01 May 1999
(3) Area Dividers. Area dividers do not provide for movement but are useful in subdividing big
areas into isolated, smaller areas, each of which can be roofed (and maintained) separately.
d. Re-entrant Corners. Expansion joints or area dividers should be used at most re-entrant
corners where stress concentrations are likely.
e. Roof Access. Roof access is essential to maintenance and inspection functions. Access
should be controlled to block unauthorized entry (provided that it does not interfere with means of
egress in an emergency).
(1) Roof Vents. On BUR membrane roof systems, roof vents have been proven to be of no
value in drying wet materials and their efficacy for pressure relief is questionable. Roof vents may be
required by the manufacturer's specification for some proprietary wet fill deck materials to augment
venting base sheets. It is best to avoid using wet decks.
(2) Edge Venting. When a BUR is installed in a re-cover two membranes will be present.
g. Roof Decks.
(1) Uninsulated Decks.
(a) Non-combustible/non-nailable decks such as poured and precast concrete should be
cured and dry (28 day dry). Test for dryness by pouring some hot bitumen on the deck. If it can be
peeled off, the deck is too wet. If moisture collects on the underside of a piece of glass placed on the
deck, the deck is too wet. Asphalt primer (ASTM D41) is used to promote adhesion and to penetrate
dirt and dust.
(b) Non-combustible/nailable decks such as precast lightweight planks and poured
gypsum require base sheets (ASTM D2626, D4897) secured with proprietary fasteners designed for
that deck type.
(c) Combustible/nailable decks such as wood, plywood, and OSB are dimensionally
unstable in the presence of seasonal moisture changes. They require mechanically attached (not
adhered) base sheets to avoid stress concentrations and splits at panel joints.
(2) Insulated Decks. Insulation may be anchored to the deck with bitumen or mechanical
fasteners. Insulation should be divorced from moisture bearing wet fill decks by the use of a nailed
asphalt coated base sheet.
h. Vapor Retarders.
(1) Vapor Retarder Location. On non-combustible decks two plies of bituminous felt mopped
to the substrate and to each other provide an excellent vapor retarder. For nailable substrates a
coated base sheet nailed, followed by a mopped felt is adequate. On steel roof decks mechanically
fastened insulation or a fire rated underlayment is required to support the retarder. Thermal insulation
is then mopped directly to the retarder. Refer to figure 2-14 to determine the need for a vapor
(2) Special Conditions for Freezers and Coolers. When a roofing system is used as the cover
of a freezer or cooler building a severe reverse vapor drive occurs. Vapor pressure is higher outside
the building and the primary vapor drive is towards the cold interior. The BUR membrane is itself the
vapor retarder. Roof vents and vented roof edges must be avoided. In addition, the membrane must
form a vapor seal with the wall retarder or ice damage will occur at this critical intersection. It may be