We are converting a worksop that is attached to a garage into a screen house. The garage has a hip roof The old structure had a shingled flat roof that rotted through. So now, we have torn out the walls and reconfigured the 2 x 4 studs for the screen house on which we want to build a gabled roof, 3/12 pitch, and attach it perpindicularly the slope of the hip roof. We plan to use polycarbonate panels over the screen house and lumber & shingles over the garage hip slope. How do we attach to the slope? Won't the rafter have to shorten as the hip narrows? Any help would be appreciated.