2. Among major building systems, roofing is something
of an anomaly.
Although roofing systems consist of some
manufactured components, they are assembled and
installed outdoors in the field, where critical variables
cannot be controlled as precisely as they are in an
indoor manufacturing facility. Roofing materials must
be transported to a site, stored in a staging area,
and then attached in widely varying temperature and
moisture conditions.
And although roofs are a major element in a
building’s initial and lifetime costs, they garner little
respect or attention from many building occupants
and owners. They are supposed to do their job with
little fuss or maintenance. People are not as aware
of roofing systems as they are of building systems
like glazing, lighting, and HVAC because they
interact regularly with the latter.
But there is more to roofing than meets most eyes.
Its performance – in terms of expansion, contraction,
MULTI-PLY ROOFING:
LIFECYCLE COST AND PERFORMANCE FACTORS
weathering, repairs, penetrations, insulation value,
and resistance to moisture and damage – is the
result of complex interactions over time among
system components. Roofs don’t stand still. They
respond to conditions above, on, and below the
roof membrane.
The principle of multi-ply redundancy provides
building owners with a way to manage these
performance factors, extend roof service life, and
reap the greatest return on their investment.
The Built-In Redundancy of Built-Up Multi-Plies
The core of a modified bitumen multi-ply system
is redundancy – a minimum of two waterproof
sheets founded on a cap sheet and a base sheet.
Multiple plies offer the clear advantage of greater
overall thickness. In addition, each ply in a modified
bitumen multi-ply system is typically thicker (up to
300 mils) than a single-ply system’s membrane (up
to 95 mils).However, the benefits of multiple plies are
greater than the sum of their thicknesses due to the
ways in which each ply works with the others.
2
continued
Granulated Cap Sheet The cap sheet is
the first line of defense against punctures, heat,
ultraviolet light, and foot traffic
Base Sheet This sheet is a second line of
defense with seams in different positions than those
of the cap sheet
Extended Service Life of the Multi-Ply System
Cover Board The dimensional stability
of a sturdy cover board enhances service life
Insulation Insulation plays an important
role in energy consumption
Roof Deck
3. To maximize overall system performance and
service life, the base and cap sheets in modified
bitumen systems can be customized with different
combinations of polymers and reinforcement
for different facilities and climates. The polymers
represent a significant technical advance over
traditional built-up roofs (BURs).
As noted by the U.S. military’s Unified Facility Criteria
assessment of modified bitumen roofing systems,
“The polymers used to modify the asphalt bitumen
improve the performance characteristics of the
asphalt. The addition of polymers increases low
temperature flexibility and high temperature stability.
MB membranes are reinforced with fiberglass,
polyester, or a combination of both.” (See box
below for more guidelines from the Department of
Defense’s Unified Facilities Criteria program.)
Critical Factors for Extended Service Life
The major elements that age all roof membranes
from above are ultraviolet light and heat. Although the
two share an obvious relationship, they are different
factors that cause different weathering characteristics
and chemical reactions. Depending on a membrane’s
3
chemical formula, it will be subject to photo-oxidation,
shrinking, loss of flexibility, and chalking.
The modified bitumen’s cap sheet is the first line of
defense against heat and ultraviolet light. With a light-
colored granular surface, the cap sheet reflects light
and reduces heat, decreasing air conditioning loads.
In northern climates, various colors of dark granules
can be specified in order to absorb light and reduce
indoor heating loads.
On the surface of the roof, the granules of the cap
sheet also provide protection against hail and foot
traffic. Mechanical contractors are frequent visitors
to many rooftops for maintenance and installation of
new equipment whenever a facility’s needs change.
Thin-film and PV solar arrays also require rooftop
traffic from contractors with tools and materials
that have sharp metal edges – not to mention the
contractors’ possible indifference to the integrity of
the owner’s roofing system.
In terms of punctures and tears, the multi-ply’s
base sheet provides a second line of defense.
Blistering heat and
ultraviolet light on the
rooftop make these
elements the major
threats to a roofing
system’s service life.
continued
DEPARTMENT OF DEFENSE FOCUSES ON EXTENDED ROOF SERVICE LIFE
The Department of Defense’s Unified Facilities Criteria (UFC) program supports the military’s facilities
and infrastructure component. UFC documents provide planning, design, construction, restoration, and
modernization criteria. The UFC program is administered by the U.S. Army Corps of Engineers, the Naval
Facilities Engineering Command, and the Air Force Civil Engineer Support Agency.
The UFC report on roofing prescribes materials and methods based on the military’s experience with
many thousands of buildings and all roofing types. UFC documents emphasize the military’s need for
extended service life and low maintenance. According to the report, asphalt built-up roofs (BUR) have
the attributes of “durability with long service life, low maintenance, and well-understood maintenance
procedures.” The report goes on to note that the modified bitumen BUR “must be considered when long
service life is required” and that “the allowable types of top surfacing for BUR membranes are granulated
modified bitumen cap sheet and aggregate.”
The latest version of the roofing criteria document, UFC 3-110-03, is available to the public online at the
Whole Building Design Guide (www.wbdg.org).
4. If surface degradation of the cap sheet occurs,
the unexposed base sheet continues to provide
protection and waterproofing.
The redundancy of the multi-ply system is a critical
consideration for the growing number of rooftop
solar applications. The service life of solar
equipment can be 20 years or more, and that life
must be met or exceeded by the roofing system
in order to protect the owner’s solar investment.
4
Redundancy also counteracts any deficiencies
that may occur during installation of components
in the field.
If moisture infiltrates the insulation and cover boards,
then trouble begins from below. If these components
lose their dimensional stability, system delamination
and other problems ensue. However, the
redundancy of multi-ply systems means that routine
maintenance is more likely to reveal potential leaks
before they affect the layers below the membrane.
Critical Factors for Repair and Maintenance
According to the old joke, roof maintenance –
especially while a roof is still under warranty –
consists of waiting for a leak. While that approach is
not a best practice for preventive maintenance, any
roof will eventually age and develop problems even
with a regular maintenance program. However, the
options for repair and replacement are not the same
with all systems. The time available to do repair and
replacement also varies with different systems.
When a problem is detected on the surface of a
multi-ply’s cap sheet, it is easy to come back and
repair the asphalt base material without doing
extensive prep work and before the problem extends
below the base sheet. In addition, a multi-ply system
offers the option of adding another cap sheet to
any smooth-surfaced system, effectively making,
say, a two-ply roof into a three-ply roof and vastly
extending an already long service life. This option
can easily add 10 to 15 years to a roof’s life while
eliminating costly tear-offs, demolition, trips to the
landfill, and replacement.
When single-ply systems wear out or develop leaks,
they are subject to catastrophic failures. Without
the property of redundancy, these systems have no
second line of defense to protect insulation, cover
boards, and deck.
Far-sighted building owners who focus on lifecycle
costs profit from the value inherent in redundant,
multi-ply roofing systems.
In the field, where
critical variables like
ambient temperature,
moisture, and
contractor quality
cannot be completely
controlled, a multi-
ply’s redundancy
counteracts any
deficiencies.
Rooftop solar arrays offer vast energy savings but demand a high level
of roofing performance. Not only must the roof’s service life match the
solar equipment’s service life but the roof membrane must withstand
the stresses of installation and maintenance by personnel likely to be
indifferent to the roof’s integrity.
The extended service life and value of multi-ply modified bitumen systems make the return on investment for building owners the most competitive available. For more information
on the performance of Xtraflex multi-ply systems, consult the Product Data Sheets at www.xtraflexroofsystems.com and contact your local Xtraflex representative.
