Tap along a tiled floor with a knuckle and the note changes. Solid, solid, solid, then a dull drum under one tile or a whole run of them. Under that tile there is air where there should be mortar.
That is a finding, not a diagnosis. Hollow tile is not automatically loose tile, and plenty of it never becomes loose — a void only matters when something loads it or something feeds it water. Telling a cosmetic void from a failing one is a mapping exercise, and you can do most of the mapping yourself with painter's tape and a phone camera.
What the tap is telling you
The tile is a small drum. Where mortar is in full contact with its back, the tile cannot vibrate and the tap comes back short and dead. Where there is a gap, the tile flexes and the air behind it rings. That is all the sound knows. It reports a void. It does not report whether the tile is still attached around that void, or whether the void is growing.
Plenty of things change the note with nothing wrong. An uncoupling membrane sits between tile and substrate with open cavities by design, so a floor over one reads hollow evenly, everywhere. A heated floor does the same across the heated area. A plywood deck over joists is a drum on its own, where the same tile on a basement slab reads dead. And nearly every tile sounds hollower at its corners than in its middle, because a corner has more freedom to move.
So the tap only works comparatively: this tile against its neighbors, the middle of a tile against its own corners, the suspect area against floor you know is fine.
Map it before you decide anything
A knuckle works. So does a coin held flat, the handle of a screwdriver, or a steel ball on a rod, which is what the trade uses for this. Tap the whole floor in a grid — not just the tile that bothered you — and tape every hollow one.
Mark the rest of the evidence while you are down there: cracked grout, crumbling grout, any tile that clicks or rocks under your weight, caulk lines that have opened at the tub or the base of the wall, anywhere water sits after a shower.
Then photograph the taped floor and date the photo. Do it again after a season. A map that has not moved and a map that has grown are different problems, and nothing else you can do standing in the room separates them as reliably.
The shape of the map names the cause
- One tile alone in an otherwise solid floor — coverage local to that tile.
- Solid centers, hollow corners, tile after tile in the same pattern — dabs.
- A large area set on one day, uniform and unchanging — trowel technique.
- A zone at the base of a wet wall, around a valve or under a niche, spreading outward — water.
- A straight line, running with the joists or across them, with cracked grout in the same line — movement.
Cause one: it was never bonded in the first place
Spot bonding is a dab of thinset at each corner and one in the middle. Fast, forgiving of an uneven substrate, and it leaves the tile bridging air across most of its back from the day it went down. The other version is combed mortar whose ridges never collapsed. They are meant to flatten as the tile is pressed and beaten in; if the mortar had skinned over, or the tile was laid on rather than worked in, the ridges stand and air stays in the valleys.
How you tell, standing there
Tap the middle of a tile, then each corner. Dab-set tile gives a solid note in the center and a hollow one at all four corners, tile after tile, in the same repeating pattern. That does not happen by accident. Standing ridges read differently — stripes of solid and hollow across a single tile, running the direction the trowel was combed.
Then look for what is absent. Press hard at a corner with your thumb, then stand on the tile edge and shift your weight onto it. Tile that is bridging but still bonded does not click, does not rock, does not change its note under load. The grout around it is intact — no hairlines, nothing powdery, no joint that stays dark after the floor dries. And as far as anyone remembers, it has always sounded like that.
That is a floor set quickly that has been holding anyway. On a small format tile in ordinary traffic it can hold for as long as the floor exists.
Where it stops being harmless
Coverage matters more as tile gets bigger, because a longer tile gives a point load more leverage over an unsupported corner — one reason tile size changes the labor and not just the look. It matters at thresholds, in front of a shower door where every foot lands in the same place, and under anything heavy on a hard wheel. A bridged tile that does crack tends to crack all the way through.
For a stable spot-bonded floor the honest answer is usually: leave it, keep the map, and do not buy a new floor on the strength of a sound.
Cause two: the bond has let go
This is the one that matters. The tile was bonded and is not anymore, or what it was bonded to has stopped being solid. The usual histories are mortar set over dust or a curing compound on a slab, over a sealer nobody ground off, over a substrate that was still damp, over paper-faced gypsum in a wet area, or over a wall that has been getting wet ever since from behind the tile rather than in front of it.
How you tell, standing there
The tells are change and collateral damage, not the sound by itself:
- The map grows. Compare the tape against your photo. New hollow tile at the boundary of an old hollow zone is a bond that is traveling.
- Grout fails in the same footprint. Sandy, crumbling, coming out with a fingernail, returning after it was repaired once — grout that keeps falling out usually points under the tile rather than at the grout.
- Something moves. A tile that clicks under weight, one sitting a hair below its neighbors, an edge chipped where two no longer meet flush.
- The note is looser. A bridged tile rings fairly cleanly. A tile that has fully released rattles — shorter, buzzier, with play in it.
- Water goes somewhere. A joint that swallows water instead of shedding it, or one that stays dark long after the rest of the floor is dry.
If the hollow zone is at the base of a wet wall
Then it is not really a tile question. Water passes through grout and thinset as a matter of course, and the layer behind them is what stops it — so hollow tile at the bottom of a shower wall, around the valve, in the inside corners or under a niche is the surface reporting on that layer. What goes behind bathroom tile covers the assembly and where it fails, and a tub surround has its own weak point at the joint where tile meets the flange. Check the ceiling in the room below too. A stain there turns a maybe into a yes.
