there's nothing between the antenna and the sky other than the roof
decking and shingles. I don't expect them to add a lot of attenuation,
but I could be wrong.

Maybe 3' makes a big difference. I have always seen a difference between
antennas in the attic or stretched out on the roof, and antennas even a few
feed above the roof. I don't know if it was shingles or what. It would
really be interesting to see what kind of shingles are transparent. I have
seen antenna farms hidden behind fake gables.

I've never "played" with it at HF, but I do know that the dielectric
constant of nearby materials affects antennas at VHF.

--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
NRA Life Member & Certified Instructor for Rifle, Pistol, & Home Firearm Safety
Also Certified Instructor for the Kansas Concealed-Carry Handgun (CCH) license