Help! I am in the process of havig a two car garage built (20Ft. wide, 20ft deep and 13ft. high) I would like a roof-top deck on top. a 7 ft. H x 16 ft W garage door. How and where can I get design/plans for it, help on figuring out if the garage needs to be built with a slope/for draining, what types of beams are needed for weight support etc.