CEMP-ET
TI 809-53
01 May 1999
CHAPTER 3
PROTECTED MEMBRANE ROOFING
REFERENCE CEGS 07550
3-1. OVERVIEW.
a. Concept of PMR System. In a protected membrane roofing system (PMR) the
membrane is protected by an overlay of extruded polystyrene insulation, a filter fabric, and ballast
(figure 3-1). Protection of the membrane is provided against accelerated oxidation and
evaporation of volatile oils, ultraviolet degradation of organic materials, thermal movement
(expansion and contraction), roof traffic, hail or other impact damages, cracking, warping or
delaminating from ice contraction, blistering and ridging, and stress concentrations over insulation
joints. Virtually all roof membrane systems may be used in a protected configuration. The slight
cost increase (5 to 15%) is mitigated by much greater durability than with exposed membrane
applications.
b. General Information. PMR systems place loose boards of extruded polystyrene foam
insulation over the roof membrane (and flashings). Most rainwater flows to the drains along the
surface of the membrane. Drainage channels (kerfs) approximately 13 mm (1/2 in. x 1/2 in.) on
the bottom side of the insulation boards facilitate water flow. Porous fabric (filter-fabric or stone-
mat) above the insulation boards keeps silt and fine stones from getting between the boards. The
fabric and overlying ballast (stones or pavers) serve to raft the buoyant boards together during
heavy rains so that they will not be displaced. The pavers should be placed on pedestals or be
ribbed on the bottom surface to facilitate drying.
(1) Climate/Weather. PMRs are especially effective in extremely cold climates for the
following reasons:
(a) Once the membrane and flashings are installed the rest of the installation
(insulation, filter fabric, and ballast) can be done in cold or wet weather.
(b) The position of the insulation on top of the membrane shifts the dew point from
below the membrane to above. The membrane now serves as the vapor retarder for the system
eliminating the need for a separate vapor retarder, which can create a potential vapor trap.
(c) The insulation keeps the membrane warm in cold weather. It stays well above
freezing eliminating ice damage. Any melt water that reaches the membrane remains as a liquid
enroute to the drains. Snow that accumulates on the surface adds to the insulation value of the
assembly. (Note: This advantage is diminished in cold, wet [maritime] climates where cold melt
water flows across the warm membrane for extended periods of time.)
(2) Logistics. Substituting cement-faced composite panels for stone ballast allows
installation even when quarries are frozen and ballast stone is unavailable. Interlocking pavers
weighing only 22 kg/m2 (4.5 psf) are available.
(3) Familiarity with the System and Site. Materials are identical to any other membrane
roof system.
(4) Life Expectancy. PMR configurations will add years to membrane systems by
protecting against abuse and weather degradation.
(5) Costs. PMR systems are slightly higher due to the relatively high cost of extruded
polystyrene (XEPS) insulation, which is the only suitable insulation, and the use of filter fabric.
On a life cycle basis PMR systems are often less expensive.
3-1
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