Selecting Rigid Cover Boards in Commercial Roofing Systems

Using a roof cover board on every project is the emerging norm—and the type matters
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Sponsored by Georgia-Pacific Gypsum
By Peter J. Arsenault, FAIA, NCARB, LEED AP

Sound Resistance

Reducing noise in a building from outside sources has become an important human wellness concern in many settings. Roof systems can be designed to reduce the noise from traffic, weather, equipment, and aircraft as a vital part of the building design. The measure of sound control in this case is a sound transmission class (STC) rating. Expressed in decibels (dB), STC is a measure of the effectiveness of airborne sound transmission reduction and is based on prescriptive ASTM test methods, including sound transmission loss (STL) testing. Ideally, the threshold of a quiet indoor environment that roof systems can aspire to achieve is 25 to 35 dB. Including a 58-inch glass mat gypsum underlayment and cover board to a typical insulated roof assembly can help increase the STC rating of the roof assembly and get closer to that quiet indoor environment. Different thicknesses and materials may achieve different ratings so it is necessary to consult manufacturers’ data for the information on their specific products.

Performance Comparison of Different Roofing Boards

As is quite clear by now, a roofing assembly needs to meet a variety of performance criteria. We have also alluded to the fact that different types of roof boards can have different levels of success in meeting those performance criteria. To help illustrate that further, the following table summarizes the suitability for six options of roof board against some of the basic criteria.

Based on the test results that indicate the performance characteristics shown in the table, it is clear that the only available product that provides high performance in all areas is coated glass mat gypsum cover boards.

Protecting Roof Performance During Value Engineering

Value engineering (VE) can be a useful tool when controlling construction budgets is important. VE can also create unintended consequences when it is based on incomplete or inaccurate information. In the case of roofing systems, it is important that all aspects of the intent behind the design are understood and accounted for. Sometimes the original intent of a roof assembly design is altered when cost pressures from the building owner lead to the removal of a rigid cover board. The incorporation of a rigid cover board in a roof assembly offers a myriad of benefits that require consideration during budget discussions. This includes the fact that cover boards are usually a smaller percentage of the total roofing assembly cost compared to other costlier components, such as the membrane and the insulation. Further, if value is found in extending the lifespan of the roof and reducing scheduled maintenance, then the cost of cover boards can pay for itself over time. Typically, when a cover board is included in a roofing assembly, it only accounts for about 15–20 percent of the total cost of the roof.

Glass mat gypsum cover boards add strength to commercial roof assemblies while providing a smooth, flat surface ideal for single-ply membranes.

Rigid Roof Boards for Different Roofing Systems and Attachments

Rigid roof boards need to be selected for compatibility with the roofing system or membrane specified. They also need to be compatible with the attachment options for the selected roof system. Using a range of suitable attachment options, glass mat gypsum cover boards enhance the performance for a wide variety of roofing systems for new and reroofing applications. That includes single-ply membranes, modified bitumen, liquid applied, membranes, built-up roofing, and even metal. For attachment methods, they are compatible with mechanically attached, fully adhered, torched, mopped, and ballasted systems.

Due in large part to the green and sustainable building movement, the demand for high-performing roof boards has gone beyond typical roof installations to include photovoltaic (PV) and vegetative assemblies. The growth of the photovoltaic industry is commonly credited to new technologies driving down the initial costs to a very attractive level. Even if some of the incentives to build photovoltaic systems expire, solar power continues to be a popular alternative energy source that has seen unprecedented growth in the United States and around the world in recent years. Low-slope commercial roofs continue to be a popular location for PV systems because they offer a secured location without access by the general public and are more likely to be unobstructed for receiving sunlight. They also allow for the generation of energy on-site where it is needed for use. PV systems are ideal for retrofit situations since there is a large inventory of existing low-slope buildings.

Many of the roofing challenges presented by adding solar PV to a roof can be resolved by the use of glass mat gypsum cover boards. Roofs that host PV systems are at an increased risk for electrical-related fires, foot traffic damage from scheduled PV maintenance, puncture from equipment and tools used for servicing, and moisture damage resulting from puncture. As we have seen, glass mat gypsum cover boards can help address all of those conditions. Further, with PV systems lasting 25 to 30 years, roof assemblies need to be designed that can last as long as the PV system.

When it comes to vegetative roof systems, they continue to grow in popularity due to their ability to increase energy efficiency, manage water discharge, reduce facility operational cost, enhance the building’s appearance, and help the building owner be positioned as an environmentally friendly organization. Thicker and heavier “intensive” vegetative roofs are giving way to lighter “extensive” systems, which are designed to be more self-sustaining and maintenance free. This new system allows contractors, architects, and property owners to take advantage of the many benefits of vegetative roofs at a fraction of the cost and weight.

An intensive garden roof traditionally places the waterproofing membrane directly over a high-load capacity roof deck, drainage layer, and insulation. Over the top of these components is a 1-foot-thick layer of soil media. The more user-friendly extensive systems decrease the amount of soil media to 6 inches or less and typically incorporate a cover board on top of the roof deck and insulation. Over the roof membrane is a root barrier, drainage board, and moisture-retention mat to provide support for the growing media on the top layer. The cover board serves as a durable substrate for the membrane and protects the more fragile thermal insulation layer from mechanical damage. A durable cover board can also increase the wind uplift performance of the vegetative system and protect the membrane from hail damage and foot and mechanical traffic during the installation process.

Conclusion

Rigid cover boards are recommended by the NRCA because they have been shown to deliver value to commercial roof assemblies of all types, sizes, and installation techniques. Design and construction professionals are slowly realizing that this is a recommendation that should be incorporated on all projects that involve roofing. The type of cover board is important, however, since different materials carry different properties and performance characteristics. As discussed, glass mat cover boards currently appear to provide the broadest and highest-performing characteristics to the widest range of roof assemblies. They excel in fire resistance, moisture tolerance, wind uplift strength, and ease of handling. They are also compatible with virtually any type of roofing system and offer some added benefits of noise-reduction and hail-resistance improvements for the roofing system. Overall, they are a preferred choice for the best practice of using roof cover boards in all commercial roofing situations.

 

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Originally published in Architectural Record

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