Selecting Rigid Cover Boards in Commercial Roofing Systems

Using a roof cover board on every project is the emerging norm—and the type matters

By Peter J. Arsenault, FAIA, NCARB, LEED AP

On most commercial, institutional, and industrial buildings, roofing is a big investment whether at the time of first construction or when it needs to be replaced. That’s the primary reason durability of the roof is so important: it translates directly into a lower total cost of ownership. It can also make a difference in terms of how the roof performs during routine and severe weather conditions to protect the building from the elements. Recognizing this need for durability, the National Roofing Contractors Association has recommended, for many years, the inclusion of a rigid cover board in all single-ply commercial roofs. However, the design and construction community has been slow in adopting this best practice for all situations and instead only tends to specify or use a roof board in specific situations, such as when a high-performance roof is necessary. Of late, new technologies, new studies, and experiences both with and without cover boards have pointed to the fact that such roof boards are a logical and economical choice. Based on all of these factors, this course looks at the value that rigid cover boards deliver to a commercial roof assembly, how they perform in adverse conditions, and how different types of cover boards compare to each other.

Evolution of Roofing and Rigid Cover Boards

We begin with a brief historic perspective of how cover boards have evolved over time to support new types of roof assemblies and meet the growing needs of building owners. Note that the terms “roof board” and “cover board” are sometimes used interchangeably. Roof board is a generic term describing a rigid board used in a commercial roof assembly for a number of reasons. When it is used directly over the deck, it is being used as a thermal barrier for fire protection (mitigates risk of internal building fire spreading via the roof). When used between the insulation and membrane, it is called a cover board. Cover boards are used to protect different parts of the assembly from a number of concerns, including external fire, wind uplift, puncture, and impact.

From the 1920s through the 1970s, there was relatively little innovation in commercial roofing systems. It’s true that there were changes during this timeframe in the materials used (e.g., coal tar pitch to modified bitumen), but the essential process of mopping tar or asphalt between multiple layers of felt material remained essentially unchanged. When insulation was used in commercial roofing through this time period, it was a soft fiberglass material that was rolled over the structural deck of the roof. Cover boards (which were called overlayment at the time) were primarily used to keep the asphalt from being absorbed into the fiberglass insulation. Popular overlayment types included perlite and wood fiber boards.

The 1980s marked the start of over three decades of innovation in commercial roofing systems when the first glass mat gypsum roof boards were introduced into the market place. These boards provided superior fire resistance, wind uplift performance, moisture resistance, and sound isolation compared to traditional overlayment materials. Also introduced in the 1980s were the first rigid insulation boards, including polyisocyanurate (ISO) boards. These boards were easier to stage and install, provided better R-value, and did not absorb asphalt. Throughout the 80s, there was a lot of innovation in these rigid insulation boards, with several companies launching new, enhanced boards. Some of the first single-ply membranes were introduced during the 1980s, but hot-mop assemblies remained very much the norm.

During the 1990s, single-ply systems slowly became more popular, gaining acceptance in the marketplace due in large part to the ease of installation and overall cost. Single-ply options provided other benefits too, including enhanced energy efficiency, easier/quicker installation on larger roofs, and ease of repair. Because of these advantages of single-ply roofing, the use of hot mopping dropped from roughly 80 percent of commercial roof projects in 1990 to about half of that by the end of the decade. As the demand for single-ply systems increased, so did the demand for glass mat gypsum cover boards. The cover boards provided a structurally strong, smooth surface to adhere the membrane and provided an additional sound barrier while, most importantly, protecting the rigid insulation boards from foot traffic, hail impact, wind uplift, and fire.

Throughout the 2000s, single-ply membranes have continued to grow in popularity, currently comprising approximately 80 percent of the overall market. In response, the demand for cover boards also increased. To feed this demand, high-density ISO (HD ISO) and gypsum fiber cover boards were introduced to the market and glass mat gypsum boards were enhanced to improve adhesive coverage. At the same time, the trend for companies to lower their carbon footprints by reducing their reliance on fossil fuels led to an increase in alternative roofing systems, such as vegetative and photovoltaic roofs.

