Ric: I built two 30" by 78" shed doors, and used 5/8" T-111 plywood; and 1x4" rough cedar doorframe and criss-cross. One door warped just a little, and the other is perfectly flat. I think if you want the doors to be true, you need to heavy up on the coonstruction and then coat all six sides to waterproof. The one of mine that is a little warped I used for the "close-me-first door" and the latches hold it shut. I had looked a a number of commercially built sheds, and this seemed to be the preferred method of construction. BobB