Low-slope roofs can have slopes as minor as 1⁄8 inch per 12 inches. These roofs employ a waterproof roofing system and are found primarily on commercial structures.

A low-slope roof system generally consists of a roof membrane, insulation, and one of a number of surfacing options. To control the application and improve the quality of low-slope roofing, a variety of specifications and procedures apply to the assembly of the roofing components.

These specifications and procedures are generally accepted and used throughout the United States. Roofing systems that meet these specifications normally can be expected to give satisfactory service for many years.

Climatic conditions and available materials dictate regional low-slope procedures, which can vary greatly in different parts of the country. Low slope roofs are essentially a custom product. They are designed for a specific building, at a specific location, and manufactured on the jobsite.

Membrane Components
Low-slope membranes are composed of at least three elements: waterproofing, reinforcement, and surfacing. Some materials within the membrane might perform more than one function. The waterproofing agent is the most important element within the roof membrane.

In BUR and modified bitumen roofing (MBR), the waterproofing agent is bitumen. In single-ply roofing, the waterproofing agent is synthetic rubber or plastic.

The reinforcement element provides stability to the roof membrane; it holds the waterproofing agent in place and provides tensile strength. In BUR, reinforcement is typically provided by organic or glass-fiber roofing felts. In MBR, the reinforcement is generally glass-fiber felt or polyester scrim, which is fabricated into the finished sheet by the manufacturer.

Polyester and other woven fabrics are used as reinforcements for elastomeric and plastomeric, single-ply membranes. Some singleply membranes do not require reinforcement because the waterproofing material is inherently stable.

The surfacing materials protect the waterproofing and reinforcement elements from the direct effects of sunlight and weather exposure. They also provide other properties, such as fire resistance, traffic and hail protection, and reflectivity.

Some single-ply membranes are self- or factory-surfaced. Aggregate, which is field-applied, and mineral granules, which are usually factory-applied, are the most common types of surfacing materials. Smooth-surfaced coatings, however, are increasing in popularity.

Membrane Classifications
Low-slope roof membranes can usually be grouped, or classified, into the general categories reviewed below. There are, however, hybrid systems that might not fit into a category, or that might be appropriate in several categories.

BUR, which uses asphalt or coal tar products, is by far the oldest of the modern commercial roofing methods. Many commercial buildings in this country have BUR roofs. The large number of 20-, 30-, and even 40-year-old BUR roofs that are still sound attests to the system’s durability and popularity.

Roofing materials continue to evolve, however, and improvements are continually being made to asphalt and coal tar pitch, the basic bitumen components of BUR. Asphalt tends to be more popular with most roofers than coal tar.

Since the first MBR membranes were manufactured in the United States in the late 1970s, they have become one of the roofing industry’s fastest-growing materials. The popularity and specification of MBR membranes has increased steadily for more than two decades. Contractors have found the materials easy to use and easily inspected. MBR systems provide a time-tested, high-performance, reliable roof.

Since they first appeared in the 1950s, single-ply materials have become increasingly popular in the United States. Whether imported from Europe or produced domestically, these high-tech products have proven themselves in a wide variety of climates during more than three decades of use.

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