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Waterproofing Glass Mat Backer Board

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Quality Matters

Waterproofing Glass Mat Backer Board

Follow these best practices for a reliable installation


By John Koenig March 5, 2021
Man installing backer board for tile
Glass mat gypsum backer board is a worthy alternative to cement-based panels, but still requires attention to detail to perform properly. | Photo: courtesy USG

Although cement backer board is one of the most commonly used tile underlayments for wet-area walls, a high percentage of production builders have switched to glass mat gypsum board products, such as Georgia Pacific’s DensShield or CertainTeed’s Diamondback Tile Backer. Here’s why I think that’s a wise move, and provide some tips for correctly installing glass mat gypsum products.

Cement Backer Board vs. Glass Mat Gypsum

Despite its popularity, cement backer board has its drawbacks:

Glass mat gypsum doesn't have these issues. The panel product is a water-resistant silicone-treated gypsum board that’s topped with a fiberglass mat surface, which creates a built-in moisture barrier. It weighs less than cement backer board, is easy to cut—simply score and snap it like drywall—and it’s mold-resistant and doesn’t require a full-surface waterproof coating.

However, a trouble-free installation depends on the right framing details, fasteners, and joint/fastener treatment.

Which Framing and Fasteners to Use

Glass mat gypsum comes in 1/2-inch and 5/8-inch thicknesses. When installing 1/2-inch-thick board, framing must be a maximum of 16 inches on center, though you can get away with 24-inch o.c. framing by installing blocking behind every seam. Assign someone—a job superintendent or quality inspector—to ensure proper stud spacing after the plumber finishes the rough-in, then add blocking, as needed. The 5/8-inch boards can be installed over 24-inch o.c. framing without blocking, but you should still inspect the framing after the plumbing rough.

Fasteners should be installed every 6 inches for walls and ceilings. Drive fasteners flush with the coated surface—don’t countersink. You can use galvanized roofing nails, but screws are better. However, screws must be corrosion-resistant, so be sure to use galvanized screws instead of regular drywall screws.

Trouble-free installation depends on the right framing details, fasteners, and joint/fastener treatment

Sealing Joints Against Water Intrusion

Even though glass mat gypsum comes with a fiberglass surface layer, fastener heads, field joints, and corners must be sealed against water intrusion. Do this by first applying an ASTM C920 flexible silicone sealant over all fasteners, field joints, and corners prior to installing 2-inch fiberglass mesh tape embedded in a skim coat of thinset material used to set tile. You should seal penetrations and abutments to dissimilar materials as well.

Glass mat backer board installed
A C920 flexible silicone sealant covering all fasteners and joints not only protects against water infiltration at those points but also shows the proper fastening pattern. | Photo courtesy Wedi

Installing the sealant and mesh tape as described can be enough to waterproof the joints. However, most tile installers prefer to have backup protection. That means an additional minimum step of finishing the fastener heads, field joints, and corners with an additional two coats of a liquid-applied waterproofing membrane such as RedGard.


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Many installers also prefer additional waterproofing in vulnerable horizontal areas such as shower benches, windowsills, or wall caps. They apply RedGard to the entire surface in these areas where moisture has the chance to collect. 

The best way to ensure these installation and waterproofing details are done right is to have the backer board installed and finished by the tile contractor, not the drywaller. If the drywaller does hang the board, sealing the joints should still be included in the tile contractor’s scope of work.  

John Koenig drives quality and performance in home building as a building performance specialist on the PERFORM Builder Solutions team at IBACOS.

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