Low-Slope Commercial Roofing

Roof Cover Boards Provide Outstanding Performance
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Robert C. Grupe, CSI, Director, Architectural and Technical Solutions, USG C

Typical Cover Board Options

There are a variety of different cover board material types available. These are some of the most popular cover board options and their specific benefits.

Fiber Board
One of the most widely used cover board materials, this board is often made from wood fiber or bagasi (sugar cane). Fiber board has a porous nature, good compressive and flexural strength, and an affinity for moisture, making it a popular choice under many membrane systems. However, fiber board is susceptible to moisture absorption, which leads to strength loss, decay or rot when it is exposed to moisture for extended periods of time.

Perlite Roof Board
This roof board is composed of expanded perlite ore, cellulose fibers, asphalt and starch binders. Traditionally, it has been coated on the top side surface with an emulsion to inhibit excessive absorption of the mopping asphalt. This roof board provides a dimensionally stable and thermally resistant surface for the application of hot-applied roofing membranes. It exhibits excellent flame spread characteristics, which may be important in the design and installation of some roof assemblies. Like fiber board, perlite cover board is sensitive to moisture.

Glass/Mineral Fiber Board
Glass and mineral fiber board is made from fiberized glass or basalt rock bonded with resinous binders and compressed to various densities. To prevent absorption of adhesives or asphalt into the porous core, a facer is applied at the factory. Some manufacturers provide boards with special facers designed for the direct application of torched-on modified bitumen membranes. The product is dimensionally stable and has a high degree of heat resistance. While generally unaffected by moisture, glass and mineral fiber insulation can lose its structural integrity if it remains wet for long periods of time. In addition, the facers may be adversely affected by exposure to moisture. Due to the inherent absorptive nature of its bottom surface, it cannot be back-mopped with hot asphalt for securement over heat-sensitive insulations.

Asphaltic Core Boards
Developed in recent years, this type of cover board is often fabricated with an ashphaltic core sandwiched between a glass fiber reinforcement. Asphaltic cover boards are designed for use primarily with hot-applied asphalt membranes and torch-on systems. Highly resistant to moisture, they are compatible with all asphaltic systems. However, the asphalt content makes the board unsuitable for use under single-ply membranes, which are affected adversely by contact with asphalt.

Glass Mat-Faced Gypsum Board
This type of board employs glass mat facings front and back that are embedded into a water-resistant treated core, providing moisture and fire resistance.

Fiber-Reinforced Gypsum Panels
This innovative new type of fiber-reinforced gypsum panel incorporates the latest cover board technology. The cover board provides exceptional resistance to both mold and moisture, along with outstanding fire performance. This high-performance product is manufactured with no face layers, providing excellent wind uplift properties. The product also features a high compressive strength (up to 20 times greater than wood fiber board or perlite), enabling it to protect the roofing system from potential damage from foot traffic or hail. It is also made from 95 percent recycled materials.

 

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Originally published in Architectural Record.
Originally published in September 2005

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