01 May 1999
h. 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). Asphalt primer (ASTM D41) should be used to promote adhesion and to
penetrate dirt and dust. 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.
(b) Noncombustible/nailable decks such as precast lightweight planks, insulating
lightweight concrete, and poured gypsum require base sheets 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 joints.
(2) Insulated Decks. Insulation may be attached with bitumen or mechanical fasteners.
Insulation should be divorced from moisture bearing wet fill decks by the use of a nailed base sheet.
(1) Vapor Retarder Location. One ply of MB felt, or two plies of saturated roofing felt mopped
to the deck and each other provide an excellent vapor retarder for non-combustible decks. On
nailable decks a coated base sheet, either modified or not, followed by one more ply of felt mopped to
the base sheet is adequate for nailable substrates. 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.
(2) Special Condition of Freezers and Coolers. When a roof 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 vapor drive will be towards the colder interior. While a MB membrane itself is an
excellent vapor retarder roof vents and venting roof edges must be avoided. In addition, the
membrane must form a vapor seal with the wall retarder or ice damage can occur at this structurally
critical intersection. It may be better to separate the cooler or freezer from the roof system by at least a
600 mm (2 ft.) air space. The vapor retarder for the cooler or freezer is then installed on the exterior of
the freezer box facing the air space.
j. Thermal Insulation and Heat Flow. Torching should not be permitted directly to any thermal
(1) Polyisocyanurate Insulation. Mopping MB membranes directly to isoboards is not
recommended. A layer of non-foam insulation should overlay the isoboard. Typically, wood fiber or
perlite board is mopped to the iso layer and the MB system is mopped to that.
(2) Aged R-factor. Some decay of R value is observed with HCFC blown foams (urethanes
and isoboards) due to diffusion of air and moisture into the cells of the foam. Manufacturers publish
aged R values to reflect this decay. Thicker foams are more thermally stable.
(3) Recover Boards and Cant Strips. Perlite, glass fiber, and wood fiber are used in re-cover
situations where upgraded thermal resistance is not essential. Non-combustible cant strips should be
used when torched flashings are used.
(4) Thermoplastic Foams. Polystyrene foam is damaged by hot asphalt or torch application.
Some attempts have been made to adhere EPS with hot bitumen by allowing some cooling prior to