01 May 1999
(4) Valleys, Crickets, Saddles. Valleys are fabricated out of membrane material. Keep
membrane seams out of the low point of the valley. Crickets and saddles help divert water to the
drains. Refer to NRCA details for their design.
6-3. DESIGN CONSIDERATIONS AND ALERTS.
a. Attachment. Adhered systems are practical for sloped roofs where ballast is impractical or
where ballast weight is unacceptable. Mechanically fastened systems are usually intermediate in price
and labor intensity between fully adhered and ballasted systems. Each insulation board on a
mechanically fastened system should have a minimum of two fasteners installed (called preliminary
fastening) before the membrane is fastened. This keeps the boards from shifting. Anchorage at roof
edges is critical to resist membrane shrinkage as well as wind flutter. Nailers and nailer installation
should be anchored to meet the wind uplift requirements of Factory Mutual (FM) Class 1-60, 1-90 or
higher, and ANSI/SPRI RP-4 recommendations (Appendix 6-1). The perimeter nailer installation must
be in accordance with Factory Mutual (FM) Loss Prevention Data Sheet 1-49 entitled, Perimeter
b. Phased Construction. Since this is a single-ply system, membrane phasing is not possible.
However, edges of completed work need to be sealed at night to prevent water entry. Temporary
ballast needs to be replaced with properly distributed final ballast as expeditiously as possible.
c. Protective Walkways. Walkways are needed if more than occasional roof traffic is expected.
These may be rubber pads partially bonded to the membrane, wood duck boards on wood sleepers, or
concrete pavers. Boards, sleepers, and pavers must be installed over protective fabric, shims, or extra
membrane for puncture protection. Thicker membranes provide added puncture resistance. Pavers
should be a minimum of 38 mm (1-1/2 in.) thick and 21 MPa (3000 psi) minimum compressive
strength, air entrained freeze-thaw resistant concrete.
d. Contamination. EPDM should not be used in areas exposed to asphalt, coal tar, grease, oil,
solvents, vegetable or mineral oil, animal fat, or steam venting. An oil resistant coating may be used
where the roofing will be exposed to petroleum, grease, oil, or solvents. Some manufacturers
recommend the use of neoprene or epichlohydrin membranes for difficult areas. Fleece-backed
sheeting is used to separate EPDM from underlying bitumen.
e. Wind Design. Follow ANSI/SPRI-4 for ballast guidelines. Stone or paver ballast may be used
in the vicinity of aircraft operations as it is much bigger than the pea size stone ballast allowed in BUR
surfacing and thus much more resistant to winds. For mechanically fastened systems, reinforced
EPDM is required. Increased fastener density is required in corners and perimeters. Follow SPRI
Wind Design Guide for Edge Systems Used With Low-Slope Roof Systems.
6-4. CONSTRUCTION CONSIDERATIONS.
a. General. Materials, including accessories, must be delivered to the job site in the
manufacturer's original unopened packages clearly marked with the manufacturer's name, brand
name, and description of contents.
(1) Temporary Ballast. For loose-laid systems temporary ballast may be needed to protect
work until final ballast can be installed.
(2) Protection. Avoid traffic over the recently completed membrane. Use a protective mat
over the membrane if crushed rock is used in lieu of rounded river gravel as ballast.
b. Pre-roofing Conference. A conference should be held prior to construction. Fire safety with
combustible adhesives and fume control should be discussed. Discuss construction of water cutoffs to