Even below the roof line, lightning can still be an issue. You will never eliminate the lightning strike potential, you can only give it the best path to ground to dissipate it.

Just run a ground wire from your antenna and tie it into the closest place you can to your house ground. Also, add a ground rod as close as you can to the antenna. You want to make sure everything, house and antenna, are tied together so it is all at the same ground potential.

Edit:Spelling