doing a 6m deep rear extension and could really do with your thoughts _/\_ Our adjacent (party wall) neighbour currently has a 3m extension with pitched roof. We are looking to match the neighbours pitched roof (with pitched internal ceiling below) and then transition to flat...

I would like to have a flat warm roof for our new extension but as the our house has a pitched roof am not sure how the abut the too as the exterior walls will be flush and so the large flat roof fascia, deck and insulation will cut awkwardly into the pitch roof and i am not sure it is even...