Cement board, also known as fiber cement board or backerboard, is a common building material used for countertops, flooring, walls, and other wet areas like bathrooms and laundry rooms. Cement board is moisture and mold resistant, providing a durable substrate for tile and other finishes. However, cement board is porous and rough, with seams between each board that need to be filled and sealed properly before applying tile or other surfacing materials. Selecting the right seam sealer is important to create a waterproof surface and prevent damage or leaks under tiles.
Page Contents
What is cement board?
Cement board, often sold under brand names like HardieBacker, Durock, PermaBase, and WonderBoard, is a mixture of Portland cement, sand, and cellulose fibers. The cement and sand give it strength and fire resistance, while the cellulose fibers provide flexibility and resistance to cracking. Cement board is available in a variety of sizes, like 3 x 5 foot panels or 2 x 2 foot squares. The boards are nailed or screwed to the wall or subfloor. Gaps are left between cement boards, which are then filled with a seam sealer.
Cement board provides a solid, stable substrate for tiling and flooring. It does not expand or contract as much as wood, helping prevent cracked grout and tile. It won’t rot or deteriorate from water exposure. It also serves as an effective backing material behind tubs and showers, protecting walls from moisture damage. Cement board has become a standard for wet areas like bathrooms, laundry rooms, pools, and more.
Should cement board seams be taped?
Yes, cement board seams should always be taped and filled with a seam sealer. The seams between cement board panels have small gaps and ridges from manufacturing and installation. If these seams are not properly sealed, they can allow moisture to penetrate behind tiles, leading to damage of the materials underneath.
Taping and sealing the seams protects the substrate and provides a watertight surface for tiles and finishes. It also helps prevent cracked grout and tiles by evening out the seams to reduce differential movement. Building codes and cement board manufacturers specify that all seams and joints should be filled and taped with a specialized seam sealer.
What can you use to seal cement board seams?
There are several good options for sealing cement board seams:
Mesh tape and thinset mortar
Fiberglass mesh tape embedded in thinset mortar is a very common and effective seam sealer. The mesh tape, usually 2 inches wide, is pressed into the thinset mortar along each seam. This fully covers and reinforces the seam while leveling uneven areas. It dries to a hard, durable finish. Mesh tape and thinset can be used for both floor and wall installations.
Alkali-resistant tape
Specialty alkaline-resistant (AR) fiberglass tapes are also available for cement board projects. These are designed to be more chemically compatible with the cement board and resist mineral salts that could corrode standard fiberglass tape. AR tape provides good seam sealing and extra insurance against deterioration.
Latex-modified mortar
Latex-modified thinset mortar adds flexibility and adhesion. Latex-modified mortar is especially recommended for floor seams that need to resist shear forces from movement. The latex makes the mortar more waterproof as well.
Silicone sealant
Using 100% silicone along cement board seams creates a flexible, waterproof seal. Silicone adheres well and won’t crack or deteriorate. It works for sealing corner joints and gaps. Silicone sealant is often used in combination with tape and mortar.
Unmodified thinset
Standard, unmodified thinset mortar also works for sealing seams, especially on walls. However, it is more prone to cracking and may not be as long-lasting as more flexible latex-modified options. Make sure to use an appropriate thinset recommended for the installation.
Epoxy
Two-part epoxy adhesives seal seams with a durable, permanent waterproof bond. Epoxies provide excellent adhesion to cement board while allowing some flexibility. Products are available specifically designed for cement board application. Epoxy seam sealers are often used for heavy-use floors.
How to seal cement board seams
Sealing cement board seams takes a few simple steps:
Prep the seams
The seams need to be clean and free of dust and debris. Lightly sanding or grinding each seam improves adhesion. Vacuum up any dust before applying seam sealers.
Apply seam sealer
Apply a continuous bead of silicone sealant, thinset mortar, epoxy, or other seam sealer evenly along each seam, covering all gaps and ridges. Spread the sealer with a trowel or putty knife 2-4 inches wide centered over the seam.
Embed the tape
For mesh or AR tape, center it over the seam and press it into the fresh sealer, smoothing it down to become fully embedded. Avoid air pockets or wrinkles.
Apply a second coat
Once the first application has dried, apply a second coat to fully seal the seams and feather out uneven areas. The sealer should cover the edges of the tape by about 1 inch on each side.
Let dry completely
Allow the seam sealer to fully cure according to manufacturer directions before applying finishes or walking on floor installations. Drying time varies by product from about 4-48 hours.
Tips for sealing cement board seams
– Carefully read and follow product instructions for application and drying times with any cement board seam sealer.
– Less is more with silicone or epoxy – a thin, continuous bead is effective without waste.
– Hold the trowel or putty knife at a 45-degree angle to get corner angles clean.
– To embed tape fully, press gently with the trowel – don’t drag it along the seam.
– Make sure the cement board is properly fastened before sealing seams.
– Wipe away excess sealer that squeezes out along edges before it dries.
– Where multiple pieces join, seal all seams before they intersect.
– Seal corner seams and gaps with silicone or epoxy for added waterproofing.
Common problems sealing cement board seams
Sealing cement board seams takes practice. Some issues that can occur include:
– Gap showing – the seam sealant didn’t fully cover the seam. Reapply additional coats.
– Tape lifting – improper embedding can cause the tape to unstick. Remove and re-tape.
– Cracking – inflexible mortar or fast drying can lead to cracks. Use latex-modified products.
– Hollow spots – air pockets under the tape weaken the seal. Press out bubbles and air pockets.
– Insufficient thickness – two coats ensures proper coverage and smoothing of seams.
– Peeling or poor adhesion – remove dust and debris before sealing for best bonding.
– White haze – epoxy or silicone sealant residue can leave white curing by-products on the surface. Wipe any excess immediately before drying.
Conclusion
Sealing the seams between cement backerboard properly is one of the most important steps of any cement board installation. Taking the time to fully seal gaps and ridges with quality waterproofing materials leads to a long-lasting substrate for tile, flooring, and other finishes. Mesh tape and thinset mortar, AR tape, latex-modified mortar, silicone sealant, and epoxy are all good cement board seam sealers. With attention to proper application procedures, the seams can form a durable surface free of cracks and moisture issues. Proper sealing prevents leaks, damage, and cracked tiles or grout down the road.