Dow Protected Membrane Roof EN - Manual (Page 17)

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1 7
Vegetative Roof
Design
THE ISSUE
Can PMR assemblies be used
for vegetative roof designs?
DISCUSSION
In a “green roof” design, the
ballast in a PMR assembly is
essentially replaced with vegeta-
tive soil and plantings – plus a
drainage layer directly on top of
the insulation (Figure 6).
Replacing conventional 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 configura-
tion, also add additional
R-value (RSI) to the roof assembly.
Vegetative roofs provide habitat
for insects and other wildlife
and often are considered in
buildings applying for USGBC or
CAGBC Leadership in Energy
and Environmental Design
(LEED
††) 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: Vegetative Roof Design
PMR assemblies are ideal for
vegetative roof designs:
• The membrane is protected
under the insulation.
• Because STYROFOAM ™ Brand
Extruded Polystyrene Foam
Insulation comes in a range of
compressive strengths, the
insulation layer can be
designed to withstand the
higher dead loads.
• STYROFOAM ™ Brand Insulation
is proven to outperform in a
moist environment.
• STYROFOAM ™ Brand 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
vegetative 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 vegetative roof designs.
Vegetation
Soil
System Filters
Filter Fabric
STYROFOAM™
Brand Foam
Insulation
Membrane
Deck
†† The U.S. Green Building Council (USGBC), a nonprofit coalition promoting high-
performance green building design, has developed Leadership in Energy and
Environmental Design (LEED), a voluntary, consensus-based standard. The LEED
rating recognizes the life cycle costing of construction. The Canada Council
(CAGBC) LEED rating system is modeled after the USGBC LEED program.
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