We get called out to look at failed tile floors regularly. Cracked grout lines running in a straight line across a room. Tiles that sound hollow when you tap them. A row that's tented up along one wall.
In nearly every case the tile was fine and the installation was fine. What was missing happened before any tile came out of the box.
1. The slab cracked, and the tile was bonded rigidly across it
Concrete slabs crack. That is not a defect — it is what concrete does as it cures and as the ground under it moves seasonally. In Pinellas County, with a high water table and expansive soil in places, that movement is real and ongoing.
If you bond tile rigidly across an existing crack with thinset, the tile has no choice. When the slab moves, the tile is the weakest point in the assembly, and it fails along the same line the crack runs. That's why the failure is almost always a suspiciously straight line rather than a random pattern.
The prevention is a crack isolation membrane — a sheet or liquid-applied layer meeting ANSI A118.12, installed over the crack and a margin either side. It uncouples the tile from the slab so the slab can move without dragging the tile with it. It costs a fraction of what replacing a failed floor costs, and it's the line most commonly missing from a cheap quote.
A membrane will not save you from a structural crack — one that's growing, has vertical displacement between the two sides, or runs the full length of the building. That needs assessing before anyone tiles anything. Any contractor who tells you a membrane fixes a moving structural crack is guessing.
2. Moisture is coming up through the slab
This one is nearly invisible until it isn't.
Water vapour moves up through concrete constantly. In newer construction a plastic vapour retarder under the slab limits it. In a lot of Pinellas housing stock, that barrier is degraded, was never installed, or was punctured during construction fifty years ago.
Tile itself doesn't care. Adhesives do. Glue-down vinyl, LVT and some engineered products will debond over time under sustained vapour drive, which is how you end up with a floor that was perfect for eighteen months and then started lifting at the seams.
The check is a moisture test on the slab — a calcium chloride test or an in-situ relative humidity probe — before selecting the material. It's cheap. If the reading is high, the answer is either a vapour-mitigating membrane or choosing a material that doesn't care, and that decision needs making before you buy flooring, not after.
3. The slab isn't flat enough for the tile you chose
Flatness tolerance tightens as tile gets bigger. A 12×12 tile is forgiving. A 24×48 large-format plank is not — and large format is what almost everyone wants right now.
Lay a big rigid tile across a slab that dips or humps and you get lippage: one edge sitting proud of its neighbour. You will feel it with bare feet every day, it collects dirt at the raised edge, and it's a stub-your-toe hazard. Levelling clips help set it flat but they cannot fix a slab that isn't.
The fix is boring and effective: grind the high spots, fill the low spots with self-levelling compound, and check with a straightedge before setting anything. On a typical Pinellas slab this runs $1–$3 per square foot. It is the single most common item cut from a competing quote to win a job on price.
The detail everyone forgets: movement joints
Tile expands. So does the slab, at a different rate. Industry guidance calls for a soft movement joint — flexible sealant, not grout — around the perimeter of every tiled room, and in field joints on larger areas.
Grout the perimeter tight to the wall instead, and on the first properly hot week the floor has nowhere to go but up. That's the tented row you occasionally see along a wall or a doorway.
How to check a quote
You don't need to become a tile setter. You need to ask three questions:
- "Have you checked the slab for cracks, and are you using a crack isolation membrane?"
- "Are you moisture-testing the slab before we choose material?"
- "What's the flatness like, and is there a levelling line in this quote?"
A quote that's meaningfully cheaper than the others is usually cheaper because those three lines aren't in it. That's not always wrong — if your slab is genuinely sound, flat and dry, you don't need any of them, and we'll tell you that and leave them off. But it should be a decision someone made after looking, not a line someone dropped to win the job.