I am building a two storey extension on the side of my semi. I want to extend the roof across the new blockwork. My mate's house has had this done, but the builders left all the old hip timbers in and attached the new ridge board to the existing joint, is this the normal way? I assumed all the old hip timbers come out, the existing ridge timber gets temporarily propped and the new ridge timber gets scarf jointed to the old one or am I wrong?