As the use of roof board products has grown over time, the choices available have also grown. Here are the most common products on the market today:

• Glass-mat gypsum cover boards are composed of a fiberglass mat with a gypsum core. They can be used with all low-slope roofing systems as a cover board as well as a thermal fire barrier. There are several manufactures of glass mat gypsum boards, all with unique qualities. Comparing the published manufacturing specifications is the best way to match the board’s performance with your design intent.
• Gypsum fiber cover board, a blend of gypsum and cellulose fibers (recycled waste paper), is compatible with most low-slope roofing systems.
• High-density polyisocyanurate (HD ISO) is made with closed-cell polyisocyanurate insulation with a high-density core. With the exception of hot asphalt and torch-applied systems, it can be used with all roofing assemblies as a cover board (not as a fire-resistant thermal barrier).
• Mineral fiber board is made of semi-rigid rock wool or fiberglass board and is used with hot-mop, torch, and single-ply roofing systems.
• Perlite boards comprise mineral aggregate board with particles of expanded perlite or vermiculite, selected cellulose binders, and sizing agents. They offer good fire performance and are used with modified bitumen (mod-bit), built-up-roofing (BUR), and some single-ply roofing systems.
• Asphaltic board has an asphalt core with fiberglass faces on both sides. It is typically utilized as a substrate for a variety of systems, including BUR, torch-applied, cold mastic, and mod-bit assemblies.

Clearly it is important to understand the capabilities of any roof board and its compatibility with the type of roofing membrane being used on a building. We will next take a closer look at the performance criteria for all of these roof boards and how they interact with complete roofing assembly systems.

Performance Criteria for Roofing Systems

A typical roof system includes different layers of materials to assure different performance criteria are achieved. The structural roof deck layer can be metal or concrete and provides the supportive base of the roof. A vapor retarder and roof board are commonly the next layers set on top of the deck to prevent unwanted moisture and fire from penetrating into the roof assembly. Rigid roofing insulation comes next, typically in multiple layers and in varying thicknesses. A rigid cover board is placed on top of the insulation for protection and to improve the durability of the roofing membrane, which is installed last over the assembly. Within this makeup, the two most expensive layers are the roof membrane and the insulation. Using comparatively inexpensive rigid cover boards to protect the expensive layers from damage and help assure their long-term performance has become a common best practice. In fact, the National Roofing Contractors Association (NRCA) roofing manual states “…for mechanically attached, single-ply membrane roof systems, designers of newly installed roof systems are now recommended to include a noncombustible cover board that is consistent with an appropriate listing or approval from a code-approved testing agency.”

In the following sections, we will look in more detail at the specific criteria for selecting an appropriate roof/cover board.

Fire Resistance

Fire resistance is very important when choosing a roof cover board and it is a critical element in effective roof design. Fire resistance in a roofing system comes from all of the roof components working together such that the choice of cover board can either enhance or detract from fire resistance. Where the decking, insulation, or membrane may be combustible, a noncombustible cover board can contribute to a better fire rating.

There are two predominate fire testing bodies in the United States that are widely accepted in regards to fire issues, including external and internal exposure, code compliance, life safety issues, and insurance premiums.

• Underwriters Laboratories (UL): The well-known UL testing and labeling programs are mainly concerned with life-safety issues for the general public—i.e., people who occupy or visit buildings. For construction products, they provide both interior and exterior classification ratings. The three exterior classifications, applicable to roofing systems, in order from most to least stringent, include simply UL Class A, B, and C. These exterior classifications are determined by conducting three specific tests following designated criteria for 1) flame spread (all decks), 2) intermittent flame (combustible decks), and 3) burning brand (combustible decks). Note that for noncombustible decks, only the spread of flame test is required. UL also identifies an interior classification of “P” assemblies that test the resistance to fires that start within the building.
• FM Approvals: Previously known as Factory Mutual, FM Approvals is the testing entity part of FM Global. Having grown out of the insurance industry, it focuses on loss-prevention issues and the damaging effects to buildings resulting from fire, wind uplift, foot traffic, and hail. The FM classifications in order from most stringent to least stringent are Class 1, 2, and 3. The sought after Class 1 rating includes a calorimeter test that measures the fuel contributed by the assembly from inside the building. Keep in mind, that lower-risk fire ratings can both ensure code compliance and help reduce insurance premiums. For example, roofing systems that qualify for a FM Class 1 rating will qualify for the lowest insurance rates from FM-affiliated insurance companies.

