Modified Bitumen Roofing Systems

Tom Irvine

In selecting a new or replacement roof membrane system, there is a long list of products from which to choose. One classification of products that is worthy of consideration is the modified bitumens.

Modified bitumen membranes were originally developed in Europe, where they have enjoyed widespread success since the early 1970s. Over the past 10 years, modifieds have continued to grow in popularity in the U.S., and are now according to many industry professionals, the fastest growing segment of the commercial roofing market. In addition to exclusive manufacturers of modified bitumens, most built-up roofing manufacturers now also market a line of these products.

Modified bitumen technology developed out of attempts to widen the serviceability range of conventional oxidized roofing asphalt. Increased resistance to brittleness at cold temperatures, greater flow resistance at high temperatures, and increased elasticity can be achieved by blending synthetic rubberized polymers with asphalt in lieu of modification by blowing processes (oxidation). Another distinct advantage is that polymer modification does not prematurely age the asphalt, as does oxidation. Therefore, an increase in service life is also achieved.

In the manufacture of modified bitumen membranes, one or more reinforcing plies, typically composed of fiberglass, polyester, or fiberglass/polyester composite, are encapsulated in polymer modified asphalt of varying weights. Sheets may then be surfaced with mineral granules or metal foils, or remain unsurfaced. The result is a membrane that combines the multi-layer benefits of a built-up roof with the installation advantages of a single-ply membrane.

Although a variety of polymer modified asphalt membranes are being introduced as technology advances, two formulations have been proven performers over time, and comprise the vast majority of products available.

SBS (Styrene-butadiene-styrene) formulations perform well in cold weather applications, possess superior elongation and recovery properties, and are quite versatile in their application in that they can be mopped in asphalt, torched, or installed with cold adhesives. Mineral granules and metal foils are generally used in surfacing this type of membrane, as resistance to ultraviolet light is somewhat limited.

APP (Atactic polypropylene) products are more suited to application in hot weather, are resistant to weathering and aging, and are more resistant to ultraviolet radiation than are the SBS membranes. The method of application is generally limited to torching. However, cold adhesive applications have been introduced by a few manufacturers. These membranes are generally manufactured without a surfacing, although granule surfaced products are available. We recommend that protective coatings be installed and maintained on the unsurfaced sheets to provide additional ultraviolet resistance and to achieve the required fire rating.

Neither APP or SBS membranes are well suited to application in ponded conditions. Furthermore, their effectiveness is limited in recover situations where latent moisture in the original roof system generally exists.

Perhaps the greatest advantage to modified bitumens is their versatility. These products can be torched, mopped, or cold applied, depending on the limitations of the project. Modifieds can be used alone in one or two-ply applications, or with conventional asphalt built-up roof systems as a cap sheet and/or flashing material. They are also an excellent material for repairing existing bituminous roofs. In addition, they are installed and repaired by a large contractor base using application methods quite similar to conventional built-up applications.

Although many types of roof membranes should be considered in selection of a new or replacement system, the flexibility and proven performance of modifieds will likely make them a contender in many situations.