Hi Vicki, I'm sure others will chime in here to clarify the exact definition, but here's my "opinion". A "roof surface" would be defined as "any" covering of a "roof", i.e., tar paper, shingles, flashing. This can be derived from the presumtion that a "roof" is the "physical" structure which would be comprised of wood, concrete, brick, or whatever material used to construct a covered environment. Anything that would go "on top" of that physical structure would be considered a "roof surface", i.e., something that "surfaces" the defined "roof". Anyway, that would be my opinion, and like I said others will surely respond.
P.S. DON'T use my name when you talk to the lawyers. LOL