· by Michael Blair, Nashville's Tile and Stone
It often happens overnight, sometimes with a bang: a row of floor tiles lifts along a line, forming a ridge or a tent, with the grout cracked on both sides. Or, quieter, a few tiles begin to sound hollow underfoot and eventually rock and come loose. Both are the same failure. The tile and the floor beneath it moved by different amounts, the bond between them gave up, and the tile, which cannot compress, buckled upward. The causes are specific, and a repair that does not address them produces the same ridge a few years later.
What makes a tile floor move
Tile expands with heat and with moisture, and so does whatever is under it, but at different rates. A concrete slab shrinks as it cures for years and moves with moisture from below; a wood subfloor swells and shrinks with humidity; the whole house moves a little on Middle Tennessee clay. Tile bonded rigidly to all of that, wall to wall with no gap at the edges and no movement joints across the field, has nowhere to go when the floor beneath it shrinks or it expands. The stress builds until the weakest bond lets go, and the tiles pop upward along the line of least resistance.
A sunroom or a room with a big south-facing window, where the floor heats sharply in the afternoon, is a classic location. So is a large open floor with tile run continuously through several rooms, and a floor over a slab that was tiled while the concrete was still new and shrinking.
What makes the bond weak
A floor that was installed with good coverage and the right thinset can hold a lot of movement. The floors that fail were often set with thinset spread too thin, spot-bonded with dabs at the corners, or laid on a slab that had a curing compound, sealer or dust on it that the mortar never bonded to. Pull a popped tile and look at the back: clean tile with no mortar on it means the bond to the tile failed; mortar on the tile and a clean slab means the bond to the floor failed; and dabs instead of full coverage tell their own story.
A wood subfloor that was too thin or too springy, a tile laid directly on plywood without a backer or an uncoupling membrane, or a slab with moisture coming up through it, all weaken the bond over time until ordinary expansion is enough to finish it.
- No movement joints across a large floor, and no gap at the walls
- Thin or spot-bonded thinset, or a slab surface the mortar could not grip
- Tile on new concrete that was still shrinking
- Sharp temperature swings on the floor surface
- Moisture from below, especially on a slab at grade
What the repair has to do
The tented tiles and every hollow-sounding tile around them come up, and the old thinset is ground off the floor so the new mortar bonds to something solid. Tapping the whole floor with a coin or a rod finds the tiles that have already let go but have not moved yet, and they come up too; leaving them in is how the ridge reappears next door. The floor surface is checked for the reason the bond failed: contamination, moisture, deflection.
Then the movement is designed in. Soft joints, filled with a flexible sealant rather than grout, go at the perimeter under the baseboards and across the field at intervals the tile industry standards set, closer in a room with direct sun. If the whole floor is being redone, an uncoupling membrane between the floor and the tile lets the two move independently, and it is the single best insurance against the problem on a slab or a wood subfloor.
Whether to repair or replace
If the tenting is in one area, the rest of the floor sounds solid, and matching tile is available, a section repair with soft joints added is reasonable. If the hollow sound is widespread, the tile is discontinued, or the floor was spot-bonded throughout, the failure will keep spreading and a full replacement done properly is the better spend. Have the tile setter tap the whole floor and tell you what share of it has already let go before deciding.
A tile floor set with full mortar coverage, on a stable substrate, with movement joints where they belong, does not tent. The ones that do were missing one of those three, and the repair is worth only what it does about the missing one.