I used reclaimed galvanized roofing from an old barn. I dug a little to set the pallets level and tied corners with 2×4 cut offs. They slide in and long deck screws. I also drove in 6 foot steaks at all 4 corners in case of support but it ended up being very sturdy. I also had the benefit of attaching it to my existing 12×12 tool shed. Regular roof rafters with old 2×4s sloped down.