If a cavity has been wet long enough there is often mold in it. We will not tell you what that means for your health — we do not test for it, diagnose it or remediate it, and neither does anyone answering that over the phone. The EPA publishes homeowner guidance on mold and moisture, and the Indiana Department of Health can point you toward testing. Our end of it is the water.
Cause three: the floor under it is moving
Tile does not bend. The subfloor spans between joists and the joists span between bearing points, and both deflect under load. Past what a tile assembly tolerates, the bond breaks along the line the floor moves on and the grout cracks in that same line. People miss this one because all of it looks like a tile problem.
How you tell, standing there
- The map is a line, not a blob. Straight, running the length of the room or square across it, often over a joist or exactly mid-bay.
- The grout is cracked along that line, and came back after it was raked out and repaired.
- The floor announces it when you walk. Bounce underfoot, a vanity door that swings when someone crosses the room.
- Load changes the sound. Tap a tile with nobody else on the floor, then have someone stand mid-bay beside it and tap the same tile again. Deflection changes the note. A coverage void does not.
- It squeaks. The mechanism behind a floor that squeaks is the same one cracking your grout.
If there is a basement or crawl space, go and look up. A joist notched deeply for a drain line, a sistered joist stopping short of its bearing, a post that has settled, a wall removed in an earlier remodel — any of them shows up as a line in a tile floor two levels of finish away.
This one is not a tile repair
Nothing done to the tile fixes deflection. Injecting adhesive into a bay that moves gives you a stiffer void that cracks along the same line. The fix is underneath — sistering, blocking, a mid-span beam, a post — and it happens before anything is set back down, which is why stiffness and flatness get sorted first rather than after the tile is ordered. If it is structural rather than a springy joist bay, that is a structural engineer's call and a foundation contractor's work, not ours. We will say so rather than tile over it.
Walls sound hollow almost by default
On a wall the tap is weak evidence. Backer board over studs has air behind it by design, so the note goes solid over each stud and hollow in every bay, and that pattern tells you where the framing is. Weight the other signals instead: grout cracked in a corner, a caulk joint that opens no matter how often it is redone, a tile that moves under your palm, dampness at the base, a stain on the far side of the wall. Change-of-plane joints move and grout does not, so the joints that fail first fail there whether or not anything is wrong deeper in.
What is a Saturday, and what is not
Yours: mapping, taping, photographing, dating, checking again after a season. Raking out and replacing a small patch of cracked grout — understanding that if it cracks again in the same place, that is the answer rather than a failure of your grouting. Swapping grout for silicone at the inside corners and the wall-to-floor joint, where a movement joint belongs. Keeping water off the suspect area while you decide.
Not yours:
- Injecting epoxy through drilled grout joints. It suits specific situations and makes the rest permanent.
- Pulling a tile in a shower for a look. Once you are through the tile you are into the waterproofing, and if you do not know what is back there you have turned a question into an opening.
- Anything on a floor with heat in it. The cable cannot be cut or spliced — see heated floors for what that changes.
- Anything where the honest answer was structural.
Replacing one tile, honestly
On a floor a single failed tile is genuinely repairable. Find a match first: dye lots vary and patterns get discontinued, so a near match reads as a repair from then on. With no attic stock, the better move is often to steal a tile from inside a closet and put the mismatch where nobody stands.
Then the grout comes out, and the tile is cut into pieces and taken from the middle outward so nothing levers against its neighbors. Neighbors chip anyway sometimes. Then the substrate has to come clean, back to something sound and dry, and that is the fork. Solid underneath, and the new tile gets set, brought to the same height and grouted. Soft, damp, or a membrane already compromised, and nothing goes back until that is dealt with.
Nobody knows which of the two it is until the tile is out. Anyone telling you from a photograph is guessing, and guesses of that kind are the ordinary route to a change order.
When the answer is that it comes up
When the map is growing, when it sits at the base of a wet wall, or when the floor moves, patching is money spent on the symptom. At that point you are not pricing a repair. You are pricing the room, because tile, substrate, waterproofing and usually the shower come out together.
We do not publish our own prices, for the reason we give everywhere on this site: a number written before anyone has seen your bathroom is a guess. What we can point at is what independent research says people here spend. Angi's Indianapolis bathroom cost research and published Greater Indianapolis contractor cost guides put a typical bathroom around $10,000, most projects between roughly $5,700 and $14,700, and full remodels across the wider metro from about $7,500 past $25,000.
Where a room lands in a spread that wide depends on what is found once the tile is off, which is the argument for looking before quoting instead of after.
What we do when we look at one
We tap it ourselves — the whole floor and the walls, not only the tile you pointed at — and mark what we find. We ask when it was set and what has happened in the room since. We take a moisture reading at the base of the walls, look at the ceiling below, check the floor for deflection, and go underneath if there is anywhere to go. Then we read the pattern: one tile, an edge, a zone at a wet wall, or a line.
Some of it we can answer from the doorway. Some cannot be answered until it is open, and we would rather say so than write a confident number over a cavity nobody has seen. When it does come apart, substrate and waterproofing get their own stage before any finish material arrives, which is how the bathroom work is sequenced anyway.
The short version
Hollow is a sound. Loose is a condition. They overlap, they are not the same thing, and the sound alone will not tell you which you have.
Tap the whole floor, tape it, photograph it, look again after a season. One quiet tile in a solid floor that has not changed is a floor you can live on. A zone at the base of a wet wall, or a straight line with cracked grout down it, is not a tile problem and will not be fixed by tile work.
If you have taped a floor and want someone to put a knuckle on it before you commit to anything, ask for an estimate and we will map it with you.
