How to Prepare Your Bathroom Subfloor Before Tile Installation

Demo the Existing Floor Covering

Before you can assess the subfloor, everything on top of it has to come off. Vinyl, old tile, carpet, and any adhesive residue all need to be removed completely. In Austin bathrooms, we frequently find multiple layers of flooring stacked on top of each other — vinyl over old tile over the original subfloor — which raises the floor height and hides problems underneath.

For tile-over-tile situations, check every square foot with a rubber mallet. Any hollow sound means the tile is no longer bonded to the substrate. Hollow tile will transmit movement to the new installation and cause cracking within months. If more than 10 to 15 percent of the existing tile is hollow, demo is the right call.

Inspect the Subfloor for Damage

Once the floor is bare, get down and look at it carefully. In Austin homes built on pier-and-beam foundations, the subfloor is typically plywood or OSB over floor joists. Check for:

• Soft spots or spongy areas that indicate moisture damage or rot • Delamination — layers of plywood separating from each other • Squeaks or movement when you walk across the floor • Water stains or discoloration from past leaks

On concrete slab — the most common foundation type in Central Texas — look for cracks, spalling, and areas where the slab has settled unevenly. Minor hairline cracks are normal and can be filled. Structural cracks that are wide, stepped, or show vertical displacement need to be evaluated before tile goes down.

Check and Correct Flatness

This is the step most DIYers skip and most failed tile jobs trace back to. Use a long straightedge (6 to 10 feet) and a level to identify high spots and low spots across the entire bathroom floor. Mark them with a pencil or chalk.

The TCNA standard for tile installation is 3/16 inch over a 10-foot span. For large-format tile (15 inches or larger), the tolerance is 1/8 inch over 10 feet. High spots are ground down with an angle grinder. Low spots are filled with floor-leveling compound (Mapei Planipatch or equivalent for small areas, self-leveling compound for larger areas).

In Austin, slab foundations that have settled over time are a common source of out-of-flat conditions. We see this frequently in homes built in the 1970s through 1990s in neighborhoods like Allandale, Crestview, and South Congress. Self-leveling compound handles most of these situations cleanly.

Install Cement Backer Board (Wood Subfloors Only)

If your bathroom is over a wood subfloor — common in pier-and-beam homes and second-floor bathrooms — you need to install cement backer board before tile. Standard plywood and OSB are not appropriate substrates for tile in wet areas because they flex with humidity and temperature changes, which cracks grout and eventually the tile itself.

Use 1/4-inch or 1/2-inch HardieBacker, Durock, or equivalent. Fasten it with corrosion-resistant screws every 6 to 8 inches in the field and every 4 to 6 inches at the edges. Tape the seams with alkali-resistant mesh tape and thin-set. Leave a 1/8-inch gap at the walls to allow for expansion.

On concrete slab, backer board is not required — you can tile directly over properly prepared concrete.

Apply a Waterproof Membrane

In any wet area — shower floor, shower walls, and the bathroom floor within the splash zone — a waterproof membrane is not optional. It is the last line of defense between water and your framing.

For shower areas, Schluter Kerdi, RedGard, or a similar sheet or liquid membrane is applied over the backer board and up the walls a minimum of 6 inches above the finished floor height. Pay special attention to corners, seams, and penetrations (drain, pipe stubs) — these are the most common failure points.

For the main bathroom floor outside the shower, a vapor barrier between the slab and the tile system is good practice in Austin, where ground moisture can migrate upward through the slab, particularly in older homes and during the wet season.

Prime the Surface

Priming is a step that gets skipped more often than it should. A concrete or backer board surface that has not been primed will pull moisture out of the thin-set too quickly, weakening the bond between the mortar and the substrate.

For concrete slab, use a penetrating concrete primer (Mapei Primer T or equivalent) and allow it to dry completely before setting tile. For backer board, a skim coat of unmodified thin-set or a dedicated primer creates a consistent, absorbent surface that bonds reliably.

In Austin's summer heat, this step matters even more — high temperatures accelerate moisture loss from mortar, and a primed surface gives you the working time you need to set tile properly.

Choose the Right Mortar

The mortar you use matters as much as the subfloor prep. For standard bathroom tile (12x12 to 12x24), a polymer-modified thin-set is the right choice. For large-format tile (18x18 and larger), use a medium-bed or large-format tile mortar (Mapei Ultraflex LFT or equivalent) — standard thin-set does not provide enough coverage and support for heavy, large tiles.

For shower floors with small mosaic tile, use a non-sag mortar that holds the tiles in place on the sloped surface without sliding. For natural stone, use a white mortar to avoid bleed-through on light-colored stone.

The mortar choice is part of the subfloor prep conversation because the wrong mortar on a correctly prepped surface will still fail. Get both right.