Preparing the Subfloor Is the Secret to Gorgeous Tile Floors—Here’s How to Do It

Ceramic tile floors are heavy and rigid. They need a solid subfloor that won’t flex or move around underneath; one that’s strong enough to hold their weight and the weight of the preparation materials involved in their installation. They also need an even surface so tiles don’t crack or come loose over time.

There are a variety of viable subfloor options for ceramic tile, and each requires different preparation. You’ll commonly find a layer of oriented strand board (OSB) or plywood laid over the joists or structural support.

If your home has a concrete foundation or basement, tile can be installed directly over the concrete slab. A newly poured concrete floor should cure for at least three months before any flooring is installed on top.

If your subfloor is made of any of the following materials, it is unsuitable for tile and will need to be removed before you take any other steps:

• Interior-grade plywood or particleboard, which can swell with water

• Vinyl in multiple layers or not fully bonded to the entire subfloor

• Drywall or green board

Regardless of what material your subfloor is made from, sweep the entire room to get rid of dust and debris.

If your subfloor is concrete, use your scrub brush to scrub the surface with a degreasing cleaner in warm water. This will remove any salts, adhesives, and other surface contaminants that may inhibit water penetration, which is key to proper bonding between your subfloor and underlayment. Rinse and dry thoroughly before moving on.

Another additional step for concrete is to fill any cracks or holes using a cement patching compound. After mixing, apply with the edge of your trowel and level each patch with the surrounding concrete. Large cracks or displaced chunks may be a sign of deeper problems that you should hire a tile professional to diagnose.

Depending on your subfloor material, sealing and leveling may occur in different orders. Regardless of your material, start by sliding your level all around the room in different directions. If you notice any dips or uneven spots, they’ll need to be addressed.

If your subfloor is wood, fill seams with caulk and roll on a layer of latex primer-sealer with your roller brush. This prepares the surface to bond with self-leveler. Tape any vents off with painter’s tape.

Then, for both wood and concrete subfloors, mix and pour a self-leveling floor compound into any dips according to the manufacturer’s instructions. This self-leveler will seek its own level and dry in a flat, even surface. Wear safety goggles and gloves when handling the self-leveler and allow it to dry completely before moving on. This can take several days.

Finally, for concrete subfloors only, apply a concrete sealer to the level surface. Because concrete is porous, it draws moisture up from the ground which can cause damage to your floors without proper protection. Sealer keeps moisture away and prevents mold growth.

The next step will vary depending on your subfloor and underlayment materials.

Attaching Cement Backer Board Underlayment to a Wood Subfloor

A cement-based backer board provides a solid surface that doesn’t flex, similar to solid concrete. Follow the manufacturer’s instructions and our tips below.

• Attach the backer board to your subfloor with a polymer-modified thin-set (also called mortar) for additional strength and stability. It usually takes a day to dry.

• Follow all directions on the packaging and wear safety glasses

• Promptly clean your mixing paddle after mixing to prevent mortar from hardening on it

• If your subfloor runs east to west, lay your backer boards running north to south

• If you need to trim a panel, measure it and score along both sides with a utility knife. Press down on one side of the score line and lift the opposite side to snap the panel.

Working quickly while the mortar is wet, use your trowel to scoop and spread enough to cover an area slightly larger than your first panel of backer board. Comb the mortar with the trowel’s notched edge and place the panel on top.

Walk gently on the panel as you drill backer board screws every eight inches around the perimeter, half an inch from each edge and two inches from corners. Some manufacturers make this easier by identifying locations on the backer board to place screws. Every screw head should be flush with the panel.

Leaving a one-quarter-inch gap at the walls for expansion and a one-eighth-inch gap between panels, continue this process to cover your room. Arrange panels in a staggered pattern in which no four corners touch.

Attaching Uncoupling Membrane Underlayment to a Concrete Subfloor

Concrete, although solid, can still move and shift over time. To prevent this movement from cracking your grout or tile, install an underlayment of crack prevention membrane. It comes in large rolls of a waffle-patterned peel-and-stick mat that covers your entire subfloor.

Start by unrolling the membrane and using your utility knife to dry cut pieces to cover your subfloor. Leave a quarter-inch of space around the perimeter of the room and any poles or other obstructions. Pull back the membrane and wipe the areas with a slightly damp sponge, then apply thin-set mortar with the notch of your trowel directly to the subfloor.

Roll your membrane back down onto the thin-set as soon as possible so the surface doesn’t dry before contact. Using a wood float or hand roller, push the entire surface, including corners and edges, into the membrane.

Once your underlayment is laid and you’ve allowed at least a day for the mortar to dry, you’re ready to install your new tile floor!