Originally used to prevent blistering incidences with built-up roof (BUR) membranes, cover boards now are recognized as good roofing practice for low-slope roof systems. Cover boards are flat or tapered stock materials ranging in size from 2 by 4 feet to 4 by 12 feet with thicknesses from 1/8 of an inch to 1 inch depending on material composition.
Cover boards often are used between roof membranes and rigid roof insulation in low-slope roof assemblies to enhance overall roof system performance. There are several cover board options available in the current roofing market.
On Aug. 10, 1978, NRCA issued Interim Alert Bulletin #4, "Membrane Blistering Over Foam Roof Insulation," in response to blistering incidences on BUR membranes. The bulletin noted using hot bitumen over some polyurethane and composite polyurethane roof insulations created degassing and vapor release, resulting in a blistered roof, and recommended the following: "Over the top surface of the polyurethane insulation, install a thin layer of wood fiberboard, perlitic board or glass fiber board insulation, staggering the joints from the layer below, and then apply the roof membrane as specified by the designer."