Negatives include paper facers deteriorating in the presence of moisture, destabilizing the product. It is not meant for use as a cover board, but rather only as a thermal barrier on the deck. It has limited R-Value, and is typically excluded from roof manufacturers’ warranties. Only Type X gypsum can be used for this purpose.  This product is relatively heavy for installers to handle. With a number of options of glass mat Type X gypsum products available, this product should not be the specifier’s first option.

Wood fiberboard — This material has a long track record of use as a cover board in the roofing industry. It is relatively lightweight with good compressive strength. It can be used with a wide range of roofing systems. It is one of the less expensive boards available on the market. Some sugarcane cellulose is made to make the board, as opposed to using just wood fibers.

On the negative side, it is highly susceptible to moisture absorption; it acts as a sponge. Loss of structural stability, rot and decay once exposed to water within the roof system. This type of board is combustible. This product readily absorbs adhesives used to attach the membrane, increasing the cost of the system installation. Used rarely in many parts of the country because of improved product alternatives.

Perlite board — This material has good heat resistance and is relatively lightweight, which the workers will appreciate.

But that comes with a cost. The structural integrity disintegrates when the board is exposed to water. It is easily breakable, and prone to corners breaking off. It cannot adhere any of the single ply or self-adhesive membrane products. It is rarely used in many parts of the country anymore due to improved product alternatives.

Mineral fiberboard — This material has good heat resistance and is dimensionally stable.

While this product does not readily absorb water, it can lose its structural integrity if wet for a long period of time. It has limited availability in the marketplace and is used rarely in many parts of the country due to improved product alternatives.

High-density polyisocyanurate board — This is a very lightweight and easy to handle material, which reduces loading time. It adds a small amount of R-Value to the roof system. Provides a firm surface for walkability.

However, it has low compressive strength relative to the more rigid products available  (100 psf vs. 600 to 1,200 psf). It has fiberglass facers on both sides, which may experience the historical facer delamination issues iso has had. Polyisocyanurate insulation has had historical problems with cupping, warping, etc. Some industry observers believe this product may be too new to determine whether that will continue to occur. It has slope restrictions for fire ratings. High Density iso is 100+ psi versus ratings of 500 psi compared to 600 / 1250 psi for gypsum-based products.   

Extruded polystyrene board — This material is extremely lightweight and transportable and easy to install. Workers can cover large areas fairly quickly. It is very water resistant due to its closed-cell structure.

On the other hand, this product provides no protection as a fire barrier. And it does not prevent damage from hail or foot traffic. The fanfold configuration can cause high and low points in the roof. And to top it off, it’s relative expensive compared to other roofing cover boards.

Plywood/OSB — Of course these products have very high compressive strength and good dimensional stability. They can improve wind uplift ratings when tested as part of a single ply roof assembly, according to ASTM D 3501, compression for wood-based structural panels.

Then again, these products are highly combustible, and efforts to chemically treat them for fire resistance have caused other issues. The fire-rated version of the product can experience problems with having adhesives bonding to it properly. These boards are relatively heavy and hard to work with. Most alarming, they are susceptible to absorbing moisture and will support the growth of mold when wet as they provide the food source that mold needs.

Asphaltic board — This is the thinnest board available, which could be helpful in case of height limitation during installation. Plus, it is very water resistant.

However, it does not add to fire resistance, nor does it improve fire ratings. It has low impact resistance, and very little compressive strength.

Conclusion

For a superior building that is high performing and long lasting, the roofing assembly must perform exceptionally well. A superior roofing assembly comes about by design. The addition of a roofing cover board helps insure the long-term viability of a roofing assembly, and helps avoid leaks and damage later on. We have discussed different roofing assemblies for low-slope roofing, and the standards that apply to them. We have examined the external forces that impact a roof, and how those forces are tested. Finally, we took an expansive look at the various types of roofing cover boards that you, as the designer and/or specifier, can choose from, and gave you the necessary information to choose the right roofing board for the best protection of the building.

Known for providing innovative solutions for sustainable design and green building practices, National Gypsum is a fully integrated manufacturer of building products. Primary emphasis is on its three core brands — Gold Bond® gypsum board, including the PURPLE® family of drywall products, ProForm® drywall finishing products and PermaBase® cement board products. http://www.nationalgypsum.com/