FAQs

Built-up roofing systems have had a long-standing popularity, due in large part to the success and proven reliability of BUR. The stock of 20, 30 and 40-year-old BUR roofs still in excellent condition attests to this fact. Specifically, BUR roofs offer:

• Multi-Layer Protection – the multiple layers of bitumen and bitumen saturated “felts” make a water-resistant barrier capable of providing many years of reliable protection from the elements.
• Thermal Performance – Built-up roofing systems exhibit exceptional resistance to the conduction of heat between the exterior and interior of a building, resulting in noticeable reductions in heating and cooling costs.
• Fire and Uplift Resistance — Built-up roofing systems are tested through Factory Mutual Research Corporation to meet very strict fire resistance requirements and ensure adequate uplift resistance under extreme wind conditions.
• Economy — Compared to other high performance commercial roofing systems, built-up roofing is one of the best investments on the market due to its competitive cost per year of expected service life.

Modified bitumen membranes — MBS — combine the features of a built-up roof with the added tensile strength from its polymer modification. Using a reinforced sheet that is prefabricated in the plant, modified bitumen systems require a less labor-intensive application and can be applied cross-platform in both commercial and certain residential applications.

A modified bitumen roofing system is composed primarily of polymer-modified bitumen reinforced with one or more plies of fabric such as polyester, fiberglass or a combination of both. Factory surfacing, if applied, includes mineral granules, slag, aluminum or copper. The bitumen determines the membrane’s physical characteristics and provides primary waterproofing protection, while the reinforcement adds strength, puncture resistance and overall system integrity.

Factory-assembled, modified bitumen membranes undergo strict quality control standards to ensure uniform thickness and consistent physical properties throughout the membrane. The finished roofing system is usually a two- to four-ply system consisting of a modified bitumen membrane and a base sheet, with additional plies for added strength if needed. The substrate often determines which ply system is best specified.

The finished roofing membrane may consist of one or more modified bitumen sheets, or it may be comprised of a combination of built-up roofing (BUR) felts and one or more modified bitumen sheets. The type of substrate and the performance objectives influence the specification of the modified bitumen membrane system.

There are modified bitumen membranes tailored to almost every type of construction design and climate: for new roofing or reroofing of commercial buildings, residential high rises, domes, spires, and most categories of low-slope or steep-roof roofing. Thus there are a variety of modifiers, and types of reinforcements and surfacings available.

Modified bitumens generally use a traditional waterproofing medium — asphalt — modified with atactic polypropylene (APP), styrene butadiene styrene (SBS), synthetic rubber or other agents that create a uniform matrix that enhances the physical properties of the asphalt. SBS and APP are the most common bitumen modifiers.

• SBS (Styrene-Butadiene-Styrene) modifies the asphalt by forming a polymer network within the bitumen. SBS gives the bitumen rubber-like characteristics and improved resistance to aging and weathering. Most SBS-modified bitumen sheets are either set in hot mopping asphalt, torch-applied or adhered with cold-process adhesives. SBS-modified bitumen sheets that do not have factory applied granule or foil surfacing need some form of field-applied ultraviolet protective coating.
• APP (Atactic Polypropylene) is a thermoplastic polymer which forms a uniform matrix within the asphalt. This enhances the bitumen’s performance by increasing its UV resistance, increasing its flexibility at low temperatures and improving its flow resistance at high temperatures. APP-modified bitumen sheets are generally applied using a propane-fueled torch. Applicators use the heat to soften the modified bitumen on the underside of the sheet. The sheet’s bottom surface becomes a molten adhesive which flows upon the substrate and then cools to form a waterproof adhesive bond. Some APP sheets can also be applied with cold process adhesives.

While modified asphalt provides the primary waterproofing characteristics of these membranes a reinforcing ply adds strength and puncture resistance. Glass fiber and polyester plies are the most commonly used reinforcing fabrics. Each has distinctive properties. Polyester has excellent elongation, tensile strength and recovery. It provides good puncture resistance and stands up well to foot traffic. Glass fiber resists flame penetration and provides excellent tensile strength and dimensional stability.

The combination of asphalt, modifiers and reinforcements determines the characteristics of a specific modified bitumen membrane. To obtain the best roofing system possible, the designer, building owner and contractor should understand the dynamics of the roof as well as the roofing system when specifying either APP or SBS modified roofs.

The architect, specifier, building owner and roofing contractor should examine the following parameters when choosing a roofing system:

• What is the building’s type and usage?
• What building code requirements apply to the system?
• What is the estimated service life of the roof system? What type of maintenance will be required to maximize the service life of the roof system?
• What kind of roof deck will be used (type, size, slope)?
• How much is the roof deck expected to move relative to the roof perimeter? Shrinkage of the deck material? Moisture content of the deck?
• Access for equipment on the roof?

The application of modified bituminous roofing is a construction process that involves the skillful arrangement of several components as specified for the process. Roof performance is determined by factors that include building design, job specifications, materials quality and suitability, application procedures and maintenance. The level of quality in the workmanship during the application process is critical.

The application of modified bitumen roofing systems is not an exact science. It’s a craft that involves people, roofing professionals, who deal with a broad range of materials, designs, customs, techniques, weather events, and climates. We recognizes the importance of these critical factors as they affect the quality of modified bitumen roofing. These factors are controlled by applicable inspection and review procedures carried out during the development, manufacturing, production, and application stages.

Since gaining acceptance in the roofing industry during the 1970s, the use of modified bitumen membrane roofing systems has increased steadily. Today modified bitumen membranes are the fastest growing roofing materials in the industry.

Considered a quality “hybrid” system, MBS provides the features of a built-up roof with the added tensile strength and elongation of a modified bitumen cap sheet, as well as the quality assurance of in-plant  membrane fabrication uniformity and control, and reduced labor requirements for installation.

In addition, MBS products undergo the same fire and uplift resistance testing standards as BUR, providing the commercial customer full assurance of appropriate fire and uplift resistance.