Frequently Asked Questions

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Low-Slope FAQs

What are some of the advantages of BUR?


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.

What is Modified Bitumen (MB) or Modified Bitumen Membranes (MBS)?


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.

What are Modified Bitumen modifiers? How do they work?


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.

What are reinforcing plies? What are they made of?

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.

How do I choose the roofing system that works best for a particular job?


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?
How is Modified Bitumen roofing applied for maximum performance?


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. ARMA 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.

Why choose Modified Bitumen roofing systems?


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.

What are some low-slope roofing terms?

APP (Atactic Polypropylene): A modifier of asphalt (see modified bitumen roof membrane) that increases the UV resistance of the bitumen as well as its flexibility at low temperatures and improves its flow resistance at high temperatures.
Asphalt: A bituminous waterproofing agent applied to roofing materials during manufacture. 
Built-Up Roof (BUR): A flat or low-sloped roof consisting of multiple layers of asphalt and ply sheets. 
Base Sheet: A saturated or coated felt installed as the first ply in some multi-ply modified bitumen roofing assemblies. 
Bitumen: (1) a class of amorphous, black or dark colored (solid, semi-solid or viscous) cementitious substances, natural or manufactured, composed principally of high molecular weight hydrocarbons and found in asphalts, tars, pitches and asphaltines; (2) a generic term used to denote any material composed principally of bitumen.
Bituminous: Containing or treated with bitumen. 
Cap Sheet: A granule-surfaced coated sheet used as the top ply of a modified bitumen roof membrane. 
Coated Sheet or Felt: (1) an asphalt felt that has been coated on both sides with harder, more viscous asphalt; (2) a glass fiber felt that has been simultaneously impregnated and coated with asphalt or coal tar on both sides. 
Cold-Applied Roofing: A continuous roof membrane, consisting of plies of felts, mats or fabrics that are laminated on a roof with alternate layers of cold-applied roof adhesive and surfaced with a cold-applied coating. 
Deck: The structural surface to which the roofing or waterproofing system (including insulation) is applied. 
Felt: A flexible sheet manufactured by the interlocking of fibers through a combination of mechanical work, moisture, and heat. Felts are manufactured principally from vegetable fibers (organic felts), glass fibers (glass fiber felts), or polyester fibers (polyester felts); other fibers may be present in each type. 
Fiberglass Mat: An asphalt roofing base material manufactured from glass fibers. 
Flashing: Pieces of metal or roll roofing used to prevent seepage of water into a building around any intersection or projection in a roof, such as vent pipes, chimneys, adjoining walls, dormers and valleys.
Granules: Ceramic-coated colored crushed rock that is applied to the exposed surface of asphalt roofing products. 
Incline: The slope of a roof expressed either in percent or in the number of vertical units of rise per horizontal unit of run. Also referred to as slope. 
Low-Fuming Asphalt: An asphalt that contains a small amount of special
polymer that, when heated, floats to the surface, creating a skim layer on
the asphalt in the kettle that traps most of the fumes.
Membrane: A roof covering or waterproofing layer whose primary function is the exclusion of water.
Modified Bitumen Roof Membrane: A continuous, semi-flexible roof membrane assembly consisting of plies of saturated felts, coated felts, fabrics or mats between which alternate layers of bitumen are applied, either surfaced or unsurfaced. 
Organic Felt: An asphalt roofing base material manufactured from cellulose fibers.
Re-covering: The process of covering an existing roofing system with a new roofing system. 
Re-roofing: The practice of removing an existing roofing system and replacing it with a new roofing system. 
Roll Roofing: Asphalt roofing products manufactured in roll form, either smooth- or mineral-surfaced. 
Saturated Felt: An asphalt-impregnated felt used as an underlayment between the deck and the roofing material. 
SBS (Styrene-Butadiene-Styrene): A modifier of asphalt (see modified bitumen roof membrane) that enhances the bitumen’s ability to resist the effects of aging the weather.
Self-Adhering Membrane: A membrane that can adhere to a substrate without the use of an additional adhesive. The undersurface of a self-adhering
membrane is protected by a release paper or film, which prevents the
membrane from bonding to itself during shipping and handling. These
membranes can be base sheets, ply sheets, cap sheets or underlayments. 
Smooth-Surfaced Roof: A roof membrane surfaced with a layer of hot-mopped asphalt, cold-applied asphalt-clay emulsion, cold-applied asphalt cutbacks, elastomeric coating, or sometimes with an unmopped, inorganic felt. 
Square: A unit of roof measure covering 100 square feet. 
Thermal Insulation: A material applied to reduce the flow of heat.
Underlayment: Asphalt saturated felt used beneath roofing to provide additional protection for the deck.
Vapor Retarder: A material designed to impede the passage of water vapor into the roofing system.