Start With the Subfloor, Not the Tiles

Almost every cracked tile, hollow-sounding corner and lifting edge traces back to what's underneath it. In a UK home, bathroom floors are usually timber — original floorboards or chipboard sheets in newer builds — and both move far more than people expect. Tiles don't bend, so when the floor flexes, the adhesive bond or the tile itself gives way.

Before anything else, check for deflection. Walk across the room and watch for bounce. If the floor feels springy, or if a glass of water on the sill ripples when you step nearby, the joists need stiffening or the boards need overboarding before a single tile comes out of the box.

• Chipboard is not a tile-ready substrate. Overboard it with a minimum of 15mm plywood, screwed at 150mm centres, or better still 6mm cement backer board fixed with flexible adhesive and screws.
• Seal and prime timber. Untreated plywood pulls moisture out of the adhesive before it cures. A coat of acrylic primer solves it in minutes.
• Fill and level. Any dip deeper than 3mm over a 2m straight edge needs a smoothing compound. Tiling over a hollow just hides the problem for six months.
• Never tile straight onto vinyl, lino or old adhesive residue. They're a parting layer, not a base.

Choosing the Wrong Adhesive

Adhesive isn't a commodity; it's an engineered product, and the difference between a £15 bag and a £30 bag is the flexibility and grip that keeps your floor intact. On a timber or backer board floor, standard-set wall-and-floor adhesive is the wrong answer.

Look for a flexible, cement-based powder adhesive, mixed by hand with a paddle and drill — not a ready-mixed tub, which relies on water evaporation to dry and simply won't cure properly under a large-format porcelain tile. For underfloor heating or a suspended timber floor over a void, step up to an S1-rated adhesive. If you're laying stone, choose adhesive with a suitable base for moisture-sensitive tiles.

Matching the adhesive to the tile matters too. Porcelain is dense and barely absorbs water, so it needs a modified adhesive with good grab. Large-format tiles over 600mm benefit from a large-trowel notch and back-buttering — combing adhesive onto the tile as well as the floor. Anything less than 95% coverage under a floor tile is a void waiting to crack.

Waterproofing Isn't Optional in a Wet Room

In a standard bathroom with a bath or a low-level shower tray, most of the floor is splash zone, not wet zone. In a walk-in shower or wet room, the entire shower footprint — and ideally the whole floor — needs tanking.

Tanking is a liquid-applied or matting-based membrane that sits between the substrate and the tiles, turning the floor into a sealed tray so any water that gets behind the tiles drains away or evaporates rather than soaking into the joists. Skipping it is the single most expensive shortcut in bathroom tiling, because the failure shows up as a stained ceiling downstairs, not a wobbly tile.

• Use a proper tanking kit with jointing tape at every internal corner, floor-to-wall junction and pipe penetration.
• Allow the full curing time on the instructions — usually 24 hours between coats.
• Don't pierce the membrane after it's applied. Even screw holes from a shower screen need sealing.

Even outside a wet room, silicone — not grout — should run around the entire perimeter of the floor and at every wall junction. Grout is rigid and will crack with normal building movement.

Layout, Expansion and Movement Joints

Poor planning creates awkward slivers of tile at doorways, which look amateurish and crack easily. Set out the room before you mix any adhesive: find the centre line, dry-lay a row, and adjust so you get cut tiles of at least half a width at every edge.

Leave a small gap — 2 to 3mm — between the tile edge and any wall, then cover it with skirting or a bead. That gap is a movement joint, and floors without one tend to tent upwards in the middle when the heating comes on. On floors larger than about 30 square metres, or any floor with underfloor heating, a proper expansion joint through the tile field is good practice, not fussiness.

If you're using underfloor heating, remember that it must be commissioned — warmed gradually, then cooled — before tiling, and never switched on within seven days of grouting.

Grout, Sealing and the Final Details

Grout choice gets treated as an afterthought, but a floor needs a flexible, fine-grained grout, ideally polymer-modified and mould-resistant. Wide joints of 3 to 5mm handle movement better than hairline joints, which crumble. Work it in diagonally with a rubber float, clean the haze off before it sets hard, and don't over-wash the joints or you'll pull the binder out and leave them sandy.

Natural stone needs sealing before and after grouting, and porcelain in a wet room benefits from a penetrating sealer on the grout lines. Finally, resist walking on the floor for at least 24 hours and don't bring heavy items in for a week. The adhesive and grout are still curing, and every bit of patience you show now is a tile that stays flat for the next twenty years.