Technical Paper No 2

Multi-Ply Roofing: LIFECYCLE COST AND PERFORMANCE FACTORS

Technical Paper No. 2

October 2014

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.

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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

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

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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).

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.

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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.