01 May 1999
(6) Occupancy Considerations. The PMR is suitable for all occupancies. They are
particularly well suited where interior relative humidity is high (> 50%). Special paper-mill designs
may be needed when the RH is > 80%.
(7) Low-Sloped vs. Steep Roofs. PMR's are used on low-slope membrane systems at
slopes up to 16.7% (2:12), but is best to limit the slope of any ballasted roof to 8.3% (1:13).
(8) Leaks. PMR systems are very durable; when properly built they require little
maintenance. However, if they contain built-in deficiencies that cause leaks the flows are difficult
to locate and repairs are complicated because everything above the membrane must be removed
to get at it. Leaks are much easier to isolate if the membrane is fully adhered. Flood testing a
protected membrane roof for 24 hours prior to adding insulation, etc., is worthwhile.
3-2. BUILDING ELEMENTS
a. Slope. Minimum slope of 2% (1/4 in./ft.) is required to insure positive drainage and to
minimize flotation potential. Maximum slope is 16.7% (2:12) due to a tendency for slippage or
sliding of surfacing ballast.
b. Low-Sloped Roofs. Protected membranes will be wet for long periods of time. Organic
roofing felts and other moisture sensitive materials should be avoided. Consideration should be
given to flood testing small sections of the roof before insulation and ballast are installed as it is
difficult to detect flaws once the system is completed.
c. Structural Considerations. Protected membranes require continuous support. On
discontinuous substrates a bridging layer of insulation or gypsum board may be needed. The
membrane is applied to this substrate. Ballasted systems may add weight as follows: lightweight
interlocking insulated pavers--22 kg/m (4.5 psf); stone ballast--49 to 98 kg/m (10-20 psf); and,
concrete pavers--42 to 107 kg/m2 (8.5-22 psf).
d. Expansion and Seismic Joints. No special requirements.
e. Re-entrant Corners. No special requirements.
f. Roof Access. Mortar surfaced insulation is only a suitable surface for light roof traffic.
Some pavers cab support heavy loads. When stone ballast is used paver walkways are
g. Roof Venting. Since the thermal insulation is above the membrane venting is not
applicable. Elevating pavers slightly above the insulation promotes self-drying, reducing the risk
of freeze-thaw degradation of the pavers and insulation.
h. Roof Decks. Noncombustible decks should be smooth and free of abrasive debris. Steel
decks require a fire rated bridging layer of roof insulation or gypsum board to support the roof
i. Vapor Retarders.
Under most conditions the PMR system serves as its own vapor
(1) Special Condition for Condensation. For extremely high internal vapor pressures
(c9mbination of high temperature and high humidity), it is possible for the roof deck to be
temporarily chilled below the dew point. Condensation may result in drippage onto equipment or
processes blow. This condition can be reached during a cold rain when the rainwater by passes
the thermal insulation on the way to the drains. In this case a separate vapor retarder is installed