Quantcast Specific Design Recommendations

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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.
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
a)  To properly function, a PMR must have fast
rainwater runoff.  Assure this by requiring a sloped membrane,


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