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 MIL-HDBK-1001/5A
percolation course, well-drained insulation, and smooth-finished
surface.
b) Locate roof drains where they will not freeze. The
first few feet of pipe from drains must be thermally protected in
cold regions. Provide weep holes in drain at each sub-assembly
level.
c) Consider using elevated scuppers in case drains
become blocked.
d) In geographical areas subject to intense rainfall,
overflow drains should be considered. Overflow drains are
smaller in diameter than regular drains. They are placed
upstream 10 to 15 feet (3048 to 6096 millimeters) and terminate
at the top of the insulation, or percolation course, if included,
with weep holes at the membrane level. The drain cage must be
fine enough to prevent infiltration by percolation materials.
8.3.2
Percolation Course. The optional percolation course is
designed to allow easy and free access of water to the drains and
the insulation above to dry. This rock course should be graded
1-1/2- to 2-inch (38- to 51-millimeter) maximum size. It must be
rescreened before sending to the roof as fines develop in
transit.
8.3.3
Flashings. Flashings for a PMR are essentially the
same as for conventional roofs. They are, however, more
vulnerable to damage, especially at cants adjacent to pavers.
They should be carried a minimum of 8 inches (203 millimeters)
above the roof's surface.
8.4
Variations. The insulation may be placed directly on
the membrane, and the percolation course can be placed directly
on the insulation with its filter fabric. The percolation course
then serves as a ballasted finish.
8.5
Other Considerations
8.5.1
Single-ply or Liquid Membrane. A four-ply fiberglass
built-up system set in hot bitumen or a modified bitumen system
are the most common membranes used in a PMR. However, a single-
ply or liquid membrane may also be used.
8.5.2
Expansion Joint. In flush joints, never drape membrane
materials in or over an expansion joint. Use preformed joint
formers.
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