Underlayment, which is the layer between a home’s subfloor and the finished flooring, prevents mold, reduces noise, and improves insulation. It also supports the weight of tiles and helps the tiles to adhere to other parts of your flooring. Read our guide below to learn about the different types of underlayment, the best tile for do-it-yourself projects, and how to install tile underlayment.

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Can You Tile on a Concrete Slab Without Underlayment?

You can adhere tiles directly to a concrete slab, but tiling professionals strongly discourage skipping the underlayment. Hidden issues can emerge even if concrete flooring seems to be stable and dry. Underlayment acts as an insurance policy and helps to prevent the issues below.

• Cracking: Concrete naturally expands and contracts as temperatures and moisture levels change. Underlayments help to absorb these changes, and minimize the risk of cracked tiles and grout.
• Efflorescence: This is the whitish, salty deposit that can sometimes form on concrete and migrate upwards. A proper underlayment or waterproofing membrane helps to prevent efflorescence from reaching and staining tile and grout.
• Moisture damage: Porous concrete can absorb moisture from the ground, even if it feels dry. This moisture can compromise the bond of the tile and lead to loosening or discoloration.
• Unevenness: Even concrete that seems level can have slight dips or imperfections. Underlayment helps to create a perfectly smooth and even surface for tiles.

You should address cracks in concrete before you install any underlayment. Unaddressed cracks and moisture issues may eventually cause tiles to crack no matter which underlayment type you choose. You may need to level the concrete before you install underlayment.

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What Are the Different Types of Tile Underlayment?

Cement backer boards

These are large, rigid sheets made of cement, sand, and sometimes fiberglass. They go above a layer of thinset, which is a mortar adhesive, and below the flooring. They help to prevent ceramic, porcelain, or format tiles from cracking

Below are some popular underlayment backer boards to consider for use on concrete slabs.

• Fiber cement backer board: Fiber cement boards are rigid more than cement boards but are easier to work with. Contractors use them for large rooms, countertops, and walls.
• Foam backer board: Lightweight foam backer boards are common for use on walls but not flooring. They aren’t as strong as cement backer boards, but they provide a fair amount of insulation.
• Gypsum backer board: Gypsum backer boards are vulnerable to moisture and aren’t suitable for use in wet areas, such as bathrooms. They are frequently made with fiberglass, which makes them durable.
• Magnesium oxide (MgO) board: MgO boards are lighter than drywall and cement boards. They are strong and resistant to fire, mold, and moisture. They don’t insulate well, though, and you should not use them in cold climates.

Backer boards are easy to cut and install, but they’re heavier and more expensive than other options.

Pro tip: We don’t recommend using oriented strand board as an underlayment for tile on concrete. OSB is made of compressed layers of chipped wood, and it can absorb moisture, swell, and compromise the stability of your tile installation. OSB is also too flexible to provide the rigid support that tiles need. Several cement backer board manufacturers, such as the makers of HardieBacker and Durock, advise against installing their products over concrete.

Membranes

Polyethylene membranes are available as rolls that you can easily cut with a utility knife. They are easier to install than traditional cement backer boards for DIY enthusiasts. Below are the main types:

• Uncoupling membranes: Uncoupling membranes prevent cracks from transferring to the tile surface by absorbing movement in the subtrate. You can apply thinset mortar below and above the membrane, which makes 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. Crack isolation membranes help to prevent existing cracks in the substrate from telegraphing up through the tile and grout.
• Waterproof membranes: You can use waterproof membranes in bathrooms and showers to prevent water seepage. Kerdi is a trusted waterproof membrane brand. You can typically install these membranes over a concrete subfloor with thinset mortar above and below.
• Liquid/roll-on membranes: You can apply liquid or roll-on products with a paintbrush, roller, or trowel. They create a seamless waterproof barrier, but only if you pair them with something thicker that can provide support.

Membranes are especially resistant to moisture, but they’re more difficult to install than other underlayments. You’ll probably need to hire a professional contractor to complete membrane installation to get the material’s full waterproofing benefits.

Poured Underlayments

Poured underlayment is the product of mixing mortar until it’s in a semi-solid, spreadable state that creates a smooth level surfaces for tile. The two types of mortar you can use as underlayment are below.

• Thinset: You can pair thinset mortar with a backer board to set tiles. You can buy this mortar as a powder, mix it with water at the job site, then apply with a notched trowel. This underlayment is DIY-friendly, but you must apply it to another layer of underlayment.
• Self-leveling: You can use self-leveling underlayment instead of backer boards. SLUs feature specific additives that make them pourable, and experienced DIYers can use them to create a perfectly level surface.

