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.