The area on any roofing material that is left exposed to the elements.

Merriam-Webster Online Dictionary
exposure (noun)
the fact or condition of being as - exposed
a) the condition of being presented to view or made known - a politician seeks a lot of exposure
b) the condition of being unprotected especially from severe weather - died of exposure
c) the condition of being subject to some effect or influence - risk exposure to the flu
d) the condition of being at risk of financial loss - minimizes your exposure to market fluctuations , also an amount at risk
the act or an instance of as - exposing
a) disclosure of something secret - tried to prevent exposure of their past
b) the treating of sensitized material (as film) to controlled amounts of radiant energy , also the amount of such energy or length of such treatment - a 3-second exposure
a) the manner of being exposed
b) the position (as of a house) with respect to weather influences or compass points - a room with a southern exposure
a piece or section of sensitized material (as film) on which an exposure is or can be made - 36 exposures per roll