I’m framing 17x10 room addition to the side of the house I want to do cathedral ceiling. So far I got plans for triple 2x12 ridge beam supported on house end by three 2x4 in the existing outside wall and three 2x6 on the other end, where wall is 6 inch. Rafters to be 2x10 that should give just enough room for R30 insulation and little space for ventilation. The plans I have are not for exactly the same size addition so I’m wondering is I could go with less lumber for the posts and the ridge beam think double not triple. And second question because existing house use trusses for roof they are made out of 2x4 to match the roof pitch and height in the addition with 2x10s is that mean that I have to have the double plate lower than the existing house so that the birth mouth does not eat half on that 2x10