Dow Protected Membrane Roof EN - Manual (Page 18)

®™Trademark of The Dow Chemical Company (“Dow”) or an affiliated company of Dow
C
O
N
D
I
T
I
O
N
S
,
I
S
S
U
E
S
A
N
D
R
A
T
I
N
G
S
–
S
p
e
c
i
a
l
C
o
n
d
i
t
i
o
n
s
a
n
d
I
s
s
u
e
s
1 7
Green Roof Design
THE ISSUE
Can PMR assemblies be used
for “green roof” designs?
DISCUSSION
In a “green roof” design, the
ballast in a PMR assembly is
essentially replaced with green
material – usually soil and
plantings – plus a drainage layer
directly on top of the insulation
(Figure 6). Replacing conven-
tional ballast with vegetation can
limit storm water runoff and, by
filtering the runoff through the
plants, also improve the quality
of the runoff. The plantings not
only ballast the insulation, they
can, depending on the configu-
ration, also add additional
R-value to the roof assembly.
Green roofs provide habitat for
insects and other wildlife and
often are considered in buildings
applying for LEED
†† (Leadership
in Energy & Environmental
Design) certification.
Many materials may be suitable
as ballast, provided they are
compatible with the insulation,
prevent flotation, shield ultra-
violet light and provide a Class
A fire-resistant roof finish.
The roof structure must also be
designed to accommodate the
dead load from the additional
weight of the plantings (including
when they are fully saturated by
rainfall and covered in several
feet of snow), plus any live load
from traffic, if applicable. It is
also important to design the
roof slope and drainage system
to accommodate rain runoff.
Figure 6: Green Roof Design
PMR assemblies are ideal for
green roof designs:
• The membrane is protected
under the insulation.
• Because STYROFOAM ™ extrud-
ed polystyrene products come
in a range of compressive
strengths, the insulation layer
can be designed to withstand
the higher dead loads.
• STYROFOAM insulation is
proven to outperform in a
moist environment.
• STYROFOAM insulation has a
high modulus of elasticity,
allowing it to perform under
long-term live or cycle loading.
Maximum recommended
dynamic (live) load is 1/10 of
the rated compressive strength
for 1,000,000 repetitions to
address creep and fatigue
guidelines.
Typically, a drainage layer is
placed over the insulation to
direct runoff to the drains, as
well as keep the top surface of
the insulation “diffusion open.”
(See “Moisture Absorption” on
page 16 for details.) This
drainage layer usually includes a
fabric over the insulation to pro-
tect the joints and keep them
open for drainage. Any stone
used for this drainage layer must
be clean and have a low per-
centage of fines. In some cases,
a drainage mat combined with a
filter fabric has also been used
successfully to create the neces-
sary air space.
For additional information on
green roof design, see:
Design Guidelines for Green Roofs,
by Steven Peck and Monica
Kuhn, B.E.S., B. Arch., OAA, an
OAA and CMHC publication,
available at http://www.cmhc-
schl.gc.ca
CONCLUSION
PMR assemblies are ideally
suited to green roof designs.
Vegetation
Soil
System Filters
Filter Fabric
STYROFOAM™
Extruded
Polystyrene
Insulation
Membrane
Deck
†† Trademark of the U.S. Green Building Council
Conditions, Issues and Ratings
S P E C I A L C O N D I T I O N S A N D I S S U E S