Dow Protected Membrane Roof EN - Manual (Page 11)

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Description
Crushed stone or washed,
rounded riverbed rock, ASTM
D448 Gradation #2, 4, 5 or 57
depending on membrane type,
building height, wind zone and
parapet height (Table 2).
Depending on the ballast
design, the range of ballast is
10 to 15 lb/ft
2 with additional
ballast around perimeters and
penetrations (15 to 20 lb/ft
2 ). In
some cases, pavers can be used.
See “Pavers” on page 12 for
additional details.
Function
Prevent uplift and prevent
flotation: The amount and
placement of ballast is based on
the following considerations:
• Design wind speed – Refer to
the ANSI/ASCE 7-95 wind
speed map, contact the local
code authority for the design
wind speed for the building
location or refer to TechNote
508: “Ballast Design Guide for
IRMA Roofs”
• Roof height – Use the worst-
case elevation (e.g., from
ground level to the highest
point of the roof)
• Parapet height – Measured
from the top of the ballast to
the top of the parapet, use the
shortest parapet height in any
variation
• Membrane type – Adhered,
loose-laid or mechanically
attached
Areas of extra ballasting:
Extra ballast, required to overcome
high wind loads and restrain
insulation during heavy rain-
storms, should be considered in
the following locations:
• Perimeter edge – 8.5' wide
band running along the
perimeter edge of the roof
insulation. As an alternate to
additional ballast, 1 to 4 rows
of concrete pavers may be
installed along the perimeter
edge (see TechNote 508).
• Penetrations through the insu-
lation – 4' wide band around
any roof penetration greater
than 4' in any direction (e.g.,
skylights, equipment pads, etc.).
• Corners – Concrete pavers may
be required with steel strap-
ping and anchors for certain
designs (see TechNote 508).
See “Pavers” on page 12 for
details about concrete pavers.
• Building exposure – Consider
the surrounding terrain and its
potential effect on the overall
wind exposure (e.g., nearby
woods versus shorelines).
• Membrane type – Adhered,
mechanically fastened or fully
ballasted. For additional
details, see “All Other
Components” on page 13.
Prevent wind scouring: The
wind performance of stone bal-
lasted PMRs has been excellent.
Only a few isolated minor scour-
ing problems have occurred,
typically limited to small areas
in a corner. In these few cases,
the ballast has blown inbound
by about 4' and piled up on the
filter fabric, creating additional
weight.
Prevent UV degradation of
the insulation: Most PMR appli-
cations use a filter fabric that
typically incorporates a UV
stabilizer. However, if no fabric
is used, the insulation must be
totally covered by the ballast to
prevent UV degradation. The
quality of the ballast is very
critical in these types of applica-
tions. Too small (fines not more
than 10 percent of mix) and
the stones may work into the
insulation joints or be moved
by the wind; too large and the
ballast may not provide adequate
cover to protect from UV light.
Provide a Class A fire-resistant
roof cover: Class A roof covering,
as defined by ULC S107, ULI
790 and ASTM E108. (See “Fire
and Wind Ratings” on page 20
for additional details.) The
requirements for Class A roof
construction cover the perform-
ance of roof assemblies and roof
covering materials when
exposed to a fire originating
from sources outside a building.
The stone ballast or pavers
provide the Class A fire rating.
Standard Sizes of Coarse Aggregate (Weight % Finer Than Sieve Openings)
ASTM D448 Nominal Size 3" 2-1/2" 2" 1-1/2" 1" 3/4" 1/2" 3/8" 3/16"
Gradation Square Openings
2 2-1/2" to 1-1/2" 100% 90-100% 35-70% 0-15% 0-5%
4 1-1/2" to 3/4" 100% 90-100% 20-55% 0-15% 0-5%
5 1" to 1/2" 100% 90-100% 20-55% 0-10% 0-5%
57 1" to No. 4 100% 95-100% 25-60% 0-10%
T A B L E 2
Components
B A L L A S T