2. Roof Deck
Insulation
Cover Board
Base Sheet
Granulated Cap Sheet
In engineering, redundancy is the principle of
duplicating critical components to increase
system reliability. In roofing, multi-ply
systems of modified bitumen provide a similar
duplication of critical components, resulting
in longer service life and return on investment
(ROI) for the building owner.
A second inherent benefit of multi-ply systems
stems from the first: multiple plies mean that each
membrane’s properties can be specialized to
perform its task in an integrated system custom-
built for a particular building’s needs, climate, jobsite
conditions, and local installation practices.
Multi-Ply System Components
The core of a modified bitumen multi-ply system is
a minimum of two waterproof sheets – a cap sheet
and a base sheet – applied over a cover board,
insulation, and the roof deck (see diagram below).
Unlike single-ply membranes that provide only a
single line of defense, multi-ply systems offer more
than one. In addition, modified bitumen membranes
MULTI-PLY ROOFING:
REDUNDANCY AND VALUE
are typically thicker (up to 300 mils) than single-
ply membranes (up to 95 mils) and they withstand
weathering without losing their thickness. In roofing,
thickness is good.
Components of Modified Bitumen Membranes
Although bitumen is a near synonym for asphalt,
multi-ply modified bitumen roofs are a significant
technical advance over the traditional asphalt built-
up roof (BUR) that is constructed in the field. The
properties of the bitumen have been enhanced
by the addition of polymers and reinforcement
during the manufacturing process, which creates a
prefabricated roll product.
Polymers – Modifying bitumen with polymers is a
technology originally developed in the 1960s
in Europe. Its use and manufacturing have since
spread around the world. There are two polymers
used to modify the bitumen membrane during
manufacturing: APP (atactic polypropylene), which
has qualities like a durable plastic, or SBS (styrene
butadiene styrene), which has rubber-like qualities
of flexibility.
Granulated Cap Sheet
Base Sheet
The Multi-Ply System
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.
2
Cover Board
Insulation
Roof Deck
continued
3. Reinforcement – Modified bitumen membranes
are reinforced with matting of fiberglass, polyester,
or a combination of the two. Fiberglass improves
dimensional stability, while polyester improves
puncture resistance.
To optimize performance and service life, base
and cap sheets in modified bitumen systems can
consist of different combinations of polymers and
reinforcement. The sheets can also be manufactured
with flame inhibitors that provide a fire rating.
Color Considerations for Look and Climate
Modified bitumen cap sheets with a colored
granular surface offer various options for reflectivity
and color matching.
For buildings where the roof is visible, such as a
lower section of roof visible to occupants on upper
floors, granules can be specified with a color that
complements the building.
In warmer climate zones where cool roofs reduce
a building’s air conditioning loads and energy
consumption, granules with white or light colors can
be specified to increase reflectivity. Dark colors are
available for buildings in northern climates in order to
reduce heating loads.
• APP (atactic polypropylene): A thermoplastic
material with very high resistance to heat
and ultraviolet light. Modified bitumen sheet
reinforced with APP is typically applied by
heat-welding with propane torches.
• SBS (styrene butadiene styrene): A rubber-like,
elastomeric material with excellent low-
temperature flexibility. It can be applied by
heat-welding with propane torches or hot-air
guns, mopping with hot asphalt, or using
cold-process adhesives or peel-back self-
adhesives on the membrane.
POLYMER MODIFIERS FOR
BITUMEN SHEETS
TPO
Cover Board
Insulation
Roof Deck
The Single-Ply System
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.
3
Selecting an Attachment Method
Modified bitumen rolls can be attached by four
different methods (two heat processes and two
cold), allowing building owners to select the method
that best suits the facility, field conditions, and local
contractor practices.
continued
4. In the heat-welding process, contractors apply and seam
the membranes with a heat gun or a propane torch.
Like a conventional BUR system, modified bitumen
can also be applied by mopping with hot asphalt.
The two cold processes, which have the benefit of
avoiding the smell of hot asphalt, involve spraying an
adhesive over the substrate or using a membrane
with a self-adhesive backing and a peel-off release
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.
4
paper. Self-adhesive bitumen membranes were
pioneered by Xtraflex and its ADESO technology.
Options for the Surface of Cap Sheets
The cap sheets of multi-ply systems are available
with various surfaces to match the needs of
particular buildings. For slip resistance, a granular
surface can be specified rather than a smooth
surface. The granules also supply greater resistance
to hail, heat and ultraviolet light. Coatings and films
are also available to increase resistance to punctures
and weathering. Metal-surfaced or granule-surfaced
sheets can be used as flashings, creating multi-ply
redundancy where leaks and problems are most
likely to occur.
The Multiple Benefits of a Multi-Ply System
The inherent depth and redundancy of multi-ply
systems supply a number of benefits that cannot be
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 incorporating vapor retarders, tapered
insulation, and multiple layers of insulation
because most modified bitumen systems are
fully adhered
• slip resistance with granule and gravel surfaces
• ease of repair and maintenance
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.
Modified bitumen multi-ply systems provide several layers of defense, unlike
single-ply membranes. When properly installed, mod bit membranes have
a stable thickness while single-ply membranes have annual thickness loss
due to weathering.
MODIFIED BITUMEN VS. SINGLE-PLY MEMBRANES
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
Color Options 11 Polyglass colors 1*
Available Surface Smooth, Granular & Film Smooth
Available Warranties 20+ Years 20+ Years
Application Method
Self-Adhered, Heat-Welded,
Cold-Applied, Hot Mopped
Fully Adhered, Ballasted,
Mechanically-Fastened
Highly Reflective Surface Yes Yes
Hail Resistant Yes Yes
Wind Rated Yes Yes
Fire Approvals Yes Yes
UL Approvals Yes Yes
*EPDM is typically black, PVC & TPO are typically white
