Benefits of Rigid Cover Boards in Commercial Roofing Systems  

Photo courtesy of Georgia-Pacific Gypsum

Roofing system performance can be enhanced by incorporating the NRCA recommendation to use a non-combustible, rigid roof board as part of all low-slope commercial roofing assemblies.

On most commercial, institutional, and industrial buildings, roofing is a big investment, whether at the time of first construction or when it is time for it 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 high-risk situations like severe weather or fire. 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 historical perspective of how cover boards have evolved over time to support new types of roof assemblies and meet the growing needs of building owners. A typical roof system includes different layers of materials to ensure 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. On top of the insulation is placed a rigid cover board for protection of the insulation and to improve the durability of the roofing membrane, which is installed last over the assembly.

Image courtesy of Georgia-Pacific Gypsum

An example of a fairly common commercial (low slope) roof assembly is shown here with the addition of photovoltaic (PV) panels and include: A) PV Panels; B) Single-ply roofing membrane; C) Gypsum-based roof board used as a cover board; D) Rigid foam insulation (typically more than one layer); E) Gypsum-based roof board used as a thermal barrier resting directly on the metal roof deck.

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 used as a thermal barrier for fire protection (mitigates the 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. 

Between 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 during 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 marketplace. 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 with 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 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% 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. These 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.

Photo courtesy of Georgia-Pacific Gypsum

The materials used as cover boards have evolved as roofing systems have evolved, along with changing performance and marketing needs.

Throughout the 2000s, single-ply membranes have continued to grow in popularity, currently comprising approximately 80% of the overall market. In response, the demand for cover boards also increased. To feed this demand, high-density polyisocyanurate (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 footprint by reducing their reliance on fossil fuels led to an increase in alternative roofing systems such as vegetative and photovoltaic roofs.

Common Types of Cover Boards

As the use of roof board products has grown over time, the choices available have also grown. Here are the most common cover board 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 manufacturers 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.
• Wood-based boards are made of wood fibers or strands combined with resins to form flat panels. While such boards are commonly used in wood-based construction, they are less common in commercial roof construction since they offer no fire resistance and may swell or deform along the edges if they become wet.

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 Adverse Conditions

Roofing is a big investment, and it protects an even bigger investment in the building and its contents. That’s why durability in any roofing system is so important: it translates directly into lower total cost of ownership. Given the number of low-slope commercial roofs found currently in service, it’s key that architects and designers understand the forces that affect the durability of a roof assembly and the high-performance options that are available in order to choose the best solution for each project.

Within this make-up, 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 predominant fire testing bodies in the U.S. that are widely accepted in regards to fire issues, including external and internal exposure, code compliance, life safety issues, and insurance premiums.

• Underwriters Laboratories Inc. (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 non-combustible decks, only the spread of flame test is required. UL also identifies an interior classification of “P” assemblies, which 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. 

Images courtesy of Georgia-Pacific Gypsum

Gypsum products have a chemical makeup that stores chemically combined water that is released with heat and helps prevent the spread of fire.

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 crystallized 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% chemically combined water and 79% inert calcium sulfate. Put another way, one board of 5/8” 4’x8’ glass mat gypsum cover board contains two gallons of crystallized 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. The combination of the release of water vapor and the calcination typically keeps the unexposed side of the roof board from getting any hotter than 100 degrees Celsius until the water is all gone. This helps prevent the spread of a structural fire.

Moisture

According to a 2016 survey of roofing consultants conducted by Ducker Worldwide out of Troy, Michigan, moisture ranked second only to puncture in destructive events that negatively impact the life span 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. The 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 condensation. Not only do roofing products get wet 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 fiberboard 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. The difference is based directly on the rate of total water absorption of the products used in a roof. 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’X4’ 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.

Photos courtesy of Georgia-Pacific Gypsum

Moisture intrusion, from whatever source, can cause damage and deterioration, as shown in this wood fiberboard cover board that has gotten wet within the roof assembly.

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.

Two of the most influential factors that influence resistance to wind uplift are flexural strength and vertical pull strength. Flexural strength is critical for both adhered and mechanically attached roofing, but especially for mechanically attached roofing. Vertical pull strength is more critical for fully adhered roofing. Independent testing is done to determine both of these strengths for different types of roof boards. While different roof boards produce different results, coated glass mat boards generally perform the best in both conditions. Note that moisture intrusion can affect wind uplift performance by weakening the cover board. Different materials will be affected differently, with tested flexural strength loss of 15 to 71 percent.

