A roof is a significant investment that protects an even larger one-the building and its contents. That is why durability in any roofing system translates directly into lower total cost of ownership.

Durability is more than just toughness. It is also the ability to protect contents and occupants from fire, wind, water, and noise. Properly chosen materials can help reduce the effects of these forces.

One option that can contribute to durability is a cover board. A cover board is placed directly below the roofing membrane and above the rigid foam insulation. In this application, the board provides the primary support for the roofing membrane and protects the insulation.

Fire Resistance

Fire resistance in a roofing system comes from all roof components working together-and the choice of cover board can either enhance or degrade fire resistance. Where the decking, insulation, or membrane is combustible, a non-combustible 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 and comply with fire code requirements. This not only protects the buildings and contents but also the occupants who live and work in them.

Wind Uplift

In extreme situations, wind generated forces trying to lift the roofing system off a building can reach over a hundred 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 requirement in many specifications. As with fire ratings, higher wind uplift ratings may qualify for lower insurance premiums, and cover boards can contribute to wind uplift durability.

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 that's installed between the membrane and the insulation can prevent this.

Cover boards can also enhance durability of roof systems using mechanical fasteners. Mechanical fasteners penetrate the insulation and transfer uplift forces directly to the rigid deck. In these systems, wind forces may try to rock the fasteners sideways, and the fasteners and plates can lose compression or even back out of the decking. A suitable cover board under the membrane can keep the fasteners from rocking and strengthen the roofing system.

Impact Resistance

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 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 rooftop traffic that can be tolerated.

Cover boards can protect against both hail and traffic damage. Installed between the membrane and the insulation, a rigid cover board with good compression strength distributes impact loads to prevent insulation compression. Cover boards also improve puncture resistance by supporting the membrane.

Water Intrusion

Water 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.

There are two key moisture issues that facility managers need to consider:

Many cover board materials, including wood fiber and perlite, will absorb and retain a high percentage of water by weight. Further, the moisture causes swelling and permanent loss of strength in the material. Other materials-such as moisture treated gypsum-absorb much less water to begin with, don't expand in the presence of moisture, and retain their physical properties through a wet-dry cycle.

Cover Board Selection

If the proper cover board can improve durability in many roof systems, how do facility managers select a cover board material? The seven most common cover board materials are:

How do facility executives compare the performance of these different materials? The charts included with this article summarize the key dimensions of fire resistance, strength, and moisture resistance on a standardized scale of one to five.

By choosing the proper coverboards, facility executives can protect their investment in both the roof system and the contents of their buildings.

Schneider is technical development manager at G-P Gypsum. Contact Schneider at rschnei@gapac.com. Have you experienced problems with your roofing system? Send comments on this article to schwartz@groupc.com.