Where this starts
Tiles that sound hollow when tapped, lift at a corner, or come away in a group are reporting a bond failure somewhere in the stack — and the stack has several interfaces, each of which fails for different reasons.
The most informative thing available costs nothing: take a loose tile off and look at the BACK of it. A tile with almost no adhesive on it means the adhesive never bonded to the tile. A tile covered in adhesive that has parted from the wall or floor means the adhesive-to-substrate bond went. Adhesive with substrate material stuck to it — paper, paint, screed, render — means the substrate itself was the weak layer and it failed rather than the bond.
Hollowness is a related but separate observation. A tile can sound hollow because there are voids under it from the outset — the adhesive was combed and the tile pressed without full coverage — and still be firmly held at its edges. That is a coverage problem, and it matters most where the tile will be loaded, impacted or wetted.
Movement is the family that produces failures in floors and in large areas rather than in single tiles. Everything moves; the question is whether the installation was given anywhere to do it. Tiles debonding in a line, tenting up along a joint, or failing across a whole bay usually point there.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
Inadequate adhesive coverage
Most commonHollow sound over parts of the tile, with adhesive ridges visible on the back and bare areas between them. Often worst at corners, which is where a tile lifts first.
How to check: Lift one tile and estimate what proportion of its back was actually in contact. Ridges with gaps between them are a coverage failure rather than a bond failure.
The substrate was not prepared or primed
Most commonAdhesive on the back of the tile, with the failure at the substrate face — and often dust, paint or a sealer film on the exposed surface behind.
How to check: Look at the wall or floor where the tile came away. Powder, a shiny film, gloss paint or old adhesive residue each identify themselves.
No movement provision, or joints filled solid
CommonFailure in a line or across a bay rather than at one tile, sometimes with tiles tenting up at a joint. Perimeter joints filled with grout rather than left or sealed flexibly.
How to check: Look at the perimeter and at any joint in the substrate beneath. Grout carried across a movement joint transmits the movement into the tiles.
The wrong adhesive for the tile or the conditions
CommonClean tile backs with adhesive left on the substrate, particularly with porcelain, large formats, or anywhere wet or heated.
How to check: Compare what was used against what the tile and the situation call for. Porcelain, large tiles, wet areas and heated floors each narrow the choice considerably.
Tiled onto a substrate that was still moving or drying
Less commonWidespread rather than local failure, on a new screed or a new extension, with the timing tied to when the substrate was laid.
How to check: The dates. A screed needs to reach a stated moisture content before it is covered, and covering it early traps the moisture and the shrinkage.
Water getting behind the tiles
Less commonFailure concentrated in a shower, around a bath, or at a floor-to-wall junction, with damp or degraded substrate behind and sometimes a smell.
How to check: Whether the area is a wet one and whether it was tanked. Tiles and grout are not a waterproof layer on their own.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
It shows the extent before anything is disturbed, and the pattern is itself diagnostic: scattered single tiles, a line, or a whole bay point at different causes.
- 2
It identifies the failure plane, which separates most of the causes above in one observation and costs one tile that was already loose.
- 3
Dust, paint, a sealer film, old adhesive, or damp material each identify a specific preparation or moisture cause rather than a general one.
- 4
Filled or missing movement provision is the cause behind most line and bay failures, and it is visible without removing anything.
- 5
A new screed, a heated floor, a timber deck and a plastered wall each impose different requirements on the adhesive and the movement provision.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where tiles are failing on a floor over a timber structure and the deflection feels noticeable underfoot — the fix is in the structure, not the tiling.
- Where the substrate behind is wet, soft or crumbling, particularly in a shower or wet room: the waterproofing has failed and re-tiling over it repeats the failure.
- Where tiles are tenting or lifting along a line, which means the installation is in compression and something is moving — worth understanding before it is simply re-fixed.
- Any external or facade tiling coming loose, where a falling tile is a safety matter rather than a repair.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.