The first requirement of a cover board is that it be compatible with the roof-cover material, installation method and approved by the membrane manufacturer as an appropriate substrate. Additionally, it must be compatible with the adhesive and attachment method. The project conditions, building type, use, time of year for installation and weather conditions also need to be considered. My firm predominately specifies the cover board be provided by the membrane manufacturer to ensure compatibility and a clear line of liability if concerns should arise.
There are numerous cover boards from which to choose. For decades, the industry cover-board workhorses were perlite and high-density fiberboard that worked well with bituminous products and hot-asphalt attachment. As the single-ply market matured, the need for a less fibrous cover board rose. In response, modified gypsum cover boards were developed.
Following are the most common cover boards in use today:
Glass-mat-faced and Fiber-reinforced Gypsum: Gypsum boards had been used in the roofing industry as a substrate and cover board but lacked resistance to moisture and the paper facer often proved problematic. Atlanta-based Georgia-Pacific and Chicago-based USG Corp. developed modified gypsum boards that have integral moisture resistance and no paper facer. They are produced in various thicknesses: 1/4, 3/8, 1/2 and 5/8 inch. The integral nature of the board makes it an excellent substrate to which a single-ply membrane can be adhered. They are produced in 4- by 4-foot and 4- by 8-foot sizes. The packaging of these boards can become quite heavy. I suggest the pallets be split prior to placement on the roof and then spread out on the roof to avoid overloading.
High-density Wood Fiber: Composed of compressed wood fiber, these boards are hydroscopic and need close attention in the field and during installation to prevent moisture absorption. Typical thicknesses are 1/2 inch and sizes are 4 by 4 foot and 4 by 8 foot.
High-density Polyisocyanurate Insulation: Through technology, and with consideration for worker ergonomics, polyiso manufacturers developed thin polyisocyanurate boards of high density, approaching 100 psi. The boards are light and easy to install. The facer on these boards is a double-coated fiberglass, which is resistant to mold and provides a good surface to which to bond.
Perlite: Perlite is one of the oldest cover boards. Manufactured of expanded perlite, the board is inert and is a good fit in asphalt attachment. The board’s small size of 2 by 4 feet makes installation easy.
Plywood/OSB Board: While not often used, plywood and oriented strand board are used when a high degree of wind-uplift resistance and/or roof-cover support is required.
Cement Board: This is a newer cover-board product. It is non-combustible and moisture resistive and tends to be designed for conditions of periodic wetting potential.
Asphaltic Core Board: These boards are thin (1/8 and 1/4 inch) and are semi-rigid. They actually are quite flexible, which allows for conformance over irregular surfaces without fracturing. They are utilized in bituminous applications (see photo, page 62).