What are the major FLAT roofing systems?

The two broad categories of asphalt roofing systems for commercial, industrial and institutional buildings are Built-Up Roofing (BUR) and Modified Bitumen Systems (MBS).

What is Built-Up Roofing (BUR)?

Built-up Roofing (BUR) is the most popular choice of roofing used on commercial, industrial and institutional buildings. BUR is used on flat or low-sloped roofs and consists of multiple layers of bitumen and ply sheets. Components of a BUR system include the roof deck, a vapor retarder, insulation, membrane and surfacing material. The components are assembled at the job site to actually form the built-up roof. At the heart of this roofing system is the roofing membrane, which consists of roofing bitumen and multiple reinforcing plies of roofing felt. Roofing bitumen is the primary adhesion/waterproofing agent used between roofing plies. Bitumen arrives at the job site in solid form, but is heated and applied as a liquid. Roofing bitumens may be either a product of petroleum refining (asphalts) or a product of the coal-cooking process (coal tar pitch).

Multiple reinforcing “plies” are asphalt-coated roofing sheets or felts installed in three or more layers to strengthen and stabilize the BUR membrane. These multiple reinforcing felts also make the membrane more pliable and resilient, protect the bitumen from water degradation, and serve as a fire-retarding element in the membrane system.

BUR roofing membranes can be protected from solar radiation by embedding gravel in the bitumen, applying a surface coating or applying a granular-surfaced “cap” sheet. Light-colored surfacing materials can be used to reflect heat from the building. In addition, surfacing agents can provide additional fire protection.

How is BUR applied for maximum performance?

It’s critical to get secure bonding of the roofing felts (plies) using bitumen. To achieve this bond the roofing contractor applies thin, uniform moppings of bitumen. This waterproofs the system and ensures proper adhesion for fusing the membrane system together. The temperature of the bitumen is critical. By heating it to the proper temperature the roofing contractor gets the right viscosity for proper mopping. The contractor heats the bitumen to its EVT or Equiviscous Temperature, the temperature at which it can be most effectively mopped into uniform layers. Each batch of bitumen should be labeled by the supplier with its EVT. Once felts are rolled into place on the heated bitumen applicators pull brooms or squeegees over the felt or use some other method to make sure that its embedded in the bitumen. The strength of the membrane depends on the type of felt used, the number of plies, overall ply construction, and the lapping of the overlaying felts. Typically, membrane ply construction is defined by headlap, endlap, and sidelap.

Headlap is the distance of the overlap that exists between the lowermost and the uppermost plies of the shingled portion of the roof membrane when measured perpendicular to the long dimension of the membrane.

Endlap is the overlap distance that is measured from where one roll of felt ends to where another begins.

Sidelap is the overlap distance along the length of the felt where one roll of felt overlaps the adjacent overlying felt.

The application of Built-Up Roofing systems is detailed work, but the professional who pays particular attention to those details such as curbs, walls, flashings or other projections that interrupt the membrane, achieves a quality, efficient, long-lasting product for the building owner.

What are some of the advantages of BUR?

Built-up roofing systems (BUR) 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 watertight 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 re-roofing 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 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 roof 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.

Why choose Modified Bitumen roofing systems?

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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.