The ideal underlayment is one that creates a stable, rigid base for your tile. Any flexibility in the underlayment can lead to cracked tiles and grout.

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What Do I Need to Consider When Picking Underlayment?

Consider the factors below when deciding which underlayment is best for your tile project:

Condition of the Existing Concrete

Carefully evaluate your existing concrete and inspect for any of the issues below before you select an underlayment:

• Cracks: Significant cracks suggest underlying issues. Underlayment won’t fix the problem, but uncoupling membranes can help you to minimize crack transfer to the tile.
• Levelness: Correct major dips or high spots with a self-leveling underlayment, which will help you to achieve flat surfaces for the tiles.
• Moisture: Dampness or signs of water intrusion call for waterproofing membranes or moisture-tolerant underlayment.

Room Location and Usage

Consider the way you use the area in question when you’re deciding which type of underlayment is best. Bathrooms, showers, and other wet areas require a waterproof membrane beneath the tile to prevent water damage to your home. An uncoupling membrane provides an extra layer of stability and protection for the home’s high-traffic areas, and minimizes the risk of tile cracking due to minor movement in the concrete.

Type of Tile 

The size and type of tile you use can influence the type of underlayment you choose. Large-format tiles need a very stable base to prevent lippage (uneven edges). Some natural stones are sensitive to moisture and prone to staining. Always consult with the tile supplier or manufacturer to determine the best type of underlayment for your specific tile.

Your Budget

Underlayment costs vary significantly. Cement backer boards can be affordable, while uncoupling membranes are more expensive. Factor in the cost of the underlayment and additional materials you might need, such as thinset mortar or waterproofing sealant. 

Don’t reflexively choose the cheapest option. Some underlayment materials have a higher upfront cost, but they might save you money in the long run by preventing cracked tiles and water damage.

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What Is the Best Tile Underlayment for DIY Projects?

Backer board and uncoupling membranes are the best underlayments for DIY projects because they’re durable and easy to install. If the project seems overwhelming, though, don’t hesitate to hire a professional to help.

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 in 1/2-inch and 1/4-inch thicknesses. You can find it at a big-box retailer such as Lowe’s, and all brands are durable and moisture-resistant.

Uncoupling membranes are easy to work with but are more expensive. They feature built-in grids that make it easy to line up your tiles perfectly during installation. You can also install them with only a basic trowel and a utility knife.

Watch the video below for a quick tutorial on how to tile a bathroom floor with contractor Angelo McRae:

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Installation Process for Tile Underlayment

You must properly install your underlayment for it to perform correctly. Read through the correct installation process below.

Preparing the Concrete Surface

The concrete surface on which you plan to work must be clean, dry, and free of any contaminants before you install your underlayment. Follow the steps below.

1. Remove any existing flooring or adhesive residue.
2. Repair any cracks or damage in the concrete.
3. Clean the surface thoroughly to remove dust and debris.
4. Make sure the concrete is completely dry before you proceed.

Applying the Underlayment

The application process varies depending on the type of underlayment you’ve chosen. The manufacturer will often provide more specific instructions, but the basics of application are below.

Laying Cement Backer Boards

1. Apply a layer of thinset mortar to the concrete.
2. Place the boards and secure them with screws.
3. Tape the seams between boards.

Installing Uncoupling Membranes

1. Spread thinset mortar over the concrete.
2. Unroll the membrane and press it into the mortar.
3. Overlap seams according to the manufacturer’s instructions.

Pouring Self-Leveling Underlayments

1. Mix the product according to instructions.
2. Pour and spread the mixture evenly across the floor.
3. Use a gauge rake to make sure the mixture is the proper thickness.

Curing Time and Next Steps

Allow the underlayment to cure fully before you proceed with tile installation. Curing times vary by product, so follow the manufacturer’s recommendations. You can begin laying your tiles after you’ve finished the curing process.

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FAQ About Underlayment for Tile on Concrete Slab

Do you need a vapor barrier for tile on concrete?

You usually 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, and minimizes 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 that have missing, chipped, or cracked tile because they’ll provide inadequate support for the new layer of tiles. Replace damaged tiles before tiling over them.

Can cork be used as an underlayment for tile?

You can technically use cork as an underlayment for tile in certain situations, but it’s not usually a good choice, especially if you’re dealing with concrete. Cork is too flexible, vulnerable to moisture, and can compress under the weight of tile.

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