01 May 1999
on the warm side of the assembly (e.g., on the deck followed by sufficient insulation to keep the
vapor retarder temperature above the dew point [paper mill design]).
(2) Thermal Insulation and Heat Flow. PMRs are slightly less efficient than conventional
roof systems because melt water can bypass the insulation. It may be appropriate to increase
the R-value by 5 to 10% to compensate for this. When feasible use two (or more) layers of
extruded polystyrene foam (XEPS) with joints offset in both directions. This is more thermally
efficient as it reduces thermal bridges through insulation gaps and causes more water to flow at
the surface rather than percolating down to the membrane level.
j. Membranes, Filter Fabric and Surfacings. BUR, MB, Single-Ply, and liquid waterproofing
systems have all proven to be acceptable substrates for PMR systems. It is desirable that the
foam not adhere to the membrane. Therefore, when insulating over sticky materials (e.g., some
liquid systems or coal tar pitch) a divorcing sheet of plastic film or non-woven fabric should be first
placed on the membrane.
(1) Filter Fabric. The filter fabric should be either a woven or non-woven pervious sheet
stabilized against UV degradation.
(2) Interlocking Pavers. Pavers should consist of freeze-thaw resistive concrete or latex-
modified, mortar faced, extruded polystyrene foam insulation. Some interlocking pavers can be
lighter than conventional pavers and provide comparable wind resistance.
(3) Ballast. Stone ballast should comply with ASTM D448, Size 4, 3, or 2, following
ANSI/SPRI RP-4, Wind Design Guide for Ballasted Single-Ply Roofing Systems. Crushed rock is
permissible. Expensive rounded river stone is not required.
k. Penetrations. Flashings and details are similar to those of conventional membrane
systems. Drains are treated differently to allow drainage not only at the membrane level but at
the surface as well. Such drains retain heat and are seldom blocked by ice. Whenever possible,
drains should daylight to the surface (figure 3-2). However, if pedestrian traffic or vandalism
concerns necessitate hidden drains the pavers should be marked so that the drains can be
inspected periodically. Wall and curb flashings can also be protected by placing extruded
polystyrene (XEPS) foam insulation over them and using metal counterflashing to hold the foam
board in place (figure 3-3).
l. Historical Roof Restoration. PMRs are used on low-slope buildings where appearance is
not critical, where a plaza deck is desired, or where important functions below warrant an
improved roofing system.
m. Aesthetics. Ballast and pavers are appealing in pattern and texture. Interlocking pavers
are generally off-white. Heavy pavers give a plaza deck appearance. PMRs are used in urban
landscaping. If vegetation or trees are desired, these must be installed in specially drained and
flashed planters. Their weight suggests that they be placed directly over structural supports. If
exposed metal strapping is used, use non-rusting strapping and fasteners.
n. Gaps, Flashings, Joints, and Sealants. Flashing details are determined by the
membrane system selected. Caution must be taken to leave a gap between horizontal pavers
and vertical flashings to avoid puncture while placing the paver. Cants are prone to such
damage. Since organic solvents in sealants attack polystyrene foam, mastics and caulks should
be allowed to cure before they contact the foam.
o. Drainage, Valley and Intersection Details. Kerfs and drainage channels should run
parallel to the drainage paths. In valleys and roof edges truncated interlocking pavers do not
interlock. They should be overlaid with additional high-density pavers (approximately 98 kg/m2
[20 psf]) or strapped to avoid wind problems.