Hard roof coverboards protect the roof membrane above and the insulation below. During this Construction Specifier "Ask Me Anything" webinar, Warren Barber and Brian Randall explain what a coverboard is, how hard gypsum and cement coverboards differ and why they matter for wind uplift, fire, hail, human impact, moisture and sound. It also covers building code, warranty requirements, board thickness and how to keep a coverboard in the specification when cost-cutting is proposed.

Key Takeaways

• A hard coverboard is board stock rated over 700 PSI that protects the weatherproofing membrane above it and the insulation below it.
• Five external factors drive the need for a hard coverboard: wind uplift, fire, hail and impact, moisture and sound.
• The International Building Code (IBC) Chapter 15 does not explicitly require a coverboard. It does set performance standards for fire, wind uplift and impact resistance, and the speakers say a hard coverboard helps a roof system meet them.
• Most 20-year system manufacturer warranties, and nearly all 30-year warranties, require a coverboard.
• Hard coverboards can earn Class A fire ratings and expand the allowable roof slope for those ratings.
• Human activity on the roof, such as maintenance crews, HVAC work, solar service and dropped tools, can affect a roof as much as or more than hail.
• Industry statistics cited in the webinar suggest about 70% of commercial roofs installed in a given year have no coverboard.

Who Are the Speakers?

• Warren Barber is Senior Manager, Gypsum and Specialty Systems Tech Cell at National Gypsum Company. His specialties include product development, claim mitigation and resolution, and sales and marketing to the AEC community.
• Brian Randall is Director of Business and Technical Development at National Gypsum Company. He has four decades of experience in gypsum panel manufacturing and joint compound R&D, and he developed glass mat face gypsum panels.
• Farheen moderated the session for Construction Specifier.

Video Transcript

This content is a summarized, edited version of the Ask Me Anything webinar from Construction Specifier, March 26, 2026. It has been edited for clarity, readability and web publication. Filler words, repeated phrases and transcript artifacts have been removed while maintaining the intent of the original conversation. 

About National Gypsum and DEXcell Roof Boards

Who is National Gypsum?

National Gypsum is well known to the construction, architectural and contractor communities. The company is in its 101st year of making drywall gypsum board and is fully integrated, with its own raw material supply including paper and rock. It also offers a line of ProForm finishing products and a cement board division called Permabase. National Gypsum has been in the coverboard business for 13 years.

What is a roof coverboard?

A coverboard is board stock that protects the weatherproofing membrane above it and the insulation below it. Early on, the material was called an overlayment or underlayment. This webinar focuses on hard coverboards, which are boards rated over 700 PSI. Their purpose is to extend roof life, improve durability and resilience, enhance overall performance, protect critical buildings and assets, and improve thermal performance.

What was used before coverboards?

Brian Randall explained that roofs in the 1970s and earlier typically had a concrete or metal base with a three- or four-ply built-up roof (BUR), often topped with river rock. A wood fiber or perlite board was often adhered beneath it. These systems were very heavy, so wind uplift was not a major concern.
In the 1970s and 1980s, R-value became a concern and insulation was added above the deck. Foam insulation can compress when someone walks on it or drops a tool, and the single-ply membranes introduced in the 1980s can be punctured. The coverboard concept emerged when Factory Mutual (FM) first listed a coverboard, at a time when no term for coverboard existed. Today, much of a roof system’s cost is in its R-value, and a coverboard protects that insulation and the membrane while adding durability and wind uplift resistance.

What DEXcell hard coverboards does National Gypsum offer?

• DEXcell Glass Mat Roof Board is used primarily for mechanically attached systems.
• DEXcell FA is used for fully adhered (glue-down) systems. It has the same core and mats as the glass mat board, plus a coating that helps spread adhesive and keeps it from absorbing into the core.
• DEXcell FA VSH (very severe hail) is a sister product designed to meet the FM single-ply very severe hail requirement.
• DEXcell Cement Roof Board comes from the PermaBASE Cement Board division. It was re-engineered for roofing with a smooth, flat surface for single-ply adhesion and no telegraphing, plus polystyrene beads in the core for flexibility and lower weight.

How are glass mat roof boards made?
Brian Randall invented fiberglass-faced gypsum products, including roof coverboards, by replacing the paper facing with fiberglass mat. During wet manufacturing, the gypsum slurry absorbs into the mat and the gypsum crystals solidify, creating a mechanical bond so the board will not delaminate. The speakers noted a warranty on DEXcell FA for parapets.

