How to Tile a Floor

Tile is rigid, so its base has to be rigid too. Walk the floor and feel for any bounce or flex — tile needs structural deflection no worse than L/360, and a springy floor will crack grout within a season. Then lay a straightedge across the floor in several directions: it must be flat to within 1/8 inch over 10 feet. Grind or sand down high spots and fill low spots with a floor patch or self-leveler. The surface also has to be clean, dry, and free of paint, adhesive, or wax that would break the mortar bond.

You cannot reliably tile over a bare wood subfloor — it needs a dimensionally stable layer first. Option one: cement backer board. Comb a thinset bed onto the subfloor with a 1/4-inch trowel, lay 1/4-inch or 1/2-inch backer board into it, screw it down every 8 inches with backer-board screws, then tape the seams with alkali-resistant mesh and skim them with thinset. Stagger the board seams so they never line up with the subfloor seams, and leave a 1/8-inch gap between boards. Option two: an uncoupling membrane, which embeds in thinset with no screws, adds crack isolation, and waterproofs the floor in one step.

Measure to find the center of the room and snap two perpendicular chalk lines that cross there. Dry-lay a row of tiles with spacers along each line out to the walls and look at the cut tiles you'd end up with at the perimeter. If a wall would land on a sliver narrower than about 2 inches, shift the whole layout half a tile so both opposite walls get a wider, balanced cut. This is the same balance-the-cuts planning that makes a tile backsplash look professional — get it right before any mortar goes down.

Add the thinset powder to water (not the other way around) and blend with a paddle on a drill until it's the consistency of stiff peanut butter — it should hold a ridge without slumping. Let it slake (rest) for 5 to 10 minutes so the polymers activate, then remix briefly. Use a modified thinset for most porcelain and ceramic over backer board, but check your tile and membrane instructions — some uncoupling membranes require unmodified thinset to cure properly. Only mix as much as you can use in about 30 minutes.

Spread thinset over a few square feet with the flat side of the trowel, then comb it into even ridges with the notched edge held at a 45-degree angle, all combed in one direction. Set the first tiles along your center lines, pressing each one down with a slight back-and-forth twist across the ridges to collapse them and seat the tile. Drop spacers in as you go to keep the joints even. Every few tiles, lift one and check the back — you want at least 80 percent mortar coverage (95 percent for large-format), and you should back-butter big tiles to get there.

Work outward from the center in small sections, kneeling on a board (never directly on set tiles). Lay a level or straightedge across every few tiles to confirm no edge sits proud of its neighbor — that height difference is called lippage, and it shows badly on a floor and catches toes. For large-format tile, snap a clip-and-wedge leveling system onto the joints and tighten until the tiles pull flush — snug, not crushed. Lippage is fixed with flat substrate and full mortar coverage, not by jamming spacers.

Measure each perimeter tile individually — walls are rarely square — and subtract the width of a grout joint. Straight cuts go fast on a manual snap cutter; use a wet saw for diagonals, L-cuts around a toilet flange, and any large or dense porcelain tile, and tile nippers for small curves around pipes. Cut wet and wear a respirator: dry-cutting tile throws crystalline-silica dust that permanently damages your lungs. Set the cut pieces with the same coverage and spacing as the field tiles.

Wait a full 24 hours before walking on the floor or grouting — the thinset has to cure or you'll shift tiles and pull lippage. Pull the spacers, then mix grout (sanded for the typical 1/8-to-1/2-inch floor joint). Pack it deep into the joints with a rubber float held at a 45-degree angle, working diagonally across the tiles so the float doesn't dig grout back out. Wait 15 to 20 minutes, then wipe the haze off the tile faces with a damp — not soaking — sponge, rinsing it often. Do not grout the perimeter or any inside corner; those joints flex and need flexible caulk instead.