Modified Bitumen Systems


Modified Bitumen Systems 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. Modifieds have proven performance on residential, commercial, and industrial applications.

Roofhelp recommends that all modified roofs be installed on slopes not less than 1/4″ per horizontal foot (stated “quarter in twelve” or “quarter-inch slope”) in order to achieve positive drainage.



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.


In order to understand what coal tar pitch is, it’s important to understand its origins and refinement methods. Coal tar can be refined from a number of sources including coal, wood,
peat, petroleum, and other organic materials. The tar is removed by burning or heating the base substance and selectively distilling fractions of the burned chemical.

Distillation involves heating the substance to a point where different fractions of the substance become volatile. The fractions are then collected by condensing the fraction at a specific temperature. A base substance can be split into any number of fractions through distillation. A good example of industrial distillation is the oil refining process. Through distillation, crude oil can be separated into fractions that include gasoline, jet fuel, motor oil bases, and other specialty chemicals. Fractionation or distillation is a tried-and-true method for breaking a substance into different parts of its composition. One of the first uses of coal tar was in the maritime industry. Trees stumps were burned and the tar fractions were collected through distillation of the tar. The tar was then used to coat wood boats. Tar was an excellent waterproofing agent and it also helped to protect the wood from insects. The tar used in roofing is a derivative of the fractional distillation of coal gas, which is sometimes referred to as coke. Many other chemicals are produced in this and subsequent fractioning processes, including naptha, creosote, benzene, toluene, and phenol. Coal tar pitch consists of a collection of cyclical hydrocarbons of various molecular weights and configurations. Every coal tar pitch can be refined to a greater or lesser degree; greater refinement yields a stiff, glassy product, while reduced refinement yields a highly viscous fluid.

Coal tar and coal tar derivatives are used in a variety of industrial applications. Tar pitch from wood is used in soap, food, and medical applications, while coal tar derivatives can be found in dyes and cosmetics. One of the predominant uses for coal tar derivatives is construction material.

Coal tar pitch has been used as the base media for coatings and adhesives for many years,including roofing bitumen. Built-up commercial coal tar pitch roofing has historically provided long lasting and sustainable protection for the building envelope. The inherent waterproof nature of coal tar pitch provides a tight waterproof roof construction. The self-healing properties of coal tar pitch tend to bridge small deformities created by debris, stress movement, hail, and other surface punctures.

Further, coal tar pitch is highly chemically resistant, resisting exposure to a large variety of acids,bases, and solvents. These properties have led to some of the most sustainable roof systems available in the market place. It is not unusual to observe coal tar pitch roofs that last up to 40 years. Today, coal tar pitch roofing adhesives can be installed hot or cold, while maintaining the same level of waterproofing protection and chemical resistance as traditional coal tar pitch roof systems. In addition, recent manufacturing advancements take the base technology another step forward. By incorporating a blend of specialized polymers with the coal tar, polymer-modified coal tar achieves low-temperature performance that surpasses that of traditional coal tar products. Tests show that modified coal tar pitch can achieve flexibility numbers as low as 30 degrees Fahrenheit. As an added benefit, the polymer additive greatly improves the elongation of the coal tar pitch base; products are currently available with elongations of greater than 2,000 percent.

Further, these new compounds have dramatically improved impact resistance. In essence, recent technology results in the first truly elastomeric coal tar pitch. Most promising of all, these new compounds improve the environmental nature of the coal tar pitch. Through the polymer blending process and the preconditioning of the coal tar pitch, many of the volatile emissions are reduced or eliminated. Independent testing by the Environmental Protection Agency methods indicates a minimum reduction of 50 percent of the volatile emissions, in comparison with traditional coal tar pitch. Significantly, a good portion of these reductions take place within the most hazardous compounds present in standard coal tar pitch.