You will find that the FCC is on your side regarding installation and use of a receiving antenna.
http://www.fcc.gov/guides/over-air-r...n-devices-rule
Your success in the attic will be limited, signal levels are low enough to make reliable reception very difficult especially when the roof is wet or covered with snow. If you decide to try, expect to need the largest available antennas. A roof mounted Antennas Direct XG-91 would not be overkill if you are trying to receive the weaker signals in the red section of your report.
You may want to look closely at the size of the XG91, then decide if you want to try one in your attic.
If you are using or considering home theater PC technology, consider the following multi-antenna option:
http://forum.tvfool.com/showthread.php?t=820
If the multi-tuner HTPC option looks like something you want to pursue, specific antenna recommendations can be made.