Several forces challenge roof-system durability, including:
• Fire
• Moisture exposure
• Sound intrusion
• Impact from hail and foot traffic
• Wind uplift.
Properly chosen cover boards can help reduce the effects of all these forces. Fire resistance is the sum of the components Fire resistance in a roofing system comes from all roof components working together—and the choice of cover board can either enhance or detract from fire resistance. Where the decking, insulation or membrane is combustible, a noncombustible cover board can contribute to a lower-risk fire rating.
Lower-risk fire ratings can both ensure code compliance, and help reduce insurance premiums. For example, roofing systems that qualify for a Class 1 rating from Factory Mutual Research (FM) will qualify for the lowest insurance rates from FM affiliated insurance companies. A UL Class A rating (for external fire exposure) can also help qualify for lower premiums. Those lower premiums are a direct payback for
enhanced fire durability. (For more details on fire ratings of roof assemblies with cover boards, refer to
the DensDeck® Technical Guide.)
Properly chosen cover boards can help reduce the effects of all these forces. Fire resistance is the sum of the components Fire resistance in a roofing system comes from all roof components working together—and the choice of cover board can either enhance or detract from fire resistance. Where the decking, insulation or membrane is combustible, a noncombustible cover board can contribute to a lower-risk fire rating.
Lower-risk fire ratings can both ensure code compliance, and help reduce insurance premiums. For example, roofing systems that qualify for a Class 1 rating from Factory Mutual Research (FM) will qualify for the lowest insurance rates from FM affiliated insurance companies. A UL Class A rating (for external fire exposure) can also help qualify for lower premiums. Those lower premiums are a direct payback for
enhanced fire durability. (For more details on fire ratings of roof assemblies with cover boards, refer to
the DensDeck® Technical Guide.)
Wind uplift is a durability factor In extreme wind situations, wind-generated forces trying to lift the roofing off a building can be hundreds of pounds per square foot (psf). For even moderate wind conditions, roofing systems must be tested to withstand at least 60 psf before
failure, and 90 psf ratings are becoming a default in many applications that use Construction Specifications Institute (CSI) or American Institute of Architecture (AIA) master specifications. As with fire ratings, higher wind-uplift ratings may qualify for lower insurance premiums, and cover boards can contribute to wind-uplift durability. Here’s how: Where the membrane is held down by a continuous coating of adhesive, wind performance relies on the strength of the substrate. If the membrane is bonded directly to a fragile, low-density insulation layer, uplift forces can literally pull the insulation apart. A cover board with good tensile strength, installed between the membrane and the insulation, can prevent this. Mechanical fasteners penetrate the insulation and transfer uplift forces directly to the rigid deck.
failure, and 90 psf ratings are becoming a default in many applications that use Construction Specifications Institute (CSI) or American Institute of Architecture (AIA) master specifications. As with fire ratings, higher wind-uplift ratings may qualify for lower insurance premiums, and cover boards can contribute to wind-uplift durability. Here’s how: Where the membrane is held down by a continuous coating of adhesive, wind performance relies on the strength of the substrate. If the membrane is bonded directly to a fragile, low-density insulation layer, uplift forces can literally pull the insulation apart. A cover board with good tensile strength, installed between the membrane and the insulation, can prevent this. Mechanical fasteners penetrate the insulation and transfer uplift forces directly to the rigid deck.
Cover boards can also enhance durability in roof systems using mechanical fasteners rather than adhesives. In these systems, wind forces may try to rock the fasteners sideways, and the fasteners can lose compression or even back out of the decking. A suitable cover board under the membrane can keep the fasteners from rocking and protect the roof. Impact resistance withstands hail and foot traffic
The third force that tests roof system durability is impact from hail or foot traffic. Without a protective cover board, impact damages both the insulation and the membrane. The rigid cells of low-density insulation foam don’t recover from impact compression, so crushing reduces the R-value and damages the bond with the insulation board’s facing layer. Insulation compression also forces the membrane to stretch, which makes it more vulnerable to puncture. Insulation damage from hail impact is obvious. The impact damage caused by foot traffic is less immediately noticeable, but more pervasive. Foot traffic, wheelbarrows and equipment carts all generate loads that compress unprotected insulation and threaten the membrane with stretching and punctures. Traffic damage comes first during installation of the roof itself, then from installation and maintenance of HVAC, PV systems, and other roof-top equipment. After construction, routine building maintenance continues the stress. This is why some roofing system warranties place a limit on the amount of roof-top traffic that can be tolerated. Cover boards can protect against both hail and foot traffic damage. Installed between the membrane and the insulation, a rigid cover board with good compressive strength distributes impact to prevent insulation compression. Cover boards also improve puncture resistance by supporting the membrane.
Moisture exposure can be external or internal. Moisture exposure can occur in the form of rain or dew during installation, as well as roof leaks once the roof is completed. Moisture can also come from condensation or migration within the assembly. All roofing components are to be protected from moisture exposure before, during and after installation. If components are inadvertently exposed to moisture, the ability to retain physical properties when the product dries is a measure of its durability and contribution to sustainability. Many products, like wood fiberboard, perlite and other materials containing organic components are physically changed by exposure. They swell, distort and are weakened. Products with a gypsum core like DensDeck Roof Boards may retain their physical properties and integrity when allowed to dry after incidental wettings. However, you should consult the product manufacturer for additional information concerning the moisture resistance and recommendations for each product.
Information presented in this article concerning roofing systems and assemblies is presented as a general guide for illustration purposes only. Please consult the appropriate system manufacturer or design authority for system specifications and instructions for any specific system or assembly. Georgia-Pacific Gypsum does not provide roofing design services. 2011 Georgia-Pacific Gypsum LLC. All rights reserved.Originally published 10/04. Rev 2/11.GP-TM Lit. Item #102271.
