Using cover boards with roofing assemblies

by Erik Missio | February 2, 2017 11:22 am

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by Derrick Hutchinson

When designing a roofing assembly, many factors need to be considered. Performance attributes, product quality, aesthetics, price, and quantity must all be carefully examined. One key product often overlooked during this process is the cover board.

A cover board is a thin substrate to which a roof membrane is adhered—however, its use far exceeds that of a typical substrate. Cover boards also serve as critical components helping extend the life of a roof. For example, using a cover board minimizes the literal and figurative impact of hail. Hail damage on a roof without a cover board can easily pierce the insulation, which will increase the heat flow through it, thereby adding to the building’s energy costs.

Cover boards also serve as barriers protecting the roof (and its very flammable insulation) from external fire. Further, they help increase the wind uplift value of a roof assembly by providing a strong, high-performing layer of defense. However, strict adherence to the recommended fastener spacing is key to this wind uplift performance. This prevents potential damage that may occur during unfavorable weather.

Based on roof evaluations found in a global property insurance company Factory Mutual’s database[2], wind uplift study results show including a cover board helps increase a building’s wind uplift and fire performance. Cover boards can double the wind uplift of certain roof assemblies or enable them to achieve the same rating with fewer fasteners.

The National Roofing Contractors Association (NRCA) has endorsed the use of cover boards in all low-slope roofs due to the insulation’s negative attributes like facer delamination, powdering, shrinkage, cupping, and edge cavitation. Cover boards help support the insulation in the event of these failures. All these uses of cover boards provide added security and lower cost of ownership for the building.

Just as cover boards are used to protect against various potential threats, they come in a variety of compositions tailored to an array of applications. With so many options for cover boards, selecting the appropriate product may be challenging at first. By keeping in mind the desired result, the process becomes easier.

Different cover boards have different physical properties

There are many types of cover boards suitable for an assortment of applications. Each has features and benefits suitable for a desired result. In the following section, various materials are explored, all of which could also be used as a thermal barrier placed directly over the roof deck. In this application, the cover board provides a fire barrier in the case of an internal fire, significantly increasing the time required to melt the insulation, which adds fuel to the flames. The cover board also serves as a substrate and vapor retarder in this application.

Gypsum fiber

A fiber-reinforced, uniform-composition cover board that can be used in most roof applications, gypsum fiber products typically provide unmatched wind uplift and hail performance (higher than any other cover board), with no risk of facer delamination. This product also protects against moisture and mold. This product costs approximately 50 cents per square foot.

Gypsum

A gypsum core and a glass-mat facer also provide protection from moisture and mold. This product is usually available with a primed or unprimed glass facer. The primed facer allows for adhesion of the membrane, while an unprimed facer is typically used for a mechanically attached single-ply application that does not require adherence to the membrane. This product costs approximately 50 cents per square foot.

Cement

Typically cementitious-based with a reinforcement mesh for added strength, these products tend to be water-durable and will not rot, warp, delaminate, or disintegrate. They may also be noncombustible, providing a suitable fire barrier for any roof assembly or parapet wall. This product costs approximately 50 cents per square foot.

Wood fiber

Often made from wood fiber and bonded together with a resin, these cover boards are typically used in hot-mop and torched applications. This product costs approximately 30 cents per square foot.

High-density insulation

Fairly new to the market, high-density insulation boards are typically composed of a compressed polyisocyanurate (polyiso) core with a coated glass-mat facer—this means they are generally lightweight products. This product costs approximately 50 cents per square foot.

Perlite

These cover boards consist of expanded perlite, cellulose binder, and mineral aggregate. They are typically used in hot-mop and torched applications. This product costs approximately 30 cents per square foot.

Asphaltic

Made of a mineral-fortified, asphalt-based core with fiberglass facers, these boards are typically employed in asphaltic roof assemblies. This product costs approximately 30 cents per square foot.

Mineral fiber board

These boards, which consist of stone wool or mineral fiber, provide good sound properties. They are typically used in asphaltic roof systems. This product costs approximately 30 cents per square foot.

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Determining which cover board is right for the job

The membrane materials and necessary roof functions usually determine which type of cover board should be used. For example, if a building is on the Florida coast and a 14.4-kPa (300-psf) rating with a single-ply membrane is needed, a gypsum fiber cover board may be the best option due to its high strength, wind uplift performance, and durability.

When selecting a cover board, it is important to keep in mind which properties are most crucial for the project’s roof system. Knowing the properties of each cover board helps in selecting the one that will provide the best value for the building. Some building codes may also dictate the fire and wind uplift properties the roof should achieve.

Cover boards are installed directly under the membrane and above the insulation. Many are fastened directly down into the deck, or they can be adhered to insulation with low-rise foam adhesive or hot asphalt.

To ensure cover boards are installed correctly, one should:

• stagger the cover board and insulation joints;
• install using approved fasteners with plates or adhesive and follow manufacturers’ directions for fastener spacing; and
• only install what can be covered by a final membrane system within the same day.

Selecting and installing the appropriate type of cover board can lead to energy efficiency and other benefits for the building. In turn, this creates a higher-performing building beneficial to all occupants.

Conclusion

Cover boards were first used as a barrier board for hot-applied and solvent-based adhesives to prevent damage to the insulation and blisters in the membrane system. These original cover boards were comparatively weak and low-performing to the products of today; their only job was to serve as a barrier board. Over the years, the industry has collected more detailed information about roofs and how storms and normal maintenance can damage surfaces. As a result, additional properties have been added to cover boards, making them stronger and thereby increasing the lifespan of a roof. Building codes have also played a role in the development of these materials. The need for higher fire and wind uplift performance has driven innovation.

Although using a cover board is an additional step, it helps create a long-lasting, durable roof. As these products become the norm in roofing systems, there is an ongoing challenge to identify what else they can do. How can cover board functions increase? Can they help make roofers work more efficiently? Can cover boards provide more value to the building owner, and if so, what is it? These questions will drive innovation and product development in the years to come.

[4]Derrick Hutchinson is a product manager for USG Roofing Solutions with more than a decade of experience in building product development and system evaluations. He is a member of the National Roofing Contractors Association (NRCA), RCI, Single-ply Roofing Industry (SPRI), Asphalt Roofing Manufacturers Association (ARMA), and ASTM. Hutchinson graduated from the University of Illinois at Chicago with a degree in physics. Before taking on the product manager role, he was a senior researcher at USG’s Corporate Innovation Center.

Source URL: https://www.constructionspecifier.com/using-cover-boards-with-roofing-assemblies/

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