Tile Fails in the Layers You Never See

Tile does not bend. Grout bends even less. So the first question on a tile floor is not which tile, it is how much the floor moves when somebody walks across it. Joist span, joist depth, spacing and what the subfloor is made of all feed into that, and plenty of houses around here were framed for a carpet and a pad, not for a stone floor and a mortar bed. You can feel it before you measure it. Stand in the middle of the room and rock on your heels. If the china rattles in the next room, that floor is going to crack tile at a joist line and there is no adhesive on the market that fixes it. The repair is structural: sister the joists, add a layer of plywood, or in some rooms pick a different floor. Better to hear that at the walkthrough than to chase a cracked joint line a year later.

Once the floor is stiff enough, the next layer is what separates the tile from the wood underneath. Plywood swells and shrinks with the humidity and tile does not, so bonding one straight to the other is asking for a failure. Cement backer board screwed down in a bed of thinset gives you a stable, mineral surface for the tile to grab. An uncoupling membrane does the same job differently, letting small movement happen below the tile instead of through it, and it eats less height where height is scarce. In a bathroom or a mudroom the waterproofing question comes next, and it is a separate layer from the tile. Grout is not a seal. Tile is not a seal. A shower pan, a curb or a floor under a laundry needs a real membrane tied into the drain, done before a single tile goes down.

What the visit covers

• Check the floor for deflection before anything gets ordered
• Add plywood or sister joists where the framing is too light
• Set cement backer board in thinset and screw it to pattern
• Lay an uncoupling membrane where height or movement demands it
• Waterproof wet areas as a layer, not with grout alone
• Dry lay the layout so the cut row lands out of sight
• Comb thinset in one direction and back butter large format
• Pull a tile early to confirm full coverage on the back
• Set electric heat mat in the mortar bed where you want it
• Grout, clean the haze, then seal the joints that need it

Layout, grout and a warm floor

Layout is decided before the first tile, not discovered halfway across the room. We snap lines off the longest sight line rather than off a wall that is out of square, dry lay a run, and put the narrow cut row where nobody stands: under the vanity, along the tub, behind the door swing. Nothing announces an amateur job faster than a two inch sliver of tile greeting you in a doorway. Coverage matters just as much and nobody checks it after the fact. Thinset gets combed in one direction so the air can escape as the tile is set, large format tile gets back buttered as well, and we pull a tile now and then to look at the back. Hollow spots under a corner are where a floor tile cracks first, usually the winter after somebody drops something on it.

Grout is a choice, not a default. Sanded grout for wider joints, unsanded for tight ones, and a high performance grout where the floor sees water or road salt, because a cement grout in an entry will absorb the gray slush that comes off boots from November on and never fully give it back. Cement grout wants sealing once it has cured, and sealing again later. Heated mats are worth mentioning here because tile is cold and an old house here is colder. An electric mat goes into the mortar bed under the tile, it runs off its own thermostat, and it turns a bathroom floor from the worst surface in the house on a January morning into the best one. It has to be planned early: the mat, the sensor wire and the height it adds all belong in the scope before anybody orders tile.