CEMP-ET
TI 809-53
01 May 1999
3-3.DESIGN CONSIDERATIONS.
a. Guide Details. Guide details for PMR systems are provided in the Low-slope Membrane
Roofing Construction Details provided by the NRCA (1996):
BUR-P-8
TP-P-7
BUR-P-16
TP-P-15
BUR-P-21
TP-P-20C
BUR-P-23A
TP-P-22
MB-P-8
TS-P-7
MB-P-16
TS-P-15
MB-P-21
TS-P-20B
MB-P-23A
TS-P-22
b. Required Details. Additional details that may be required in design drawings include
pavers used in valleys, at edges, and where truncated; protected wall flashings; and, where
mortar-surfaced insulation needs metal strapping for high wind resistance.
3-4. CONSTRUCTION CONSIDERATIONS AND DESIGN ALERTS.
a. General. For loose-laid membrane systems (which are not recommended with PMR
systems), temporary ballast may be needed until the insulation and final ballasting are installed.
Loose-laid membranes require an airtight deck. Air barriers may satisfy this requirement.
(1) Overloading the Structure. Piles of ballast should not be accumulated on the roof.
Verify structural load capacity and distribute weight uniformly.
(2) UV Degradation. Protect insulation from extended exposure to sunlight.
(3) Drain Freeze-Up. Place drains within heated portions of a building and cover with
insulation to avoid freeze-up.
(4) Ponding. Slope must be adequate, 2% (1/4 in./ft.) minimum.
(5) Leak Detection. Consider flood testing prior to application of insulation and ballast.
(6) Heat Loss. Increase design R to allow for water-cooling in high rainfall areas. Use
two layers of insulation with joints offset.
(7) Extreme Vapor Drive. Use paper-mill design for high (> 85%) interior humidities.
(Paper-mill design requires sufficient thermal insulation beneath the membrane to keep the deck
warmer than the interior dew point temperature. This will require a deck-top vapor retarder and
rigid board insulation prior to installation of the PMR system.)
(8) Self-Drying. Provide for airflow between top of XEPS and pavers.
(9) Protective Fabric. Use specified filter fabrics.
(10) Protected Flashings. Consider protected flashing designs.
(11) Paver Displacement. Use strapping or overlayed, heavy pavers where interlock of
lightweight pavers is ineffective such as at valleys, perimeters, and penetrations.
b. Pre-roofing Conference. Agenda items for the pre-roofing conference should correlate to
the Chapter that covers the specific roof membrane being used. Flood testing procedures must
3-4
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