Bathroom Tile Installation: Walls, Floors, Showers, and Waterproofing

Waterproofing First

In a shower or tub surround, water gets behind tile through grout joints, cracked caulk lines, and any penetration (showerhead, valve, niche). The tile is not waterproof — it's the membrane behind the tile that keeps water out of the wall. This is the most important concept in bathroom tile work.

Sheet membrane systems (Kerdi, GoBoard, Hydro Ban Board) are waterproof panels that replace traditional cement board. You tile directly to them. They're the simplest approach for showers because the waterproofing and the substrate are the same product. Seams are sealed with waterproof tape and thinset.

Liquid-applied membranes (RedGard, Hydroban, AquaDefense) are painted onto cement board or other substrates. Two coats, applied by roller or brush, create a seamless waterproof barrier. This is the most common DIY approach because you can use standard cement board and just coat it.

Cement board alone (Durock, HardieBacker) is NOT waterproof. It's moisture-resistant — meaning it won't disintegrate when wet — but it allows water to pass through to the framing behind it. If you use cement board in a shower, you must apply a waterproof membrane over it.

Substrate Preparation

Bathroom floors: standard plywood subfloor needs to be overlaid with cement board (1/4 inch for floors over 3/4 inch plywood) or a similar tile-compatible substrate. Tile directly over plywood will crack at the joints as the wood moves seasonally. The cement board is fastened with cement board screws and the seams are taped with alkali-resistant mesh tape bedded in thinset.

Shower walls: framing must be plumb and flat. A straightedge held against the studs should show no gaps greater than 1/4 inch over 8 feet. Shim or plane studs as needed before installing the substrate. Install the substrate (cement board, foam board, or sheet membrane) with the proper fasteners for the product. Leave a 1/8 inch gap at the bottom where the wall meets the shower pan — this joint gets caulked, not grouted.

Existing tile: you can tile over existing tile if it's firmly bonded and the surface is flat. Sand or grind the existing tile to roughen the surface, clean thoroughly, and use a modified thinset for the bond. This avoids the demolition mess but adds thickness — check that the new tile surface won't create a lip at transitions to adjacent flooring.

Layout Strategy

Measure the area and plan your tile layout before mixing any thinset. The goal is to avoid narrow slivers of tile at the edges and ensure the pattern looks balanced. Mark center lines on the wall or floor and do a dry layout with spacers to see where the cuts fall. Adjust the starting point so that cuts on opposite edges are roughly equal and at least half a tile wide.

For shower walls, start the layout from the most visible wall (the back wall or the wall you face when entering the shower). Center the pattern on that wall and work outward. The least visible cuts end up in the corners and at the bottom behind the curb.

In a tub surround, set the first row at the top of the tub (not the bottom) and work up. The tub edge may not be perfectly level, so the bottom row gets cut to fit the tub profile. Starting from the tub and working up would put the uneven cuts at the ceiling where they're visible.

Thinset and Application

Modified thinset (polymer-modified mortar) is the standard for bathroom tile. Unmodified thinset is used only when the manufacturer specifically requires it (some membrane systems need unmodified). For walls, use a non-sag formula that won't slide down the wall before it sets.

Trowel selection: the thinset bag specifies the notch size for your tile size. Generally, 1/4 x 1/4 inch square notch for mosaic and small tile (up to 6 inches), 1/4 x 3/8 inch for medium tile (6 to 12 inches), and 1/2 x 1/2 inch for large tile (12 inches and up). Using a notch that's too small leaves voids under the tile; too large wastes material and makes the tile sit too high.

Back-butter large-format tiles (anything over 12 inches) in addition to spreading thinset on the substrate. This ensures full coverage — large tiles are more prone to voids because the thinset ridges can collapse before the tile is set. Full coverage is especially important in showers where any void can become a water pocket.

Grouting

Sanded grout for joints 1/8 inch and wider. Unsanded grout for joints less than 1/8 inch. Epoxy grout is more expensive but virtually waterproof and stain-proof — worth considering for shower floors where the grout is constantly wet. Standard cement grout absorbs water (which is fine if the waterproof membrane behind the tile is doing its job).

Mix grout to a peanut-butter consistency — stiff enough to hold its shape but smooth enough to pack into joints. Let it slake (rest) for 10 minutes after mixing before using. Work it into the joints with a rubber float held at a 45-degree angle, pushing diagonally across the joints. Wipe the excess with a barely damp sponge in diagonal strokes, rinsing frequently. Don't over-wipe — you'll pull grout out of the joints.

Caulk (not grout) at all changes of plane: where the wall meets the floor, where walls meet in corners, where tile meets the tub or shower pan, and around fixtures. Grout cracks at these joints because the two surfaces move independently. Caulk flexes. Use 100% silicone caulk color-matched to the grout.

Tools for Bathroom Tile

Cutting: a manual snap cutter handles straight cuts on most wall tile. A wet saw (rent for $50 to $80 per day) handles all cuts — straight, angled, and notched — on floor tile, porcelain, and natural stone. Tile nippers for small notches around pipes and corners. A diamond hole saw for pipe penetrations.

Setting: notched trowel (size per tile), margin trowel for mixing and back-buttering, rubber float for grouting, tile spacers (1/16 to 1/8 inch for walls, 1/8 to 3/16 inch for floors), 4-foot level, laser level or chalk lines for layout. A tile leveling system (clips and wedges) is worth the cost for large-format tile — it prevents lippage (adjacent tile edges at different heights).

Prep: cement board screws and drill/driver, alkali-resistant mesh tape, mixing paddle and bucket for thinset and grout, sponges, bucket for rinse water, painter's tape for caulk lines.

Frequently Asked Questions