How is DEXcell cement roof board constructed?

The cement board core has polystyrene beads that reduce weight and add strength, fiberglass scrims on the face and back, and a wrapped edge. National Gypsum markets it as “the board to get whenever it’s wet.” It has superior moisture resistance, and the FM listings achieved with DEXcell FA are also achieved with cement board. It works in coastal and high-wind applications and can be torched or hot mopped. The speakers said it is not as widely accepted as gypsum coverboards but resonates well with the consultant community.

Why Use a Hard Coverboard? Five External Factors

Where should hard coverboards be used?

National Gypsum’s guidance is that any building with expensive people or expensive equipment in it should use a hard coverboard. Examples include hospitals with immune-compromised patients, schools, office buildings and data centers, which cannot go down even for seconds without losing revenue. The speakers also pointed to retail stores such as Walmart or Target, where a roof failure or wind event can close the store and cost a day’s revenue.

What are the benefits of a coverboard?

Adding a hard coverboard generally gives higher wind uplift and storm resistance. It also extends the service life of a single-ply membrane, and the speakers noted that the NRCA has recommended coverboards. Hard coverboards have been proven since they came into existence in the mid-1980s.

How does a hard coverboard help with wind uplift?

Wind uplift is the most prevalent reason to use a hard coverboard. Building height, wind speed and topography all matter. As wind hits a wall, it travels up and over the roof and creates negative pressure that pulls on the roof. Typical failure points include the plates in a mechanically attached or heat-welded system, the membrane seams in an adhered system, or the interface between the mat and the core.

How does a hard coverboard improve fire resistance?

FM and Underwriters Laboratories (UL) rate roofs for fire from inside, such as the FM calorimeter test, and for fire from outside, such as spread of flame. A hard coverboard made of gypsum or cement board can give a Class A rating, the highest fire resistance available. It also prevents burn-through from the top. Some membranes earn a Class A rating only on a very low slope, but with gypsum and cement boards the rating can extend to slopes of 5:10 or 5:12. Slope matters because a greater slope creates more opportunity for flame spread.

How does a coverboard protect against hail and human impact?

Hail gets the attention, but the speakers said human impact on a roof makes a bigger difference. Foot traffic, moving equipment, dropped tools and heavy HVAC units all stress the roof, especially on data centers with significant air conditioning equipment. FM’s very severe hail rating is not confined to the central region of the FM hail map, and baseball-sized hail has been found in many other states. With a BUR or two-ply modified bitumen system, the half-inch DEXcell FA can often provide a very severe hail rating. For single-ply systems, National Gypsum offers 5/8-inch DEXcell FA very severe hail, described as “a gypsum board on steroids.” The speakers said these ratings can be provided anywhere in the U.S. The FM very severe hail test shoots a 2-inch ice ball at about 110 miles per hour. The speakers said FM’s change from severe hail to very severe hail was far more than a doubling of severity, so National Gypsum had to develop new approaches to pass it.

How does a coverboard help with moisture?

The coverboard is not the waterproofing layer, but it helps mitigate moisture issues if water gets into the roof. Some coverboards are highly absorbent. National Gypsum engineered its core with silicone oil to help prevent the passage of moisture, so boards can get wet and dry out. The speakers noted that a roof is typically 8 to 9% of the cost of new commercial construction, yet moisture accounts for about 90% of roofing litigation.

Can a coverboard help with sound isolation?

Yes. National Gypsum has several dozen tests showing that adding layers of a high-density product like its cover boards, along with low-density insulation, provides high sound resistance. This is useful, for example, for a school near an airport. The speakers noted that acoustic performance depends more on what is below the deck, but the added mass of gypsum above the deck also helps the Sound Transmission Class (STC) rating.

Which roofing systems and warranties include DEXcell?

Specifiers often ask who offers warranty and testing approvals. DEXcell can be included in the warranty systems of nearly 20 roofing system manufacturers, so it can be specified in most roofing systems with confidence. The speakers said it took about 10 years to accumulate all that testing.

Can I request a lunch and learn?

Yes. National Gypsum has an architectural team in the field that delivers lunch and learn presentations. It goes to market with independent manufacturers’ reps, many of whom are certified to give AIA lunch and learn presentations on its behalf. Attendees can also email the speakers to request an attendance certificate for the webinar.

The video is also available for view on Construction Specifier's website