Techanical Paper No 1

Multi-Ply Roofing: REDUNDANCY AND VALUE

Technical Paper No. 1

May 2014

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.

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Cover Board

Insulation

Roof Deck

continued

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.

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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

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.

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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

XTRAFLEXModified Bitumen

Single PlyEPDM • 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