Multi-Ply Roofing:
REDUNDANCY AND VALUE
Technical Paper No. 1
May 2014
MULTI-PLY ROOFING:
REDUNDANCY AND VALUE
In engineering, redundancy is the principle of
are typically thicker (up to 300 mils) than single-
duplicating critical components to increase
ply membranes (up to 95 mils) and they withstand
system reliability. In roofing, multi-ply
weathering without losing their thickness. In roofing,
systems of modified bitumen provide a similar
thickness is good.
duplication of critical components, resulting
in longer service life and return on investment
Components of Modified Bitumen Membranes
(ROI) for the building owner.
Although bitumen is a near synonym for asphalt,
multi-ply modified bitumen roofs are a significant
A second inherent benefit of multi-ply systems
technical advance over the traditional asphalt built-
stems from the first: multiple plies mean that each
up roof (BUR) that is constructed in the field. The
membrane’s properties can be specialized to
properties of the bitumen have been enhanced
perform its task in an integrated system custom-
by the addition of polymers and reinforcement
built for a particular building’s needs, climate, jobsite
during the manufacturing process, which creates a
conditions, and local installation practices.
prefabricated roll product.
Multi-Ply System Components
Polymers – Modifying bitumen with polymers is a
The core of a modified bitumen multi-ply system is
technology originally developed in the 1960s a minimum of two waterproof sheets – a cap sheet
in Europe. Its use and manufacturing have since
and a base sheet – applied over a cover board,
spread around the world. There are two polymers
insulation, and the roof deck (see diagram below).
used to modify the bitumen membrane during
manufacturing: APP (atactic polypropylene), which
Unlike single-ply membranes that provide only a
has qualities like a durable plastic, or SBS (styrene
single line of defense, multi-ply systems offer more
butadiene styrene), which has rubber-like qualities
than one. In addition, modified bitumen membranes
of flexibility.
continued
The Multi-Ply System
Granulated
Sheet
GranulatedCap
Cap
Sheet
Base
BaseSheet
Sheet
Cover Board
Cover
Board
Insulation
Insulation
Roof Deck
Roof
Deck
The multi-ply has a minimum of two plies consisting of a base sheet and a cover sheet. The seams of each ply are installed so that they are not on top
of each other. Granular coatings on cap sheets offer additional resistance to punctures, heat and ultraviolet light as well as provide slip resistance.
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Reinforcement – Modified bitumen membranes
Selecting an Attachment Method
are reinforced with matting of fiberglass, polyester,
Modified bitumen rolls can be attached by four
or a combination of the two. Fiberglass improves
different methods (two heat processes and two
dimensional stability, while polyester improves
cold), allowing building owners to select the method
puncture resistance.
that best suits the facility, field conditions, and local
contractor practices.
To optimize performance and service life, base
continued
and cap sheets in modified bitumen systems can
consist of different combinations of polymers and
reinforcement. The sheets can also be manufactured
POLYMER MODIFIERS FOR
BITUMEN SHEETS
with flame inhibitors that provide a fire rating.
Color Considerations for Look and Climate
• APP (atactic polypropylene): A thermoplastic
Modified bitumen cap sheets with a colored
material with very high resistance to heat
granular surface offer various options for reflectivity
and ultraviolet light. Modified bitumen sheet
and color matching.
reinforced with APP is typically applied by
heat-welding with propane torches.
For buildings where the roof is visible, such as a
lower section of roof visible to occupants on upper
• SBS (styrene butadiene styrene): A rubber-like,
floors, granules can be specified with a color that
elastomeric material with excellent low-
complements the building.
temperature flexibility. It can be applied by
heat-welding with propane torches or hot-air
In warmer climate zones where cool roofs reduce
guns, mopping with hot asphalt, or using
a building’s air conditioning loads and energy
cold-process adhesives or peel-back self-
consumption, granules with white or light colors can
adhesives on the membrane.
be specified to increase reflectivity. Dark colors are
available for buildings in northern climates in order to
reduce heating loads.
The Single-Ply System
TPO
Cover Board
Insulation
Roof Deck
A single-ply system, whether manufactured from TPO (shown here), PVC or EPDM, offers only one membrane of limited thickness to defend against
weathering and punctures. With a single line of defense, any installation deficiencies in the field are more likely to lead to leaks and other problems.
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In the heat-welding process, contractors apply and seam
paper. Self-adhesive bitumen membranes were
the membranes with a heat gun or a propane torch.
pioneered by Xtraflex and its ADESO technology.
Like a conventional BUR system, modified bitumen
can also be applied by mopping with hot asphalt.
Options for the Surface of Cap Sheets
The cap sheets of multi-ply systems are available
The two cold processes, which have the benefit of
with various surfaces to match the needs of
avoiding the smell of hot asphalt, involve spraying an
particular buildings. For slip resistance, a granular
adhesive over the substrate or using a membrane
surface can be specified rather than a smooth
with a self-adhesive backing and a peel-off release
surface. The granules also supply greater resistance
to hail, heat and ultraviolet light. Coatings and films
are also available to increase resistance to punctures
MODIFIED BITUMEN VS. SINGLE-PLY MEMBRANES
and weathering. Metal-surfaced or granule-surfaced
sheets can be used as flashings, creating multi-ply
Modified bitumen multi-ply systems provide several layers of defense, unlike
redundancy where leaks and problems are most
single-ply membranes. When properly installed, mod bit membranes have
likely to occur.
a stable thickness while single-ply membranes have annual thickness loss
The Multiple Benefits of a Multi-Ply System
due to weathering.
The inherent depth and redundancy of multi-ply
systems supply a number of benefits that cannot be
XTRAFLEX
Modified Bitumen
Single Ply
EPDM • TPO • PVC
Layers of Roof Membrane
2 or more
1
Climate Regions
Unlimited
Limited
Typical Thickness
Up to 300 mils
40 - 95 mils
11 Polyglass colors
1*
Smooth, Granular & Film
Smooth
Available Warranties
20+ Years
20+ Years
• ease of incorporating vapor retarders, tapered
insulation, and multiple layers of insulation
because most modified bitumen systems are
fully adhered
Application Method
Self-Adhered, Heat-Welded,
Cold-Applied, Hot Mopped
Fully Adhered, Ballasted,
Mechanically-Fastened
• slip resistance with granule and gravel surfaces
Highly Reflective Surface
Yes
Yes
Hail Resistant
Yes
Yes
Color Options
Available Surface
duplicated by single-ply systems. These include:
• greater strength
• greater resistance to punctures, hail, tears, and
abrasion
• less vulnerability to performance issues as a result
of installation deficiencies in the field
• thicker membranes with additional waterproofing
• excellent resistance to wind uplift
• multiple plies at flashings and critical locations
where most leaks occur
• ease of repair and maintenance
The extended service life and value of multi-ply
modified bitumen systems make the return on
Wind Rated
Yes
Yes
Fire Approvals
Yes
Yes
UL Approvals
Yes
Yes
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.
*EPDM is typically black, PVC & TPO are typically white
In the 1960s Polyglass became a leading manufacturer in Europe of modified bitumen roofing membranes.
The company’s technology and products were introduced to the U.S. in 1992.
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