Modified Bitumen Roofing Systems

Modified bitumen roof membranes are made from either APP (plastic) or SBS (rubber) modified asphalt with polyester and/or fiberglass reinforcement scrims. The Modified Bitumen Roof Membrane can be installed torch applied, where the back of the sheet is heated with an open flame and welded to the base ply, hot applied with hot asphalt or fully adhered with a cold adhesive. The Modified Bitumen Roof Membrane comes with a granulated surface in various colors or smooth surfaced-usually finished with an aluminum reflective coating. Modified Bitumen Roof Membrane are used as cap sheets with the built-up roof systems as well.

In Europe, where modified membranes were first introduced, they were installed with a two ply application method. The American manufacturers specified a single ply application to be competitive with others roof systems and performance has been mixed depending on attention to detail in the installation and application conditions such as roof slope, ponding and runoff restrictions. More stringent requirements addressing these conditions have been implemented in recent years and more manufacturers and roofing companies are specifying multiple ply applications with the Modified Bitumen Roof Membrane.

The Modified Bitumen materials make an excellent roof with the right application specifications and sufficient attention to detail.  Click here for technical information.

For more information please call our roofing professionals at (303) 971-0256 or Click Here to request a free quote.

Modified bitumen roofing - Commercial Roofing - The Flat Roof Roofing Company - Calebs Roofing

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Bitumen, also known as Asphalt, is a sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleum and in some natural deposits, it is a substance classed as a pitch. Neither of the terms Asphalt or Bitumen should be confused with tar or coal tars.

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.

Two types of MB roofing membranes:

  1. 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.
  2. APP polymer-modified bitumen membranes typically are heat-welded or torch-applied. Consumers should be cautioned that it is not recommended to torch-apply 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.”

Modified Bitumen (MB) is asphalt that has had modifiers added to it to give it plastic or rubber-like properties. The most common types of modifiers being used are APP (Atactic Polypropylene) and SBS (Styrene Butadiene Styrene). Rolls of modified bitumen membrane come in widths of 36″ (0.9 m) to approximately 39″ (1 m) and cover an area of approximately 100 square feet to 112 square feet per roll. Surfacings for these roll materials consist of a smooth surface, or mineral granules, aluminum, copper, or an aggregate such as gravel or slag that is set it hot asphalt.

Modified Bitumen roof systems consist of one, two, or three ply systems. The type of substrate will often determine the type of system being installed. Modified membranes can also be installed in conjunction with built-up roof materials (such as multiple plies of fiberglass felt) to form a “hybrid” roof system. Modified has proven performance on residential, commercial, and industrial applications.

In order to create roofing grade asphalt, asphalt flux is air-blown at elevated temperatures which converts the flux to roofing grade asphalt. In the early 1970’s, the Italians, lacking the blowing equipment, were looking for a product that would convert asphalt flux into a usable roofing product. They discovered that if Atactic Polypropylene (APP) – a by-product of propylene polymerization – was added to asphalt then it gave the asphalt some plastic properties. They found that by adding about 30% of APP modifier, they could stretch the modified asphalt up to fifty percent of its original length before it would break.

Next came the need to make it into a usable roll product. Some type of reinforcement would be needed. They looked into various reinforcement materials and decided on a polyester mat because polyester would accommodate the APP modified asphalt’s elongation properties whereas the more commonly used woven glass mats would not. The reinforcement material is dipped into the hot modified bitumen mix, then goes through a rolling cylinder, cooled, and then wound into a roll.

APP membranes are applied using a torch. The back of the sheet has extra asphalt on it which, when heated, bonds to the substrate. This was especially convenient for the smaller, more cut up roofs because less room and equipment is needed on site to torch-apply a membrane than is necessary for application using hot bitumen.

While APP was being looked into in southern Europe, northern Europe was experimenting with a different type of modifier called Styrene Butadiene Styrene (SBS). The French and Germans found that if they added 10%-15% of SBS rubber to asphalt, the asphalt’s characteristics changed to those of the rubber additive. They learned that they could stretch the SBS modified asphalt up to six times its original length and that, unlike the APP, it would return to its original size when allowed to relax.

There are a wide range of reinforcements used in SBS roofing materials. These include fiberglass or polyester mats and scrims, or combinations of both. The fiberglass mats range in weight from 1.0 to 2.5 pounds per 100 square feet or around 50 to 125 grams per square meter. Polyester reinforcements range in weight from 3.5 to 5.0 pounds per 100 square feet or 170 to 250 grams per square meter. The type of reinforcement used depends on the material’s performance requirements.

SBS membranes can be hot asphalt applied, torch applied, or cold process applied.