Underlayment for Tile on a Concrete Slab (2024 Guide)
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Underlayment—the layer between a home’s subfloor and the finished flooring—serves an important purpose if you are installing tiles over a concrete floor. In addition to preventing mold, reducing noise, and improving insulation, it supports the tiles’ weight and helps with adhesion.
Our guide covers the different underlayment types and factors to consider when choosing one. We’ll also recommend the best tile underlayment for a DIY flooring project.
What Are the Different Types of Tile Underlayment?
Homeowners have a few options to consider when it comes to underlayment. We’ll go over the most popular types below.
- Foam backer board: Lightweight foam backer boards are typically used for walls rather than floors. They aren’t as strong as cement backer boards but provide a fair amount of insulation.
- Gypsum backer board: Gypsum backer boards are vulnerable to moisture and therefore not suitable for wet areas, such as a bathroom. They are often made with fiberglass for increased durability.
- Fiber cement backer board: Fiber cement boards are rigid but more workable than traditional cement boards. Contractors often use them for a large room’s entire floor area. They are also sometimes used for countertops and walls.
- Magnesium oxide board (MgO): MgO boards are lighter than drywall and cement boards. They are quite strong and resistant to fire, mold, and moisture. However, they’re poor insulators and not recommended for homes in colder climates.
- Uncoupling membranes: By absorbing movement in the substrate, uncoupling membranes prevent cracks from transferring to the tile surface. Thinset mortar is applied both below and above the membrane, making it suitable for concrete or plywood subfloors. Schluter-Ditra is a well-known brand of uncoupling membranes.
- Crack isolation membranes: This type of membrane is similar to an uncoupling membrane but thinner. They help prevent existing cracks in the substrate from telegraphing up through the tile and grout.
- Waterproof membranes: You can use waterproof membrane in bathrooms and showers to prevent water seepage. Kerdi is a trusted waterproof membrane brand. These membranes are typically installed over a concrete subfloor with thinset mortar applied below and above them.
- Liquid/roll-on membranes: Unlike other membranes, liquid or roll-on products are applied with a paintbrush, roller, or trowel. They create a seamless waterproof barrier but must be paired with something thicker to provide support.
- Self-leveling : You can use self-leveling underlayments (SLUs) instead of backer boards. Unlike traditional mortar, SLUs have specific additives that make them pourable. They offer excellent results but do require some skill to apply correctly. Most can be applied in layers thinner than an inch.
with a wood subfloor or a concrete subfloor. - Thinset: Thinset mortar is primarily used for setting tiles and paired with a backerboard. It’s bought as a powder, mixed with water at the jobsite, and applied with a notched trowel. Thinset mortar is primarily used for setting tiles and paired with a backerboard.
The ideal underlayment is one that creates a stable, rigid base for your tile. If there’s any flexibility or “give” in the underlayment, it can lead to cracked tiles and grout.
As a result, it can absorb moisture, swell, and compromise the stability of your tile installation. Additionally, OSB is too flexible to provide the rigid support tiles need. Several cement backer board manufacturers, including the makers of HardieBacker and Durock, advise against installing their products over concrete.
Can You Tile on a Concrete Slab Without Underlayment?
Technically, yes, you can adhere tiles directly to a concrete slab. However, skipping the underlayment is strongly discourage by tiling professionals.
Even if a concrete floor appears perfectly stable and dry, hidden issues can emerge later. Using a suitable underlayment acts as an insurance policy and helps prevent the following issues:
- Cracking: Concrete naturally expands and contracts with temperature changes and moisture. Underlayments like uncoupling membranes help absorb this movement, minimizing the risk of cracked tiles and grout.
- Moisture damage: Porous concrete can absorb moisture from the ground, even if it feels dry at the surface. This moisture can compromise the bond of the tile, leading to loosening or discoloration. Waterproof membranes or moisture-resistant underlayments add protection.
- Unevenness: Even seemingly level concrete can have slight dips or imperfections. Underlayment helps create a perfectly smooth and even surface for tiles, ensuring a more professional finish.
- Efflorescence: This is the whitish, salty deposit that can sometimes form on concrete and migrate upwards. A proper underlayment and/or waterproofing membrane helps prevent efflorescence from reaching and staining tile and grout.
You should address cracks in concrete before installing any underlayment. Unaddressed cracks and moisture issues may eventually cause your tiles to crack regardless of the underlayment you choose. You may need to level the concrete before installing underlayment. Leveling a floor is important for most finished floor installations.
Things To Consider When Choosing Underlayment for Tile
There’s no single “best” underlayment for every tile project. To make the right choice for your home, consider the following factors:
Careful evaluation of your existing concrete will help you select the best underlayment to protect your tiles.
- Cracks: Significant cracks suggest underlying issues. Underlayment won’t fix the problem, but uncoupling membranes can help minimize crack transfer to the tile.
- Levelness: Major dips or high spots will need a self-leveling underlayment to create a perfectly flat surface for the tiles.
- Moisture: Any dampness or signs of water intrusion requires a waterproofing membrane or moisture-tolerant underlayment.
Consider where your tile installation will be located and how the area is used. Bathrooms, showers, and other wet areas absolutely require a waterproof membrane beneath the tile to prevent water damage to your home. For high-traffic areas of the home, an uncoupling membrane provides an extra layer of stability and protection, minimizing the risk of tile cracking due to minor movement in the concrete.
Large-format tiles need a very stable base, while some natural stones are sensitive to moisture. Always consult with the tile supplier or manufacturer to determine the best type of underlayment for your specific tile.
Underlayment costs vary significantly, from affordable cement backer boards to more expensive uncoupling membranes. Factor in not just the cost of the underlayment itself, but also any additional materials needed, such as thinset mortar or waterproofing sealant. While some underlayments might have a higher upfront cost, they could save you money in the long run by preventing cracked tiles and water damage.
What Is the 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 project seems overwhelming, don’t hesitate to hire a professional for this step.
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 up your tiles perfectly during installation. Plus, they don’t require any special tools. A basic trowel and utility knife are all that’s needed.
Before starting, always watch instructional videos and carefully read the manufacturer’s instructions for your chosen underlayment. For a quick tutorial on how to tile a bathroom floor, check out the video below with contractor Angelo McRae:
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.
We recommend hiring 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.
FAQ About Underlayment for Tile on Concrete Slab
Do you need a vapor barrier for tile on concrete?
Yes, you generally need a vapor barrier for tile on concrete because concrete is porous. A vapor barrier creates a waterproof layer between the concrete and your flooring, minimizing the risk of moisture-related issues.
Can I tile over the old flooring?
Yes, you can tile over the old flooring—but only 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 before tiling over them.
Can cork be used as an underlayment for tile?
Although cork can be used as an underlayment for tile in certain situations, it’s not usually a good choice. Cork is too flexible, vulnerable to moisture, and can compress under the weight of tile. However, you could use dense cork in a dry area with small-format tiles and an additional layer of underlayment.
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