Currently Reading

Waterproofing Glass Mat Backer Board

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.


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.

Related Stories

Quality Matters

Do Concrete Slabs Really Need Welded Wire Mesh?

In some soil conditions, eliminating wire in the slab can be a false economy ... at best

Quality Matters

Proper Flashing Practices for Fascia

How to avoid water damage to homes and save your reputation by employing proper flashing practices at fascia returns


The Leading Cause of New Home Structural Failures? Fill Dirt

As the leading cause of new home structural failures, fill dirt is one of the most important structural elements of a home. And when fill dirt…


More in Category


Create an account

By creating an account, you agree to Pro Builder's terms of service and privacy policy.

Daily Feed Newsletter

Get Pro Builder in your inbox

Each day, Pro Builder's editors assemble the latest breaking industry news, hottest trends, and most relevant research, delivered to your inbox.

Save the stories you care about

Lorem ipsum dolor sit amet lorem ipsum dolor sit amet lorem ipsum dolor sit amet.

The bookmark icon allows you to save any story to your account to read it later
Tap it once to save, and tap it again to unsave

It looks like you’re using an ad-blocker!

Pro Builder is an advertisting supported site and we noticed you have ad-blocking enabled in your browser. There are two ways you can keep reading:

Disable your ad-blocker
Disable now
Subscribe to Pro Builder
Already a member? Sign in
Become a Member

Subscribe to Pro Builder for unlimited access

Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.