Modified Bitumen

Modified Bitumen

Courtesy of the NRCA Website

Polymer-modified bitumen sheet membranes

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Material and system description

Polymer-modified bitumen or modified bitumen (MB) sheet membranes were developed in Europe in the early 1960s and have been in use in the U.S. since the mid 1970s. Polymer-modified roof membranes are composed of reinforcing fabrics that serve as carriers for the hot polymer-modified bitumen as it is manufactured into a roll material. MB roof system membranes are composed of multiple layers, much like BUR membranes. MB roof systems typically are installed as a two-ply system and almost always are fully adhered.

There are two types of MB roofing membranes:

SBS polymer-modified bitumen membranes commonly are installed in hot moppings of asphalt (similar to BUR systems) or cold adhesive. Some SBS modified membranes are self adhering; that is, they contain an adhesive backing.

APP polymer-modified bitumen membranes typically are heat-welded or torch-applied. Consumers should be cautioned that NRCA does not recommend torch-applying a modified bitumen membrane sheet directly to a wood deck.

Generally, APP modifiers impart a plasticized quality to asphalt, and SBS modifiers impart a rubberized quality to asphalt. MB membranes and EPDM. a thermoset membrane, often are confused by consumers because of colloquialisms used by roofing contractors. MB and EPDM membranes are sometimes called rubber roofs.

Surfacings for MB membranes include aggregate surfacing, mineral surfacing, metal foil-laminate surfacing and smooth liquid-applied surfacing.

A roof system composed of a built up roof membrane with 2 plies or 3 plies and a polymer-modified bitumen membrane cap sheet is commonly referred to as hybrid system. NRCA considers this type to be a polymer modified bitumen membrane system.

Material standards

NRCA does not make any recommendations about which MB products or manufacturer to use; however, NRCA does recommend that MB meet standards established by the American Society for Testing and Materials (ASTM). The following ASTM standards are applicable to APP polymer modified bitumen and SBS polymer modified bitumen products.

APP polymer modified bitumen products:

ASTM D6222, Standard Specification forAtacticВ Polypropylene (APP) Modified Bituminous Sheet Materials Using Polyester Reinforcements

ASTM D6223, Standard Specification for Atactic Polypropylene (APP) Modified Bituminous Sheet Materials Using a Combination of Polyester and Glass Fiber Reinforcements

SBS polymer modified bitumen products:

ASTM D6162, Standard Specification for Styrene Butadiene Styrene (SBS) Modified Bituminous Sheet Materials Using a Combination of Polyester and Glass Fiber Reinforcements

ASTM D6163, Standard Specification for Styrene Butadiene Styrene (SBS) Modified Bituminous Sheet Materials Using Glass Fiber Reinforcements

ASTM D6164, Standard Specification for Styrene Butadiene Styrene (SBS) Modified Bituminous Sheet Materials Using Polyester Reinforcements

ASTM D6298, Standard Specification for Fiberglass Reinforced Styrene Butadiene Styrene (SBS) Modified Bitumen Sheets with a Factory Applied Metal Surface


When purchasing a new roof system, there will be two warranties to consider. First, there will be the manufacturer’s warranty. In general, these warranties cover defects in the manufacture of the roof membrane. Please read NRCA’s consumer advisory bulletin addressing roofing warranties for more information. Once the project is complete, be sure the contractor provides you with a certificate for your records.

Second, the roofing contractor will provide you with a warranty covering his workmanship. Typically, this will cover installation and related issues. The warranty should contain what items are covered and what will void them. Many contractors offer one year or two years of coverage; however, there is no industry standard.

Modified Bitumen

Modified bitumen roofing membranes are composite sheets consisting of bitumen, modifiers and reinforcements. The term "modified bitumen" encompasses a broad range of materials, with each specific material differing from the others with respect to the modifiers and reinforcements used. Although modified bitumen and thermoplastic membranes are both softened by heat, they differ because the modified bitumen varieties are typically bonded to substrates by torch application or asphalt.

Modified bitumens (primarily asphalt) have found many uses in practical roofing applications and have maintained significant market share today thanks to modifiers like SBS (sequenced butadiene-styrene) and APP (atactic polypropylene). Manufacturers began blending SBS polymers with traditional roofing mastics to impart elastomeric characteristics for use at roofing joints that demanded higher performance to maintain watertight integrity.

The joints most effected were the pitch pocket penetrations and edge fascia metal joints because the coefficient of expansion and contraction of these details exceeded the limits of the typical fiber reinforced mastic, they began to exhibit cracks and splits that accelerated after a nominal amount of field exposure. As roofing chemistry became more sophisticated, various formulations and viscosity ranges were developed for specific uses such as: elastomeric sealants/mastics for high movement joints and terminations. These improvements for bituminous material durability and extensibility (especially at lower temperatures) were achieved by adding polymers (rubber and plastics). Reinforcements have evolved from the true rag felts to pulp, glass, and more recently, to heat-stabilized polyester blends and combinations of glass and polyester.

Modified products were developed for different reasons, safety was not the least of them. Hot bitumens were very hazardous to work with in confined areas and to bond to vertical surfaces. Now most products incorporate a polyurethane base for vertical application and have a low VOC that are compliant with air quality regulations and worker safety concerns. These materials are usually applied with a notched trowel to achieve the appropriate thickness for waterproofing integrity, and when cured, offer excellent MVT resistance and crack spanning capabilities.

