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Worker laying white tile on concrete.

Underlayment for Tile on a Concrete Slab (2024 Guide)

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Author Icon By Angela Bunt Updated 01/14/2024

Underlayment is a flooring layer between a home’s subfloor and finished floor. It prevents mold, reduces noise, and helps with insulation, but its most important purpose is supporting the top layer of flooring. This is especially important for heavier tiles. An underlayment can help with tile adhesion if you’re installing over concrete.

Learn more about the different underlayment types, factors when choosing one, and what we consider the best tile underlayment for do-it-yourselfers (DIYers).

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Types of Tile Underlayment

Homeowners have a few options to consider when it comes to underlayment. There shouldn’t be any give to the underlayment regardless of the tile you choose. Both your tiles and grout may crack if there is.

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We don’t suggest using an OSB (oriented strand board) underlayment. This synthetic material is made from compressed layers of chipped wood and is too flexible to support tile properly. In addition, several cement backer board manufacturers, including the makers of HardieBacker and Durock, do not recommend installing their products over concrete.

Backer Boards

Membrane

Poured

Backer boards are a combination of cement, crushed glass particles, and sometimes fiberglass. Professional builders and homeowners use them to help prevent ceramic, porcelain, or format tiles from cracking. They go above a layer of thinset (a mortar adhesive) below the tile.
There are several types of backer boards, including the following:
  1. Foam backer board: Lightweight foam backer boards aren’t as strong as cement backer boards but provide a fair amount of insulation.
  2. Gypsum backer board: This is best for low moisture areas. Gypsum board is often made with fiberglass to increase its durability.
  3. Fiber-cement backer board: Fiber cement boards are similar to cement backer boards but are more flexible. Contractors often choose it for a large room’s entire floor area. It’s also sometimes used for countertops and walls.
  4. Magnesium oxide board (MgO): MgO boards are similar to drywall and cement boards but are lighter, quite strong and resistant to fire, mold, and moisture. However, they’re poor insulators and not recommended for homes in colder climates.
Polyethylene membranes come in rolls that you can easily cut with a utility knife. They’re used instead of backer boards and are easier to work with than other materials.
DIY homeowners have three types to choose from:
  • Uncoupling membranes: Intended to absorb substrate movement so tiles are less likely to crack. You apply thinset below and above the membrane, and you can use it with a concrete or plywood subfloor. Schluter-Ditra is a popular company selling uncoupling membranes.
  • Crack isolation membranes: Very similar to an uncoupling membrane but less thick. It can help prevent tile and grout lines from cracking.
  • Waterproof membranes: You can use products such as Kerdi in North America in bathrooms and showers to prevent water seepage. Kerdi is a trusted waterproof membrane brand. Typically, you apply a layer of thinset over the concrete subfloor and another layer over the membrane before installing the tile floor.
Poured underlayment involves mixing mortar until it’s in a semi-solid, spreadable state. It’s a high-quality underlayment but requires some skill to spread evenly. It must be 1–2 inches thick to create a proper tile base.
There are two mortar types to consider if you’re going this route:
  • Self-leveling mortar: You can use self-leveling mortar with a wood subfloor or a concrete subfloor. You may need to create a mortar bed during the curing process depending on the product.
  • Thinset: You’ll need a notched trowel to apply thinset mortar. It’s bought as a powder and mixed with water at the jobsite. No backer board is needed if the concrete subfloor is in good condition.

Can You Tile on a Concrete Slab Without Underlayment?

Yes, you can tile on a concrete slab without underlayment, but it’s generally not recommended. A floor may not appear to have moisture or substrate issues, but they may not have materialized yet. Therefore, before you install any tile, it’s best to use at least one of the underlayment options discussed above to prevent cracking and moisture-related issues.


One underlayment option may seem like the best choice because of what it inherently offers, but it may not be suitable for your home project. Here are some things to consider when selecting an underlayment:

The best underlayment for a project largely revolves around the state of your concrete slab. However, most homeowners will find that a cement backer board or an uncoupling membrane is the best option for their project. Both provide moisture protection and are strong enough to support heavy tiles.

You may need to level the concrete before installing underlayment. You may even need to raise the concrete floor if there’s significant deterioration. Leveling a floor is important for most finished floor installations. The slope will also need to go in a controlled direction if you’re installing tile in a shower or bath.

You should address cracks in concrete before installing any underlayment. Cracks may indicate structural issues, and they can provide a gateway for moisture movement. Your tiles may crack if you leave these unaddressed.

Sloping to a drain can be difficult, especially if you have no experience installing underlayment. You may want to outsource this one to a professional tile setter. You may also need a sealant applied to prevent moisture damage.


Best Tile Underlayment for DIY Projects

Backer board and uncoupling membranes are often the best underlayments for DIY projects because of their durability and easy installation. However, if the job seems like it may be too much, consider hiring a contractor to install your home’s underlayment.

Backer board is particularly easy to cut and secure. It comes in standard sizes, such as 3 feet by 5 feet or 4 feet by 8 feet, and you can get both at a 1/2-inch thick or a 1/4-inch thick. You can find it at a big-box retailer such as Lowe’s, and all brands provide some measure of moisture prevention and durability.

Uncoupling membranes are also easy to work with but are typically more expensive. A benefit of uncoupling membranes is the built-in grids in their design. This makes it easy to line your tiles perfectly during installation. Plus, they don’t require any special tools. A basic trowel and utility knife are all that’s needed.


Our Conclusion

Many underlayment options exist, but cement backer boards or uncoupling membranes are a good choice for most tile installations. You’ll need a simple layer of thinset below and above the underlayment for both options.

Hire a professional if your project area requires a slope or the concrete slab is damaged. This will ensure the job is done right. If you feel confident installing the tile portion but not the underlayment, consider getting multiple quotes for your underlayment installation.

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FAQ About Underlayment for Tile on Concrete Slab

Do you need a vapor barrier for tile on concrete?

Yes, you need a vapor barrier for tile on concrete because concrete is porous. A vapor barrier stops the vapor drive of water, which can damage any flooring installed over the concrete.

Can I tile over the old flooring?

Yes, you can tile over the old flooring but only do so if the flooring is in good condition. You shouldn’t tile over sections with missing, chipped, or cracked tile because they’ll provide inadequate support for the new layer of tiles. Consider replacing damaged tiles first before tiling over them.

Can cork be used as an underlayment for tile?

Yes, you can use cork as an underlayment for tile. Not only is it good for reducing the transfer of sound, but it also helps prevent cracking. Half-inch cork underlayment is also considered a great acoustic barrier.

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