01 May 1999
loading and shrinkage forces. Refer to membrane termination requirements provided by ANSI/SPRI
RP4. Proprietary metal gravel stops should be used. Follow industry details.
(3) Expansion Joints. Because of the flexibility of thermoplastic membranes, the membrane
material (supported by flexible foam) is sometimes run directly over the curbs of the joint without a
change of material. Raised joints should be used rather than flush joints at water level.
(4) Sealants. Sealants are used primarily at seam edges and to seal the interface of
termination bars and vertical walls. Only compatible materials should come in contact with the
q. Drainage, Valley and Intersection Details.
(1) Drains. Roof drains should be sized in accordance with plumbing codes. Secondary
(overflow) drains or scuppers should be designed in accordance with ANSI/ASCE 7-95. Peripheral
drainage is acceptable in areas where heavy ice buildup is not expected. Interior drains avoid drain and
leader freeze-up. Primary roof drains should be recessed below the roof line by using deck sumps
(uninsulated decks) or by creating a sump out of roof insulation in order to properly collect water. The
roof membrane may be bonded to the drain bowl using specified adhesives or welded to a special
polymer coated metal drain flange.
(2) Valleys and Crickets. Valleys and intersections are fabricated out of membrane materials.
Crickets and saddles help divert water to the drains. Refer to NRCA details for saddle design.
Building intersections benefit by the use of expansion joints or area
7-3. DESIGN CONSIDERATIONS AND ALERTS.
a. Attachment. Single-ply membranes do not require attachment to the substrate; however,
adhered systems are convenient for steeply sloped roofs. Mechanically fastened systems are
generally less labor and material intensive than fully adhered systems. Anchorage at roof edges is
critical to resist membrane shrinkage as well as wind flutter. Nailers and nailer installation should be
anchored to resist the wind uplift of Factory Mutual (FM) 1-60 or 1-90 (or higher). The perimeter nailer
installation must be in accordance with Factory Mutual Loss Prevention Data Sheet 1-49 entitled,
Perimeter Flashing and ANSI/SPRI RP-4 recommendations (Appendix 6-1).
b. Phased Construction. All components, from the thermal insulation through completed
membrane, should be completed in a single day. Edges should be sealed at the end of a day's work to
preclude moisture penetration.
c. Protective Walkways. Walkways are needed if more than occasional roof traffic is expected.
These may be proprietary plastic pads partially bonded to the membrane, or wood duck
boards/concrete pavers installed over protective fabric, shims, or extra membrane.
d. Compatibility. Thermoplastic sheets containing plasticizer need a separator layer when
installed over polystyrene insulation or insulations containing bitumen. In re-cover or reroofing
separation from bitumen-contaminated walls and penetrations may require refacing the contaminated
surface with plywood, using fleece backed sheets, polymer-clad metal base flashings, or using
bitumen-resistant sheets where needed i.e., at drain flashings.
7-4. CONSTRUCTION CONSIDERATIONS.
a. General. Thermoplastic membranes require special heat welding equipment. Contractors may
need to use portable generators to obtain adequate electrical power.