I am planning to build a concrete studio that will be built into the side of a hill on two sides. I want the roof to be flat so it can also be used as a patio. It will be a small studio, maybe twelve feet by twenty feet. The walls will be six inches thick. I can't find any information on how far I can span with concrete for the ceiling, what the rebar layout should look like or if I have to design concrete headers into the ceiling slab. Any ideas?