Cover Board

A cover board is a rigid panel installed between the insulation and the roof membrane in a low-slope roof. It is typically 6-13 mm (1/4-1/2 in) thick. While insulation boards are chosen mainly for thermal value, a cover board is chosen for strength and surface quality.

Functions

• protects softer insulation from foot traffic, equipment, hail and dropped tools
• provides a smooth, sound substrate for adhering membranes, improving wind-uplift resistance
• separates incompatible materials, for example polystyrene from solvent adhesives or plasticised membranes
• improves fire performance of assemblies over foam insulation
• serves as a recovery board over an existing roof before a new membrane is installed

Common types

• Glass-mat gypsum board: non-combustible, moisture-resistant facers; widely used under adhered single-ply.
• High-density (HD) polyiso: lightweight, moisture-tolerant cover board made by the polyiso industry.
• Wood fibreboard: traditional, used especially under hot-mopped BUR and modified bitumen.
• Perlite board: an older mineral board used under bituminous systems.
• Cement and mineral boards, dense stone wool and asphaltic core boards in some systems.

Installation

Cover boards are mechanically fastened along with the insulation or adhered with foam or hot adhesive. Joints are offset from insulation joints. For adhered membranes, the bond strength of the board's facer to its core often governs the wind rating of the whole assembly, so boards must be listed in the tested system.

Recovery and fire

In reroofing, a recovery board laid over a sound existing membrane provides a clean, flat surface for the new roof. Non-combustible gypsum-based boards are also used to help foam-insulated assemblies achieve fire classifications in listed systems.