Bathroom tile installation, starting under the tile

Assess what the tile is going onto

A tiled floor is only as stable as the deck under it. Too much deflection between joists and the tile or grout cracks regardless of how well it was set — which is why a bouncy floor gets stiffened, or an uncoupling membrane specified, first. On walls, the check is flatness against the tile size chosen.

Watch for: Tile laid over a springy floor "because the tile is thick". Tile is strong in compression and brittle in bending. If the floor moves underfoot before tiling, it will move under the tile.

Replace the wrong substrate rather than tile over it

Standard drywall — including green-tinted moisture-resistant board — is not a shower backer. Wet areas get cement backer board, a foam board system, or another substrate rated for the location. Outside the wet area, a properly prepared wall takes tile directly.

Watch for: Greenboard behind a shower. It is moisture-resistant, not waterproof; it is a very common shortcut; and once soaked it turns soft and the tile comes away in sheets. If you ask one question about a shower, ask what is behind the tile.

Waterproof the wet area as a system, not as a product

A shower needs a continuous waterproof layer — a bonded sheet membrane, a liquid-applied membrane, or a proprietary board-and-band system — carried up the walls, into every internal corner, across the floor, and detailed where the drain passes through it. Corners and the drain penetration are where failures start, not the middle of a flat wall.

Watch for: Products from different systems mixed on the same shower, or a membrane that stops at the wall-floor junction instead of being carried through it. Manufacturers warrant their own system installed their own way; a hybrid assembly is warranted by nobody.

Build the fall to the drain — and check it with water

A shower floor must slope consistently toward the drain across its whole area, including under a bench and into the corners. On a mortar bed it is formed by hand; on a foam tray it is preformed. A linear drain lets the whole floor fall one way, which is why curbless showers usually use one.

Watch for: Ponding. Water sitting in a low spot after the shower is off means the fall is wrong, and no grout or sealer fixes it — it is a rebuild. Ask for a water test on the finished pan before tiling.

Set out the tiling before setting any tile

Where the cut tiles land, whether the wall courses line up with the floor, whether the niche and the valve fall on a joint or mid-tile — all of it is decided by a dry layout. Large-format tile is less forgiving because there are fewer joints to absorb error.

Watch for: A layout that leaves a sliver cut at the most visible edge. Ask to see the setting-out marked on the wall before tiling starts; a chalk line is much easier to move than a tiled wall.

Bed the tile with full support behind it

Adhesive is combed and the tile bedded so its back is genuinely supported rather than sitting on ridges with voids underneath. Floor tile and large formats need more coverage than small wall tile. Hollow-sounding tile is unsupported tile, and unsupported tile cracks where it is stood on.

Watch for: The "five dabs" method on floor tile or large-format wall tile. Fast, fine on the day, and it leaves voids that crack under load or fill with water in a shower.

Grout in the joints, flexible sealant everywhere things move

Grout fills the joints between tiles in the same plane. Every junction that can move — internal corners, the wall-to-floor line, around a tub or tray, and where tile meets a different material — takes a flexible sealant instead, in a joint deliberately left for it. Large floors also need movement joints designed in, usually at the perimeter.

Watch for: Grout run hard into every corner. Corners move seasonally and grout cannot; it cracks, and a cracked corner is an open path into the wall. On floors, a missing perimeter joint shows up later as tenting — a ridge of tiles lifting off the deck, which is a full re-lay to fix.