Webinar Q&A

Advantages of using coverboards

Attendee questions

Advantages of Using Coverboards on Low-Slope Roofing Assemblies Webinar on September 15, 2021

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Are coverboards useful in fire-prone areas? Or are they not relevant unless the roofing material itself resists fire?

Fire-resistant coverboards protect the combustible roof system components. 5/8” DEXcell® Glass Mat Roof Board and 5/8” DEXcell® FA Glass Mat Roof Board are approved in fire-rated designs. UL P designs are tested fire-rated roof-ceiling assemblies. Both DEXcell Glass Mat Roof Boards and DEXcell Cement Board meet FM Class 1 and UL Class A fire ratings for roofing systems up to unlimited slope per UL 790/ULC S107. Use these products as part of class A, B or C roof covering that has been tested in accordance with UL 1256, ULC CAN-S126, or FM 4450. No additional thermal barrier is required as per IBC 2603. In addition, these products are approved components in specific UL fire-rated designs.

Is polyiso the most moisture resistant insulation?

Of the three insulations discussed in the presentation: expanded polystyrene, extruded polystyrene, and polyiso, the R-value of the polyiso is most affected by moisture intrusion. In addition, the polyiso will deform after prolonged exposure to moisture. For these reasons, it is recommended to install a fiberglass mat-faced or cementitious hard coverboard.

Can plywood be used on the interior face of a parapet?

Using plywood on the roof-side of the parapet is a common installation when dimensional stability and fire ratings are not required. It is recommended to verify with the system manufacturer what substrates are approved with the specified system.

Is it true that a FM rated roof assembly is only required in roofs that are insured by Factory Mutual?

Yes, Factory Mutual (FM) requirements only apply to buildings they insure. However, their requirements have become industry design standards for all commercial roof assemblies, regardless if the roof is insured by FM.

Hasn't R-value of polyiso been derated to its LTTR value? R=5.3+/-?

Long-Term Thermal Resistance (LTTR) is related to R-value. LTTR represents a product’s average thermal resistance over a 15-year period while R-value is the value of thermal resistance for the product as a new product. For detailed information on this LTTR value, refer to THIS article on Polyisocyanurate Insulation Manufacturers Association’s (PIMA) website. The R-value of each DEXcell® Roof Board is published on each product submittal sheet.

How are the recommended thicknesses of the boards established for use and location? Why wouldn't we use just 1/4" everywhere?

The thickness of the coverboard is determined by the architect, roof consultant, and system manufacturer based on specific project requirements and roof systems. Some considerations for using a thicker board are; flute spanability, thermal protection, acoustics, severe hail areas, and when a vapor barrier is specified in the system.

I am in an area prone to hail. Can you offer some comments on which of the coverboards you described will best protect low slope roof systems?

Currently, there are several options available for multi-ply systems (mod-bit) including DEXcell FA Glass Mat roof boards for both adhered and mechanically attached systems. The redundancy of layers in a multi-ply system offers a greater opportunity to protect the membrane and the insulation below, especially in adhered systems. It is significant that DEXcell FA is also listed in mechanically attached systems because protecting the membrane over the fastener and plate is difficult. Single-ply systems are much more difficult to protect from hail damage. Options currently being tested and explored include OSB, plywood, mineral fiberboard, and glass mat roof boards.

Are there applications that would require more than one layer of a coverboard?

No, there are no specific requirements to add more coverboard layers. However, if an increased acoustical value of the system is needed then adding mass helps to increase the STC/ OITC value. Refer to pages 20-21 of the DEXcell Roof Board Technical Guide for more information on STC/ OITC rated assemblies.

Can gypsum products be used as a thermal barrier on the roof deck? If so, which one would you suggest.

Yes, the system attachment method dictates the proper products for the assembly. The best product to use as a thermal barrier in a roof system is DEXcell Cement Board because it is unaffected by moisture. DEXcell Glass Mat and DEXcell FA Glass Mat are both available as Type X products in 5/8” thickness and can be used in fire-rated roof-ceiling assemblies.

I live in an area with minimal wind speeds and no hail so why should I spend the additional money for a fiberglass mat roof coverboard?

In addition to weather-related events, coverboards protect an owner’s roof system investment. Coverboards reduce the impact of human foot traffic on insulation due to routine maintenance and equipment installations. System failures often occur where equipment has penetrated the roof membrane system. Utilization of a moisture-resistant, hard coverboard can help mitigate the negative effects of moisture intrusion into the roof assembly. Coverboards contribute to the longevity of the roof by protecting the roof membrane above it and everything in the system under it.

What is the PSI for glass mat gypsum coverboards?

Fiberglass mat gypsum coverboards have a high compressive strength compared to most coverboard options. DEXcell Glass Mat and DEXcell FA Glass Mat coverboards have a compressive strength of 900 psi.

What is the bending radius for 1/4”, 1/2” and 5/8” fiberglass mat coverboards?

Each thickness of fiberglass mat coverboards have a different bending radius. Refer to the submittals for each product for guidance, DEXcell Glass Mat Roof Boards have a bending radius of four to six feet depending on the thickness. In addition, the DEXcell Cement Board has a published bending radius of five feet. These tight bending radii give the consultant and contractor design and installation options. Barrel roofs are a great opportunity for the use of fiberglass mat and cementitious roof boards.