01 May 1999
f. Re-entrant Corners. Expansion joints should be used strictly as the structure demands.
Single ply systems have adequate elongation and flexibility to withstand typical stress concentrations
found at re-entrant corners. Area dividers may be preferred to serve as permanent physical
separators between this particular membrane and other membrane materials such as BUR, SPF, MB
g. Roof Access. Roof access is essential to maintenance and inspection functions; however,
unauthorized access should be controlled. Heavily trafficked areas should have walkways or pavers
h. Roof Venting. Roof vents have been proven to be of no value in drying wet materials and their
efficacy for pressure relief is questionable. However, pressure-reducing vents have been used in
single-ply systems vents to reduce under-membrane uplift pressure. They are only effective if the
substrate is airtight.
(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 placing a piece of glass or membrane material on the
deck. If moisture collects on the underside, the deck is too wet. Primer is used to promote adhesion
and to penetrate dirt and dust. A fleece backed membrane is more suitable over rough substrates.
Some specifications allow hot asphalt in lieu of solvent adhesives.
(b) Non-combustible/nailable decks such as precast lightweight planks and poured
gypsum require fasteners designed for that deck type.
(c) It may be necessary to tape or grout deck joints to provide a smooth substrate.
(d) Combustible/nailable decks such as wood, plywood and OSB accommodate either
adhered or mechanically fastened systems.
(2) Insulated Roofing Systems.
(a) Insulation may be attached to the substrate with bitumen, special adhesives, or
mechanical fasteners. Care must be taken not to spill asphalt on the single-ply membrane, nor to
traffic on it with work boots or equipment.
(b) Insulation should be divorced from moisture bearing wet fill decks by the use of a
nailed base sheet. Some plasticized thermoplastic membranes require a divorcing slipsheet between
the top surface of the insulation and the membrane (if that surface contains asphalt or if the insulation
is unfaced polystyrene).
(1) Installation. Two plies of bituminous felt mopped to the substrate and to each other
provide an excellent vapor retarder for non-combustible decks. Where bitumen is to be avoided, metal
foil, kraft paper or plastic films (i.e., 4 mil polyethylene) with taped laps may be suitable following the
recommendations of the manufacturer.
(2) Coolers and Freezers. 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
primary vapor drive will be towards the colder interior. Most thermoplastic membranes are only poor
vapor retarders, especially if mechanically attached where moist air can migrate laterally. Thicker
membranes would have lower permeance, or it may be feasible to install a foil layer with taped seams