To meet the needs of extensive lightweight slab construction, hot-applied modified asphalts appeared in the construction market in the mid 1970s. Unlike roofing asphalts, these materials were blends of non-blown asphalts and elastomeric polymers, primarily SBS. The advantage over conventional waterproofing configurations for on-grade waterproofing of plaza decks is that multiple layers of reinforcing felts are not needed. Unlike roof deck constructions, the concrete pours are stable. Typically, the hot-applied modified waterproofing systems are reinforced with a single layer of compatible, heat-stabilized, polyester fabric. Some Department of Transportation divisions accept hot modified bitumen materials for bridge deck repair as well as other concrete joint remediation.

The current generation of asphalt hot-melts is derived from the success of the modified product developments in the waterproofing industry. However, unlike the strict heating parameters required (oil-jacketed kettles) for hot-melt waterproofing bitumens, current hot-melt roofing adhesives are not as sensitive to heating as their predecessors. The modified bitumen adhesive can be installed with traditional means and methods, such as a roofing mop or felt-laying machine. The key to temperature flexibility is the result of the use of SBS polymers.

The unique benefit of using compatible hot-melt modified bitumens with MB felts is that all components of the system are elastomeric. Although the commonly accepted recommendation for MB ply installation is to use Type III or IV asphalt, system elongation, cold bend flex temperature, and strain energy are superior with the use of hot-melt adhesives and SBS modified plies and surfacing sheets.

Cold process roofing had its beginnings in the mid 1970s with asphalt cutbacks employed as the adhesive between coated base sheets. The application was developed for those projects where kettle locations were restricted or the fumes were highly objectionable. The materials are applied at ambient temperature or slightly heated to facilitate application through spraying. Rarely does the application temperature reach 100 F. A cold asphalt cutback is modified with calcium carbonate, a were highly objec-variety of reinforcing fibers and other thixotropes. When a typical roof cut is examined, the core appears to be homogenous. It is difficult to discern the ply lines.

Not all coated base sheets were equal when it came to cold process applications. Some roll manufacturers used "softer" asphalt in their coating process. This required extended cure intervals on the roof when used with cold-process adhesives. These systems were often very fragile to any type of foot or mechanical traffic. Often the roof bled profusely or was not able to sustain even minimal traffic for some weeks. If an acrylic or aluminum coating is specified, most manufacturers recommended a cure of at least 30 days. This would require some additional preparation in the form of washing or priming due to the accumulated surface contaminants. The more common finishing method was to broadcast ceramic granules or specific roofing aggregates into a "flood" coat of adhesive. Currently, there are engineered, coated sheets available that combine glass fibers and polyester fabrics that allow for a continuous, rapid installation. Often the coating asphalt on these ply sheets is unblown asphalt or is slightly modified to address compatibility issues with the adhesives. Cold process adhesives can be delivered in tanker trucks, refillable containers, or barrels. The material is then pumped to the roof with pneumatic or hydraulic spray delivery systems. For space-restricted projects, materials can be provided in 5-gallon pails. The materials can then be brush or squeegee-applied. On production projects, contractors often use propane-powered heat-exchange units to ratings by approved, field-applied coatings. There are torch grades of SBS membranes, but they are usually thicker than the hot-applied membranes. The additional thickness is to protect the reinforcing fabric from heat damage during installation. Polyester carriers (or reinforcements) are easily damaged by torch applications and most "Torch Grade" SBS have glass reinforcements. Most torched membranes use the APP modified, which melts at a higher temperature than the rubber modified materials. As restrictions increase with respect to open flames, kettle odors, and congested roof areas, the popularity of cold-applied systems is growing. Many manufacturers offer specially-formulated, modified bitumen roll adhesives. Unlike the solvent-actuated mechanism of traditional cold-process adhesives, SBS cold-applied systems rely on the high-performance, low, or no-solvent adhesive to bond the modified bitumen plies to the substrate and themselves.

A Torch Down Modified Bitumen roof, or as it is commonly known a 2-ply SBS roof system, is rapidly becoming the most popular roofing system in the market today. Consisting of polyester or fiberglass reinforcing mat sandwiched between two layers of rubberized asphalt, two ply roof system are durable and quick to install. Typical Modified Bitumen Roof System is composed of five basic components that can be assembled in a number of conbinations:

    Surfacing — provide UV protection and weathering surface that can be either factory or field applied, Adhesive — actual waterproofing material and bonding agent thet is either hot or cold applied, SBS or APP Modified Bitumen Membrane — a factory fabricated sheet consisting of a co-polimer bitumen typically reinforced with polyester or fiberglass, Rigid Board — either a modified or non modified base or ply sheet under a modified bitumen top, Insulation — provides R-Value and stable substrate for roof system, Flashing — provides waterproofing around roof perimeter, equipment and projection

Installers who attempt to install torched down roofing without proper training generally show their lack of experience in the work that they do, but improperly installed torch down roof will likely fail within the first two years of use. Applicator quality is one of the only sure ways to eliminate costly replacement. The quality of different two ply SBS systems varies and unlike some items, cost is not always the best measure of performance.

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