MIL-HDBK-1001/5A
a PMR membrane in a perpetually cold climate is obviously below the major insulation. (An optional thin layer of minor insulation can level the membrane substrate and reduce heat/energy losses from convective and evaporative cooling following a rain. For additional energy savings, a layer of expanded polystyrene or other type of insulation under the membrane may be more cost effective than excessive extruded polystyrene above the membrane.) In perpetually humid tropical climates, the best location for the membrane is above the major
insulation, such as for a conventional system, where the membrane serves as both roof and vapor retarder. PMR is excellent for areas with high ultraviolet exposure.
8.3 Specific Design Recommendations. PMR system design requires attention to the following aspects:
a) Drainage: slope to drains minimum 1/2 inch per foot
(4 percent) for new construction and 1/4 inch per foot (2
percent) for reroofing projects;
b) Deck: Refer to par. 8.2.2; design to support ballast and live loads;
c) Membrane: built-up bituminous membrane; modified bitumen; single-ply sheets; liquid applied;
d) Percolation course: Optional, 1-1/2- to 2-inch (38- to 51-millimeter) minimum washed rock;
e) Insulation: use only extruded polystyrene conforming to ASTM C 578 Rigid, Cellular Polystyrene Thermal Insulation, Type IV, above the membrane;
f) Filter fabric: non-rotting material (i.e. porous mat of polyester, polypropylene, polyethlene, etc.);
g) Ballast: 3/4- to 2-1/2-inch (19- to 64-millimeter) washed rock, spread at a 10 to 20 psf (49 to 98 kilograms per square meter) density; concrete pavers, concrete wearing surface, or composites of polystyrene and latex mortar. Large arrays of flat bottom pavers should be elevated on pedestals to allow for drainage.
8.3.1 Drainage
a) To properly function, a PMR must have fast rainwater runoff. Assure this by requiring a sloped membrane,
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