When selecting roof boards from the available choices to improve the fire ratings of roofing systems, gypsum-based products offer the best known, tested, and proven fire- and heat-resistance characteristics That’s because gypsum boards have a built-in fire resistance due to their chemical makeup. Pure gypsum naturally occurs in the form of crystals, known technically as calcium sulfate dihydrate (CaSO₄•2H₂O). That means it contains two particles of crystalized water per molecule of calcium sulfate. As a result, gypsum (including synthetic manufactured gypsum) is inherently fire resistant since, by volume, gypsum is made up of 21 percent chemically combined water and 79 percent inert calcium sulfate. Put another way, one board of ⁵⁄₈-inch 4-by-8-foot glass mat gypsum cover board contains 2 gallons of crystalized water. During a fire, that chemically combined water is gradually released as water vapor while the remaining gypsum becomes calcined acting as an insulating layer.

Moisture

According to a 2016 survey of roofing consultants, moisture ranked second only to puncture in destructive events that negatively impact the lifespan of a commercial roof assembly. Of course, puncture and moisture can go together since a punctured membrane often leads to a costly moisture issue. Essentially, there are three potential sources of moisture intrusion in a roofing system. First, it can be introduced during construction. Since roof construction is completed outdoors and the weather conditions can’t be controlled, a contractor may need to work under damp conditions to satisfy construction schedules, or to protect the building’s interior. In these instances, rain and dew may become trapped and retained in roofing materials. Second is a roof leak, whether during construction or after it is complete. Even the smallest penetration in a roof membrane allows water to enter the roof assembly. The third source is vapor drive. Roofing products get wet not only from the outside (i.e., from rain and leaks in the membrane) but also from moisture originating inside the building. When high-humidity indoor air meets the cooler underside of the roof membrane, the water vapor can reach the dew point and condense into liquid water.

When water does enter a roof assembly, it can adversely affect the performance of the components: insulation can lose its R-value, roof boards can lose their strength, fasteners can corrode, and even plywood and oriented strand board (OSB) can warp, delaminate, and rot. Wood fiber board that is exposed to condensation over time can even turn into wet pulp, resembling oatmeal. By contrast, products that have the ability to hold up to water intrusion can survive and continue to perform. A good indicator of a cover board’s ability to withstand water is the total water absorption test. This can be determined following the procedures of ASTM C473-15, which is a standard 2-hour soak test with results typically calculated as a percentage weight gain. Translating that into volume for a 4-by-4-foot roof board, test results show that a glass-mat gypsum board absorbs 0.1 gallon of water, while a gypsum fiber board absorbs nearly 2.0 gallons. In order to minimize the amount of water absorption in roof boards and contribute to a more resilient roof, it is recommended to specify roof boards that have been tested not to exceed 5 percent maximum total water absorption by weight.

Moisture can also affect the flexural strength of some of the more commonly used rigid cover boards. Flexural strength is a strong indicator of the board’s ability to withstand wind uplift. When different types of boards are tested for this trait, moisture has a detrimental effect on each type of cover board but to varying degrees. ASTM C1177 is the test to determine flexural strength when different roof boards are exposed to moisture. This testing has shown that the leading glass mat gypsum board maintains more of its strength and lasts longer than other alternatives before reaching the saturation point. It is worth remembering that all tested boards lose strength when wet so avoiding exposure to moisture before, during, and after installation is important. Note too that products with low vapor permeability will dry out slower than products with high vapor permeability.

Wind Uplift

Wind uplift resistance is defined as the ability of the roofing assembly to resist blow-off and minimize property damage. Wind-generated forces that are trying to lift the roof off a building can amount to the equivalent of hundreds of pounds per square foot (psf). FM guidelines are often used to address wind uplift and design roof assemblies. While wind uplift is an important consideration for almost all projects, it is particularly crucial for buildings where wind conditions are a common issue, such as coastal regions and hurricane-prone areas.

Cover boards can contribute to wind uplift durability in a variety of roofing system configurations. For fully adhered membranes, where the membrane is held down by a continuous coating of adhesive, wind performance relies on the strength of the substrate. That means if the membrane is bonded to a fragile, low-density insulation layer, uplift forces can literally pull the insulation apart. A strong, rigid cover board installed between the membrane and the insulation can prevent this from happening. For mechanically fastened membranes, cover boards can also enhance durability. In these cases, the plates and fasteners transfer the uplift forces exerted on the membrane down to the structural deck below. To address this and protect the roof, a suitable cover board positioned under the membrane will hold the fasteners while maintaining compression and strength.

