General Description

Modified bitumen sheets are composed primarily of polymer modified bitumen reinforced with one or more plies of fabric such as polyester, glass fiber or a combination of both. Assembled in factories using high quality control standards, modified bitumen sheets are made to have uniform thickness and are manufactured to have consistent physical properties throughout the sheet.

The finished roofing membrane may consist of one or more modified bitumen sheets, or it may be comprised of a combination of built-up roofing (BUR) felts and one or more modified bitumen sheets. The type of substrate and the performance objectives influence the specification of the modified bitumen membrane system.

There are modified bitumen membranes tailored to almost every type of construction design and climate: for new roofing or reroofing of commercial buildings, residential high rises, domes, spires, and most categories of low-slope or steep-roof roofing. Thus there are a variety of modifiers, and types of reinforcements and surfacings available.


Asphalt used in manufacturing modified bitumen systems is generally modified with one of two modifiers, Atactic Polypropylene (APP) or Styrene Butadiene Styrene (SBS). These modifiers create a uniform matrix that enhances the physical properties of the asphalt.

APP is a thermoplastic polymer that forms a uniform matrix within the bitumen. This matrix increases the bitumen's resistance to ultra-violet light, increases its flexibility at high and low temperatures, and increases it's ability to resist water penetration.

SBS modified membranes resist water penetration and they stretch and recover well even at cold temperatures. This makes them particularly applicable where there may be movement or deflection of the underlying deck.

Reinforcing Plies

While modified asphalt provides the primary waterproofing characteristics of these membranes a reinforcing ply adds strength and puncture resistance. Glass fiber and polyester plies are the most commonly used reinforcing fabrics. Each has distinctive properties. Polyester has excellent elongation, tensile strength and recovery. It provides good puncture resistance and stands up well to foot traffic. Glass fiber resists flame penetration and provides excellent tensile strength and dimensional stability.

What do I have to Know?

The combination of asphalt, modifiers and reinforcements determines the characteristics of a specific modified bitumen membrane. To obtain the best roofing system possible, the designer, building owner and contractor should understand the dynamics of the roof as well as the roofing system when specifying either APP or SBS modified roofs.

The architect, specifier, building owner and roofing contractor should examine the following parameters when choosing a roofing system:


The application of modified bituminous roofing is a construction process that involves the skillful arrangement of several components as specified for the process. Roof performance is determined by factors that include building design, job specifications, materials quality and suitability, application procedures and maintenance. The level of quality in the workmanship during the application process is critical.

The application of modified bitumen roof systems is not an exact science. It's a craft that involves people, roofing professionals, who deal with a broad range of materials, designs, customs, techniques, weather events, and climates. ARMA recognizes the importance of these critical factors as they affect the quality of modified bitumen roofing. These factors are controlled by applicable inspection and review procedures carried out during the development, manufacturing, production, and application stages.

For More Information on Modified Bitumen Roofing Systems

For ARMA publications on Modified Bitumen Roofing Systems, go directly to the section of this website on Publications.

For specific information on Modified Bitumen Roofing Systems click on any of the following quality manufacturers of these systems:

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