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.
Photo of a mineral-surfaced MB roof system
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.
NRCA does not make any recommendations about which MB products or manufacturer to use; however, NRCA does recommend that MB meet standards established by ASTM International.
APP polymer modified bitumen products:
ASTM D6222, "Standard Specification for Atactic 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.