Dow Protected Membrane Roof EN - Manual (Page 12)

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Components
B A L L A S T
Design Approach
Refer to TechNote 508 for the
recommended amount and
placement of ballast. The ballast
design depends on:
• Type of membrane (adhered,
loose-laid or mechanically
attached)
• Building height
• Design wind speed
• Site exposure
• Parapet height
• Gravel stop height
Specification
ASTM D448 Gradation #2, 4, 5
or 57 washed free of fines or
stones.
Spread stone ballast uniformly
over installed insulation to
provide minimum weight or
thickness.
Spread additional ballast
around the roof perimeter for
a width of 8.5' to increase ballast
weight or thickness.
Spread additional ballast
around any penetration for
a width of 4' around any pene-
tration that exceeds 4' in any
direction.
Installation Notes
Make sure that proper provi-
sions have been specified to seal
off openings in the roof deck
and any perimeter blocks. This
will prevent air from getting
below the roofing membrane
and billowing it.
For PMR installations without
a fabric, ensure that the ballast
does not contain too many small
stones (fines not more than 10
percent of mix) as they may
work into the insulation joints or
be moved by the wind.
Conversely, too many large
stones may not provide ade-
quate cover to protect the insu-
lation from UV light where a
fabric is not used.
If ballast has been moved by
wind scour, repair is simple. Just
replace the insulation (if neces-
sary), re-lay the filter fabric and
replace the ballast. A small paver
can be added, if required.
If a gravel stop is required,
the height of the gravel stop at
a building perimeter should be
at least 2" from the top of the
ballast.
Supporting
Documentation
TechNote 508: “Ballast Design
Guide for IRMA Roofs”
National Research Council
of Canada report by Kind and
Wardlaw. Report on PMRs using
a 30' x 30' wind tunnel, with
various ASTM gradation/sizes
of ballast. (See reports NRC LTR-
LA 269, NRC LTR-LA 234, NRC
No. 15544.)
ANSI/SPRI RP-4 Wind Design
Standard for Ballasted Single-Ply
Roofing Systems (for mechanically
attached and loose-laid PMR)
ASTM D448 Standard
Classification for Sizes of
Aggregate for Road and Bridge
Construction
ANSI/ASCE 7 Minimum
Design Loads for Buildings and
Other Structures (includes Basic
Wind Speed Map)