A notable example of loss due to wind uplift was evidenced when Hurricane Katrina struck New Orleans in August 2005. The well-documented Superdome, which did not include a cover board in the roof assembly, suffered a complete loss of the roof. Meanwhile, the New Orleans Arena located right across the street used a coated glass mat gypsum cover board and did not experience any noticeable loss. While the two can’t be compared as equals (both were roughly the same age, but the Superdome had a much larger roof surface, at a different pitch and height), the contrast in the level of damage is so stark, it stands as a good representation of how a gypsum cover board can enhance a roof assembly’s wind uplift performance.

Hail and Puncture

As noted, surveyed roofing consultants rank punctures as the most frequent event that reduces a roof’s lifespan. Both high-speed events (hail, flying debris, etc.) and low-impact puncture events (foot traffic, dropped tools, etc.) can understandably lead to multiple issues, including water intrusion, building down time, decreased R-value, reduction in wind resistance, and of course costly repairs.

Related to hail, glass mat gypsum cover board that is only ¹⁄₄ inch thick required a 2-inch hail stone in testing to fracture the cover board. Note that fleeceback membranes (i.e., single-ply membranes with a fibrous coating on the underside) were found to help mitigate the risk of hail damage, especially when used with a gypsum-based cover board. The significance of this testing lies in the fact that the frequency of hail storms that includes stones less than 2 inches is dramatically higher than storms with stones 2 inches or larger. Thus, choosing a cover board that can withstand large hail stones provides a great advantage.

Independent tests have also been conducted to measure various roof assemblies’ resistance to puncture by simulating heavy tools being dropped on the roof and by simulated airborne debris. These test results indicated that ¹⁄₂-inch HD ISO provided only marginal improvement over an assembly with no cover board. Meanwhile, ¹⁄₄-inch glass mat gypsum board has been found in testing to provide more than three times the puncture resistance compared to no cover board at all.

Foot Traffic

Although damage from hail impact is obvious, the damage caused by foot traffic is less immediately noticeable but can be even more pervasive. Foot traffic on adhered roof systems stresses the adhesive bond between the membrane and cover board as the materials deflect under load. On mechanically attached systems, the fasteners can be unduly stressed and loosened. On any system, stepping on debris on top of the membrane can cause a puncture. While a building is under construction, multiple trades end up using the roof deck as a staging and storage area. Foot traffic, wheel barrows, and equipment carts all generate loads that compress unprotected insulation and threaten the membrane with stretching and punctures. Foot traffic damage can occur first during installation of the roof itself, then from installation of rooftop items such as HVAC equipment and antenna systems, among others. Post-construction, routine maintenance of rooftop equipment continues the stress; this is why some roofing system warranties place a limit on the amount of rooftop traffic that can be tolerated.

Building owners rarely realize how much foot traffic their roofs are exposed to, but national surveys show that 68 percent of low-slope systems host HVAC and other equipment. The maintenance, emergency servicing, and inspection visits to all of that rooftop equipment create foot traffic by individuals who can be unaware of the damage caused by their boots and tools. Personnel with tool boxes and equipment also increase the likelihood of impact damage to the insulation caused by foot traffic. Reinforced pathways are often designated for maintenance personnel, but they are only effective if they are actually used instead of being bypassed for a shorter route across the roof.

Different roofing membranes are more durable than others based on their thickness. For example, an 80-mil TPO roofing membrane is reasonably expected to hold up better than a 45-mil TPO membrane. However, the cost difference between the two thicknesses is noticeable, and the heavier membrane brings different installation needs. A proven alternative is to use a glass mat gypsum cover board, which will allow a 45-mil TPO membrane to perform as well as an 80-mil TPO membrane alone in terms of puncture resistance, the type that results from foot traffic. Testing conducted by Intertek in York, Pennsylvania, in accordance with ASTM D4833 determined that with a thinner board and membrane, the tested glass-mat gypsum assembly delivered more puncture resistance at a lower overall cost.

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 ⁵⁄₈-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.

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.

Peter J. Arsenault, FAIA, NCARB, LEED AP, is a nationally known architect, consultant, continuing education presenter, and prolific author advancing building performance through better design. www.pjaarch.com, www.linkedin.com/in/pjaarch