Dow Protected Membrane Roof EN - Manual (Page 19)

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1 8
Finding Leaks
in a PMR
THE ISSUE
Is it more difficult to locate a
leak with a PMR or conventional
roof assembly?
DISCUSSION
Building upon years of in-field
experience, the majority of roof
leaks in PMR systems occur at
flashing as opposed to the inte-
rior field area. The field area is
protected from physical abuse,
UV attack and thermal cycling –
all factors that are the primary
causes of roof failures – by both
the insulation and ballast over
the membrane. However, some-
times interior field leaks do
occur.
Concrete decks: For PMR
installations on concrete decks,
generally the membrane is fully
adhered to the deck. This simpli-
fies leak detection because the
leak is localized. For example,
the leak in the interior will be
exactly where the hole in the
membrane is located. If the
membrane is not adhered, the
water can run under the mem-
brane for many feet before
entering the building – just like
in a conventional roof.
Steel decks: For PMR installa-
tions on steel decks, a layer of
insulation or other substrate
(e.g., drywall) is placed first to
provide a base for the mem-
brane – exactly the same as in a
conventional roof. The same
type of leak detection effort is
required for both PMR and con-
ventional roofs on steel decks.
Wood decks: On wood decks
with a PMR installation, the
membrane is typically a felt
layer and two or three plies
mopped on top. In a conven-
tional installation, the insulation
is fastened to the deck and then
the membrane is applied. Both
of these approaches will allow
the water to run to the deck
joints prior to entering the
building.
CONCLUSION
Not only do PMR assemblies
have fewer leaks in the first
place, PMR assemblies over
concrete decks with bonded
membranes have definite advan-
tages when isolating any leaks
that do occur. Both conventional
and PMR roofs over steel or
wood decks require the same
leak detection strategies. In
addition, because PMR roofs are
easier to repair and typically all
of the original materials can be
reused (ballast and insulation),
this environmentally friendly
feature can save money.
Low Temperature
Applications
THE ISSUE
PMR assemblies should
not be used in low temperature
applications because of the
potential adverse effect on the
STYROFOAM
™ extruded poly-
styrene insulation.
DISCUSSION
In a low temperature applica-
tion (e.g., freezers), the interior
space has a low temperature and
low water vapor pressure
(humidity). In contrast, the
warm outside temperature and
higher water vapor pressure
causes a vapor drive toward the
interior space. Unless addressed,
this vapor can condense in the
insulation and lower the R-value
of the system. It can also con-
dense on the membrane and
freeze, gradually forming a layer
of thick ice.
Typically in low temperature
applications, the membrane is
placed on the “warm side” – or
the exterior in a conventional
roofing application.
CONCLUSION
In low temperature applications
(e.g., freezers), a conventional
roof may offer performance
benefits.
High Temperature
Installation
THE ISSUE
In high temperature locations,
PMR assemblies should not
be covered with a dark fabric
prior to laying the ballast
because of the potential adverse
effect on the STYROFOAM
™
extruded polystyrene insulation.
DISCUSSION
Like many insulations, higher
temperatures may cause permanent
distortion and/or long-term
creep. The maximum use
temperature for STYROFOAM
™
extruded polystyrene insulation
is 165°F for continuous use,
with short-term exposure up to
190°F.
Typically, this concern arises
in warmer locations (e.g., south-
ern U.S.) when STYROFOAM
insulation is placed underneath
a dark fabric prior to laying the
ballast. Given the right condi-
tions, the temperature on the
top of the insulation may reach
close to the upper limits for
polystyrene insulation and cause
some distortion. Experience has
shown that when the STYROFOAM
insulation is exposed to both
direct sunlight and an outdoor
air temperature over 90°F, distor-
tion of the foam can occur in as
little as 30 minutes when a
heavy fabric is over the insulation.
To prevent this phenomenon
during hot weather, temporarily
place white opaque polyethylene
film on the fabric until the
ballast is laid.
CONCLUSION
In high temperature locations,
the temporary use of white
opaque polyethylene film laid
on the fabric until the ballast is
laid will prevent any distortion
of the insulation.
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