01 May 1999
beneath the single-ply membrane. Vented edges and roof vents should not be used. The single-ply
membrane or foil underlayment needs to be hermetically sealed to the wall vapor retarder system to
be effective. 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 positioned on the exterior of
the freezer box facing the air space.
Thermal Insulation and Heat Flow.
(1) Polyisocyanurate (ASTM C1289) Insulation. For adhered systems the facers must be well
adhered to the isoboard and compatible with the bonding adhesive used.
(2) Wood Fiber and Perlite. For adhered membranes wood fiber (ASTM C208) may be an
acceptable substrate. Perlite board is generally unacceptable when solvent-based adhesives are
used, due to its lower peel strength.
(3) Thermoplastic Foams. Polystyrene foam (MEPS) (ASTM C578) with a minimum density
of 16 kg/m (1 pcf) or XEPS should be used. Plasticized PVC membranes require a separator layer.
(4) Thermal Bridging. When mechanical fasteners are used to secure the membrane there is
some conductive heat flow through the fasteners. This is obvious on a roof with frost or light snow and,
in a colder climate, might lead to condensation or corrosion problems.
l. Membranes and Surfacings. Thermoplastic membranes are compounded for weather
resistance. Most thermoplastics should not require coating during their lifetime. However, some
Hypalon sheets have been observed to chalk and craze excessively. Special surface preparation
and the use of primers and coatings may extend their life and reflectivity.
m. Penetrations and Flashings. Flashing, perimeter securement, and detailing of thermoplastic
systems is critical. Construction detail plates are available from the NRCA, SMACNA, and individual
manufacturers. Pitch pockets and pipe flashings are handled by using molded boots that are clamped
to the penetration and welded to the membrane or by field fabrication. Since contact with bitumen
must be avoided, pourable sealers should be non-asphaltic. Polymer-coated galvanized steel is used
as flashing in some systems. The field membrane sheet is heat welded directly to the coated flashing
n. Historical Roof Restoration. Single-ply membranes may be used in historical renovations.
The very light weight of single-ply adhered and fastened systems may be useful in reroofing
historically significant buildings that have only marginal load capacity. Pavers installed on pedestals or
fleece pads provide an attractive appearance for pedestrian ways.
o. Aesthetics. In typical low-slope applications aesthetics is rarely of concern. Exposed light
colored membranes are reflective but because they are so thin and flexible they telegraph irregularities
in the substrate. On highly visible applications special attention must be given to smoothing of the
substrate. Thermoplastic membranes in various colors have been used to give domed stadiums very
p. Gaps, Flashings, Joints, and Sealants.
(1) Gaps. Gaps in insulation in excess of 6 mm (1/4 in.) should be filled in with thermal
insulation by inserting pieces of insulation replacing broken boards or taping joints. Gaps in nailable
decks are bridged with sheets of galvanized steel.
(2) Flashings. Flashings for single-ply systems do not require cant strips to reduce the angle
at the base of the wall or curb. NRCA details, modified to specific job conditions, should be followed.
Base flashings should extend not less than 200 mm (8 in.) above the roofline. Special polymer coated
metals may be used as base flashing and roof edging. Anchorage must be adequate to resist wind