My personal experience with the iPhone compass app is that it can be off by as much as 180°, seriously.
An alternative to a compass: Rerun your TV Fool report starting with
http://www.tvfool.com/?option=com_wrapper&Itemid=90. Put your address in, then switch to 'Satellite' or 'Hybrid' view. Zoom in close, move the pointer to the exact location of your antenna. Check the box labeled "Show lines pointing to each transmitter". Now look for landmarks on or near the lines, such as the corner of a neighbors house, or a tree, etc. When you are on the roof, look for that land mark to orient yourself and the antenna.
This gets you close, you still will want to fine tune the aim to maximize the number of stations received reliably.