Definitely be careful shooting from your roof unless it is flat. This will cause your body - shoulders, and hips - to also be at an angle. Shooting from this position, you might not see any difference as far as your sight pins from standing on the ground. However, go 12 to 15 feet in the air and stand on a flat surface - you'll be able to tell because now you have to bend at the waist. If you don't you're guranteed to shoot high.