I'm re-roofing my garage in a few weeks, and it's my first crack at roofing. I will be tearing off the existing three layers of roof (cedar shake plus two asphalt) before replacing. The garage is a smaller 2-car, approx 20x20 with a hip roof. So... any idea how big a dumpster I'll need? I may also end up using it to dispose of plaster waste from a 11x11 room, 3/4" thick plus wood lath.