A roof 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 translates directly into lower total cost of ownership. Durability is more than just toughness. It's also the ability to protect contents and occupants from fire, water and noise.
Several forces challenge roof system durability, including:
- Wind uplift.
- Impact from hail and foot traffic.
- Water intrusion and mold growth.
- Sound intrusion.
Fire ResistanceFire 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
Wind UpliftIn 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 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.
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 ResistanceThe third force that tests roof system durability is impact from hail or foot traffic. Without a protective cover board, impact can damage 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 of HVAC, antenna systems, and other rooftop equipment. After construction, routine building maintenance continues the stress. This is why some roofing system warranties place a limit on the amount of rooftop 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.
Water IntrusionWater is another force that tests roof-system durability. Moisture can come from leaks or from internal condensation caused by the daily and seasonal temperature cycles of the building.
When moisture is retained in a roofing system, it can initiate a vicious cycle; the retained moisture reduces the R-value of the roof assembly. The reduced R-value accelerates the rate of condensation within the roof, which generates even more moisture and lowers the R-value even further. In the worst case, the roof system can become completely saturated. A wet roofing system can deteriorate and collapse if the cover board does not provide wet strength.
Where a cover board is part of a roofing system, there are two key moisture issues:
- What are the moisture absorption characteristics of the board?
- Does absorbed moisture damage the integrity or mechanical strength of the board?
Mold GrowthMold requires four factors for growth: spores, proper temperature, moisture and an organic food source. The first two, spores and proper temperatures, are universally present in roof systems. The third, moisture, can be partly controlled through proper building design.
Adequate roof ventilation and vapor retarders can help reduce condensation and moisture retention in the roof system but can't eliminate moisture entirely. One element that we can practically limit is the organic food source. A cover board that contains primarily inorganic elements, such as gypsum board with fiberglass facings, can help to reduce mold growth when compared to other roof board substrates with high levels of organic materials.
Sound IntrusionThe sound transmission properties of a roofing system are not part of the direct protection the roof offers, but sound transmission affects the quality of life inside the building. In general, people like it quiet inside occupied buildings, and studies have shown that students perform better in a quiet environment.
Keeping out noise from aircraft, traffic and equipment is one of the tasks of a roof system. Adding one or more layers of high-density material, such as a gypsum cover board, can help attenuate outside noise and significantly raise the Sound Transmission Class (STC) of the roof assembly.
Cover Board SelectionIf the proper cover board can improve durability in many roof systems, how do you select a cover board material? All of these factors must be considered together:
- Fire resistance.
- Strength to resist both wind uplift and impact.
- Water intrusion characteristics.
- Sound transmission qualities.
- Asphaltic board - fiberglass-faced asphalt board in a variety of thicknesses.
- Mineral fiberboard -lightweight, semi-rigid rock wool or fiberglass board.
- Plywood or oriented strand board (OSB) - plywood incorporates thin sheets of veneer in layers, while OSB is made up of cross-laminated layers of oriented, resin-bonded wood strands.
- Wood fiberboard - organic fibers bonded with resins.
- Perlite - mineral aggregate board with cellulose binders and sizing agents.
- Paper-faced gypsum. Gypsum core with paper facers on both sides.
- Glass-mat faced gypsum board - treated, high-density gypsum board with fiberglass mats embedded on both sides (such as DensDeck® Roof Board).
Comparing PerformanceHow do you compare the performance of these different materials? Summarizing the key dimensions of fire resistance, strength and moisture resistance, we find that the top two performers in the category of fire resistance are paper-faced gypsum board and glass-mat faced gypsum board.
The top two performers in the strength category are plywood/OSB and glass-mat faced gypsum board, while the top two performers in moisture resistance category are asphaltic board and glass-mat faced gypsum board.