asphalt-saturated felt or other sheet material
(may be self adhering) installed between
the roof deck and the roof system, usually
used in a steep-slope roof construction.
Underlayment is primarily used to separate
the roof covering from the roof deck, to
shed water, and to provide secondary weather
protection for the roof area of the building;
(2) One or more layers of felt, sheathing
paper, nonbituminous saturated felt, or
other approved material over which a steep-slope
roof covering is applied.