Section 5: BITUMINOUS BUILT-UP ROOF MEMBRANE
Material Description. As the weatherproofing component
of a built-up roof system, the built-up bituminous membrane
comprises three elements: bitumen, felts, and surfacing.
Bitumen. Bitumen resists water penetration in the
interply adhesive films mopped between felt layers and the
bituminous surface-coating or flood coat. The two types of
bitumens are blown (i.e. oxidized) asphalts and coal tar pitch.
Felts. Felts stabilize and reinforce the membrane.
Felts prevent excessive bitumen flow when the bitumen is warm and
semi-fluid and distribute contractive tensile stress when the
bitumen is cold and glasslike. Felt plies separate the interply
bitumen moppings and provide the membrane's multiple lines of
Surfacing. Surfacing of built-up membranes comprises
one of four basic types: aggregate, smooth, mineral surfacing, or
Flashing. Flashings seal the joints where the membrane
is interrupted or terminated. Base flashings, essentially a
continuation of the membrane, form the edges of a watertight
"tray." Counterflashings shield or seal the exposed joints and
edges of base flashings.
Recommended materials are the
a) One ply of granular surfaced modified bitumen
sheet, over 2 plies of glass-fiber felt.
b) Three-ply glass-fiber felt (sometimes reinforced
and granular surfaced).
c) Metal base flashings should be restricted insofar
as practicable to vent-pipe seals, gravel-stop flanges, and drain
flashings, where there are no practicable alternatives.
Counterflashings. Counterflashings (sometimes called
"cap flashings") are usually made of metal: aluminum, galvanized
steel, stainless steel, copper, or lead sheets designed to shield
exposed base flashing joints and shed water from vertical or
slanting surfaces onto the roof.