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.

Photo courtesy of Georgia-Pacific Gypsum

During Hurricane Katrina in 2005, the Superdome, shown on the left, had a roof system without a cover board that was severely damaged. Across the street, the New Orleans Arena roof, which included a glass-mat gypsum cover board, remained intact.

Hail and Puncture

As noted, surveyed roofing consultants rank punctures as the most frequent event that reduces a roof’s life span. Both high-speed events (hail, flying debris, etc.) and low-impact (foot traffic, dropped tools, etc.) puncture events can understandably lead to multiple issues, including water intrusion, building downtime, decreased R-value, reduction in wind resistance, and, of course, costly repairs. Without a protective cover board, impact from hail can damage both the insulation and the membrane and may result in loss of roof use and loss of contents. Adding a cover board as a protective substrate between the foam insulation and the roofing membrane can moderate both these problems.

A rigid cover board with good compressive strength distributes impact to prevent insulation compression. Cover boards also help improve puncture resistance by supporting the membrane. But the board requires the right degree of compression strength. Wood fiberboard typically resists compression loads of 20 to 30 psi. That’s too low to protect the foam insulation from large hail. For installations where hail is a concern, a better choice would be a cover board that can handle compression loads in the 500–900 psi range (e.g., fiberglass mat gypsum board), which is hard enough to protect the foam insulation from the impact of very large hailstones but can still flex to cushion the membrane itself.

The protection provided by cover boards to roof assemblies in areas prone to hail activity is sizeable. Independent testing revealed that ¼-inch glass mat gypsum cover boards delivered 2.5 times or 150 percent more puncture resistance than no cover board at all per ASTM D5635. This testing, conducted in August of 2014 at Jim D. Koontz & Associates lab, measured the size of hail needed to fracture a cover board and fracture the insulation board beneath the cover board. Glass mat gypsum cover boards performed best in this testing, requiring a 2-inch hail stone (comparable to a hen’s egg) to fracture the cover board and/or insulation board. In contrast, either a 1-inch or 1.5-inch hail stone (comparable to a quarter or a walnut) was needed using gypsum fiber cover boards or HD ISO. A 1- to ½-inch difference does not sound like a huge swing in performance, but the frequency of hailstorms that include stones less than 2 inches is dramatically higher than storms with stones 2 inches or larger.

Fleeceback membranes are a great way to increase the performance of a roof assembly, but they are not an adequate substitute for a cover board. Third-party testing results using ASTM D 5635 Dynamic Puncture Resistance have shown that a glass mat gypsum board outperforms fleeceback membranes by 12.5–17.8 percent, depending on the membrane thickness.

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, wheelbarrows, 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 toolboxes 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.

Sound Resistance

Although sound transmission properties of a roofing system are not part of the physical protection of the roof, sound transmission does affect the quality of life inside the building. Reducing the noise from traffic, equipment, and aircraft can be a vital function of a roof system. Adding one or more layers of high-density material between the roof insulation and membrane can help attenuate outside noise and significantly increase the sound transmission class (STC) of the roof assembly.

Adding one or more layers of high-density cover board can provide greater sound isolation and reduce the disruption of high-level noise intrusion. For example, a 38-dB STC-rated roof assembly, incorporating 5/8-inch fiberglass mat gypsum underlayment and cover board, reduces 65-dB outside noise down to 27 dB inside. That level is well below acceptable background sound levels within most spaces.

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 which 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 life span of the roof and reducing scheduled maintenance, then the cost of cover boards can pay for themselves 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.

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 chart summarizes the suitability of 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.

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 re-roofing 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.

Sustainability Design Support

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 secure 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 and have been recommended by FM Global for inclusion per FM-1-15 Standard for Solar Roof Systems. Further, with PV systems lasting 25–30 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. The ability for vegetative roofs to hold water helps a building owner’s efforts to minimize stormwater runoff but also adds a considerable amount of weight to the roof assembly, up to more than 200 pounds per square foot. Choosing a cover board with high marks in compressive strength helps the assembly support the extra weight. 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

Image courtesy of Georgia-Pacific Gypsum

A vegetative roof assembly can include: A) Growing medium; B) Moisture retention mat; C) Drainage layer; D) Root barrier; E) Waterproofing membrane; F) ½ inch glass mat gypsum cover board; G) Insulation resting directly on the metal roof deck.

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 into 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 who has authored nearly 300 continuing education courses focused on advancing building performance through better design. www.pjaarch.com, www.linkedin.com/in/pjaarch