Dow Protected Membrane Roof EN - Manual (Page 11)

C
O
M
P
O
N
E
N
T
S
–
B
a
l
l
a
s
t
1 1
Components
B A L L A S T
Specification
Sound, hard, washed stone or
gravel, freeze-thaw resistant,
with a 1" (25 mm) clear size, free
of fines or stones smaller than
5/8" (15 mm) or larger than 1-3/8"
(35 mm). Most stones should
be 3/4" to 1-1/4" (19 mm to
32 mm) size.
PMR (without fabric): Spread
stone ballast uniformly over
installed insulation to provide
minimum weight or thickness
(Table 2).
PMR (with fabric): Spread
stone ballast uniformly over
installed fabric and insulation to
provide minimum weight of 10
lb/ft
2 (50 kg/m 2 ) or 1-1/2" (38 mm)
thickness.
Note: Use either weight or thickness,
but not both.
Spread additional ballast
around the roof perimeter for a
width of 4' (1.2 m) to increase
ballast weight or thickness. Refer
to Table 2 for either weight or
thickness, depending on the
thickness of insulation installed.
Spread additional ballast around
any penetration for a width of 2'
(0.6 m) if the penetration is
greater than 4' (1.2 m) in any
direction.
Installation Notes
Make sure that proper provi-
sions have been specified to seal
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 adequate
cover to protect the insulation
from UV light where a fabric is
not used.
Store ballast in dry, protected
areas and protect it with breath-
able tarps on top and side surfaces.
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" (50 mm) from the top of
the ballast.
Supporting
Documentation
National Research Council of
Canada report by Kind and
Wardlaw. Report on PMR assemblies
using a 30' x 30' (9 m x 9 m)
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 mechani-
cally 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)