By "FM is lower frequecies and will be more foriving than upper frequecies", may I assume you are not including AM in that comment?
FM has a much shorter wavelength. Car antennas, for example, are best for FM when set at 31" long; for AM, the longer, the better. In home, a di-pole seems to work well for FM.
For AM, there's no substitute for wire, wire, wire. The longer, higher plain pl' wire you have, the better. Long wire = long wavelength = low frequencies.
Also, you should ground the AM GRD terminal. (Occasionally, the reception will be better without the ground, but that's rare, ground loops not withstanding.)
Personal opinion: The weather protection afforded an attic antenna beats the gain seen by having it a few feet higher, shingles and nails included. Metal is, of course, a different story.
Man, the thought just hit me. Imagine having a copper roof. Now there's an AM antenna for you! Now imagine it being hit by lightning. Better have a good lightning arrestor! (Still, I'd try it!)
Larry
www.fineelectricco.com