Expansion joint. A flashed, structural separation between two
building segments, designed to permit free movement without
damage to the roof system.
Exposure. Transverse dimension of a felt not overlapped by an
adjacent felt in a built-up roof membrane. Correct felt exposure
in a shingled, built-up membrane is computed by dividing the felt
width minus 2 in. by the number of plies - e.g., for four plies
of 36-inch-wide felt, exposure = 36-2/4 = 8 1/2 inches.
Fabric. Woven cloth of organic or inorganic filaments, threads,
Fallback. Reduction in bitumen softening point, sometimes caused
by refluxing or overheating in a closed container.
Felt. Flexible sheet manufactured by interlocking of fibers with
a binder or through a combination of mechanical work, moisture,
Felt layer. Spreader-type, wheel-mounted equipment for laying
felt and simultaneously dispensing hot bitumen.
Finger wrinkling. Wrinkling of exposed felts in small, finger-
sized ridges parallel to the longitudinal direction of the felt
roll, caused by transverse moisture expansion of the felt.
Fishmouth. Membrane defect consisting of an opening in a
membrane felt edge lap.
Flashing. Connecting devices that seal membrane joints at walls,
expansion joints, drains, gravel stops, and other places where
the membrane is interrupted. Base flashing forms the upturned
edges of the watertight membrane. Cap or counterflashing shields
the exposed edges and joints of base flashing.
Flashing cement. A trowelable, plastic mixture of bitumen and
asbestos (or other inorganic) reinforcing fibers, and a solvent
(a stiffer, more sag-resistant material than plastic cement).
Flash point. Temperature of combustible material, as oil, at
which there is sufficient vaporization to support combustion of
Flux. Bituminous material used as a feed stock for further
processing and as a material to soften other bituminous