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A cover board is a thin layer of insulation (such as perlite or wood fiberboard or dense polyisocyanurate) or glass mat gypsum roof board, preferably pre-primed. Plywood and oriented strand board are occasionally used if required for high-wind speed uplift warranties. Cover boards should be placed over the primary thermal insulation (typically one of the plastic foam insulations) in order to provide enhanced physical characteristics, such as improved fire and compressive resistance, prevention of delamination of facers due to traffic on the roof, provision of improved wind uplift resistance, and to avoid blistering or avoid a compatibility problem.
Cover boards are also commonly used in re-covering to improve the application surface; to cover joints between insulation boards; and to provide a separation layer between the existing and new roof membranes. When used in this application they are often referred-to as “recovery board.”
Cover boards in commercial construction are typically glass mat gypsum roof board (preferably pre-primed) or perlite. Some materials used for cover boards are sometimes specified for use as an underlayment board directly over steel roof decks in order to provide a thermal barrier to provide fire protection between steel decks and certain types of plastic foam insulation (the IBC specifies thermal barrier requirements) or to provide a smooth surface for a vapor retarder. Note that perlite will absorb water more readily than glass mat gypsum board; this may be a consideration if there is a leak.