Cover Boards for Low-Slope Roofing Systems

Resist fire, wind uplift, impact and hail, moisture, mold and foot traffic by specifying the best cover board for the project
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Cover Boards and Roof Types

Workers roll out the white roof membrane prior to attaching it to the roof system over the cover board.

Photo courtesy of National Gypsum®

Workers roll out the white roof membrane prior to attaching it to the roof system over the cover board.

 

With the need for and desirability of cover boards established, the question then arises: Which cover board serves the project the best? The first issue will be determining which cover board is compatible with the specified roofing type.

Glass mat gypsum board — A water-resistant treated gypsum core with embedded fiberglass mats on both sides. Common thicknesses are 1/4 inch, 1/2 inch and 5/8 inch. It is compatible with all roofing types: single ply, built up, app mode bit, SBS modified bitumen and fluid applied roofs. (Standard: ASTM C1177)

Cement roof board — A noncombustible, water-durable, high-performance cement-based roofing cover board that is compatible with all low-slope roofing types. Common thickness is 7/16 inch. (ASTM C1325)

Fiber-reinforced gypsum board — This is a high-density gypsum reinforced with cellulose fibers. Common thicknesses are 1/4 inch, 3/8 inch, 1/2 inch and 5/8 inch. It is compatible with all roofing types: single ply, built up, APP modified bitumen, SBS modified bitumen and fluid applied roofs. (ASTM C1278)

Paper-faced gypsum board — This is a gypsum core with paper facers on both sides. It is for use as a thermal barrier. But must be Type X product. Common thicknesses are 1/2 inch and 5/8 inch. Compatible only with single-ply roofs. (ASTM C1396)

Wood fiberboard — Wood fibers or cane fibers bonded together with resins make up this product. Common thicknesses are 1/2 inch and 1 inch. It is compatible with single ply, built up, SBS modified bitumen and fluid applied roofs. (ASTM C208)

Asphaltic board — This is an asphalt core with fiberglass facers on both sides. It is 1/8 inch thick and is 4 ft. x 4 ft. (ASTM D6451)

Mineral fiberboard — Bonded fiberglass or rock wool with an adhered kraft paper face Common thicknesses are 1/2 inch to 1 inch. (ASTM C726).

Perlite board — Expanded volcanic materials combined with organic fibers and binders with an asphalt emulsion coating on the top side Common thickness is 1/2 inch. (ASTM C726).

Extruded polystyrene board (fanfold) — Extruded polystyrene beads formed into a fanfold board. It is commonly 1/4 inch thick, and comes in lengths of 4 ft. by 50 ft. (ASTM C578)

High-density polyisocyanurate board — This is composed of a polyisocyanurate foam core with fiberglass facers on both sides. It is high density with 100+ psi as compared to normal iso board with 16 psi to 20 psi. Common thickness is 1/2 inch. (ASTM C1289)

Plywood/OSB — Plywood is created by combining thin layers of wood veneer, while OSB (oriented strand board) is non-veneer panels made up of compressed layers of cross-laminated wood strands. Common thicknesses are 1/2 inch and 5/8 inch.

Noise and light flanking reduction using an air transfer silencer

Photo courtesy of National Gypsum®

A worker is attaches the fasteners through to membrane into the roof system along the seam of the membrane. The seam of the adjacent membrane will overlap and will be adhered to the membrane below it, creating a redundancy at the seam overlaps.

 

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Originally published in March 2014

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