Can Cement Board Get Wet? (Find Out Now!)

Jessica Stone
by Jessica Stone

Whether you’re planning to install new tile flooring, a shower enclosure, or tile countertops, you’ll likely end up using some form of cement board. When used as a tile backer board, cement board performs better long-term than conventional paper-faced gypsum products, as it does not physically break down or form mildew in the presence of moisture. Not to mention, cement board delivers a stronger bond and foundation for tiles. Though, when it comes to cement board, there’s one question you may be asking: “Can cement board get wet?”

This is a valid question, and one that may seem a bit confusing to answer. For starters, cement board is technically not waterproof, it’s actually water-resistant. However, this does not mean that the boards can’t get wet. In fact, since cement board absorbs moisture well, have excellent drying properties, and will not disintegrate when exposed to water, they can get wet.

Though, the reason that you may consider waterproofing your cement board installation is to protect the framing underneath. When installed in areas that are exposed to higher levels of moisture, a barrier or sealant should be implemented to prevent water from seeping through to the wood or metal studs underneath.

With this in mind, let’s dive in deeper to further understand whether or not cement board is waterproof, whether or not it can get wet, and some best practices for waterproofing cement board – if you choose to.

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Is Cement Board Waterproof?

Cement board consists of a combination of cement and reinforcing fibers in varying thicknesses that are formed into sheets to be used as backing board for a number of wall and flooring projects – most notable tile. These boards, available from numerous manufacturers, advertise themselves as being a waterproof solution for floors, walls, and countertops, the product itself is technically not waterproof.

Instead, most cement boards are technically considered to be “water-resistant.” Even though the boards will absorb water, they resist deterioration due to moisture very well. The main concern with cement boards not being waterproof has nothing to do with the boards themselves, but rather the substrate that you install the cement board over.

Since tile and grout are also not waterproof, some form of barrier or sealant must be used to prevent moisture from seeping through the cement board to the wood or metal studs underneath. While the waterproof vs. water-resistant distinction may be a bit confusing to understand, here’s what some of the most popular manufacturers of cement board say regarding waterproofing:

  • WonderBoard Cement Backerboard: “WonderBoard is unaffected by water but it is not waterproof. If the area behind the backerboard must be kept dry, a moisture barrier or waterproof membrane…”
  • PermaBASE Cement Board: “PermaBASE Cement Board is not a water barrier; consult local building code for moisture barrier requirements.”
  • USG Durock Brand Cement Board: “If waterproofing is desired, use USG Durock™ Brand Waterproofing Membrane or USG Durock™ Brand Liquid Waterproofing Membrane.”
  • HardieBacker Cement Board with HydroDefense Technology: “HardieBacker® Cement Board with HydroDefense™ Technology is the first and only waterproof cement backer board.”

The new cement board with “Hydrodefense” from Hardiebacker is the only cement board that claims to be fully waterproof. All other brands insist that their boards are either not waterproof or do not serve as a “moisture barrier.”

Can Cement Board Get Wet?

Whether you accidentally exposed cement board to water prior to installation or you’re concerned amount the impact of moisture on boards that have already been installed, you may be wondering whether cement board can get wet. Fortunately, while cement boards are technically not waterproof, they are engineered for applications that may be exposed to water.

As such, cement boards will not lose strength or disintegrate if they get wet. In fact, many manufacturers claim that the boards can be submerged underwater indefinitely and still maintain their structural integrity. This has to do with the fact that although cement boards do absorb water, they have exceptional drying properties.

With that said, if you did accidentally get your cement boards wet prior to installation, you should let them dry completely before you proceed with the install.

Cement Board vs. Green Gypsum Board

Like cement board, green board is water-resistant, instead of waterproof. This stronger form of gypsum board is highly efficient for use in bathrooms, kitchens, laundry rooms, and anywhere else that frequently experiences moisture in the air.

However, it is not the type of backer that should be used behind tile in a shower or any other areas that will come in direct contact with water. When saturated, green board weakens much more than conventional gypsum board. Fortunately, there are other options for walls come into contact with water – like cement board.

While cement board and green board are often compared to each other, they are very different products that serve distinct purposes. Cement board is the better solution for use in showers, tub surrounds, and locations that will get wet. However, it needs to be installed along with a moisture barrier, which takes more time and increases the cost of installation when compared to green board.

Green board, on the other hand, is considered a more affordable and quicker solution for areas of the home that may get damp, but not saturated. For these situations, it’s best to stick to cement board.

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How to Waterproof Cement Board

Contrary to popular belief, both tile and grout are not waterproof. Even if you use a sealant, some moisture is still going to penetrate beneath. For this reason, it is absolutely crucial that you take the necessary steps to protect the underlying materials from water and moisture damage. One way to do this is to use cement board as the tile underlayment. However, when you choose to use cement board, you also must install a vapor barrier underneath or a sealant on top to protect the materials underneath.

For shower floors and sides, in addition to tile flooring, installing a vapor membrane underneath the cement board is often the best course of action to ensure that no moisture reaches the studs. In this case, you don’t need to apply a sealant to the top of the board, as this may trap moisture in between the two layers. If you choose to use a sealant instead of a vapor barrier, there are a couple things you should keep in mind:

  • Liquid sealants have to be applied on thick enough to provide complete coverage to the entire surface. This means multiple coats will be required.
  • When cement boards are installed in a shower, the entire area must be treated with the liquid sealant to prevent moisture from getting beneath the floor or behind the walls.
  • With cement board sheets, seams need to be combined to appropriately prevent leaks and also bonded to the surface underneath.

In other words, waterproofing cement board is as simple as installing a moisture barrier – which may take the form of liquid waterproofing on the surface of the board, a thin plastic sheet underneath the cement board, or a waterproof sheet membrane on the surface of the cement board.

However, when it comes to horizontal surfaces, more extensive waterproofing is recommended. Acceptable methods for waterproofing these areas is two coats of liquid sealant, a waterproof sheet membrane, or any of the many foam waterproof products out there on the market.

Jessica Stone
Jessica Stone

Jessica considers herself a home improvement and design enthusiast. She grew up surrounded by constant home improvement projects and owes most of what she knows to helping her dad renovate her childhood home. Being a Los Angeles resident, Jessica spends a lot of her time looking for her next DIY project and sharing her love for home design.

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