Overview
The new tile is only as good as the old tile underneath it
An overlay asks the existing tile to become the foundation for the new floor. That means every quality of the old installation carries forward: tile that is firmly bonded, flat and dry can be a sound, stable base — in some ways better than a bare slab, because it has already proven it does not move. Tile that is hollow-sounding, cracked or loose cannot be fixed by covering it; the new tile and mortar bond to the old tile, and if the old tile lets go of the floor below, the new surface goes with it. On our Jacksonville estimates, the first check is always a sound check: we tap across the floor and listen for the hollow spots that show where the old bond has already failed, then check any cracked tile to see whether the crack is in the tile alone or telegraphing movement from below.
Flatness is the second gate, and it gets stricter as tile gets bigger. A floor that looked fine under small, older tile can have dips and humps that a large-format tile cannot follow — the bigger the piece, the more a low spot shows up as an uneven edge against its neighbor and a hollow pocket under its middle. We check the surface with a long straightedge in several directions, because a floor can read flat along the room and still dip across it. Small variations can be corrected in the overlay preparation; a floor that waves from area to area usually has to come up so the base underneath can be leveled properly, which is also the honest moment to check what the slab or subfloor underneath looks like.
Height is the check homeowners feel every day after the job. Tiling over tile adds the thickness of the new mortar bed and the new tile on top of the floor you already have, and that added height lands in specific places: doors that have to swing over the finished floor, the toe space and dishwasher opening in a kitchen, toilet and vanity connections in a bathroom, and the transition where the tiled room meets wood, LVP or carpet in the hall. None of those is a reason to rule an overlay out on its own — transitions and a shaved door bottom solve most of them — but they have to be measured before the decision, not discovered with the new floor half laid. In older Jacksonville homes in Mandarin, Riverside and Southside, where floors have sometimes already been layered once, this measurement is often the one that settles the question.
Moisture is the quiet disqualifier, especially in Florida. A floor can sound solid and still be sitting over moisture that has been working underneath it — around a bathroom in a humid house, at a kitchen sink line, or over a slab that wicks ground moisture after heavy rain. Tiling over a damp or mold-marked base seals the problem in instead of solving it. Loose grout that keeps darkening, a musty smell that cleaning never removes, or tile that has started tenting or popping are all signs the failure is below the surface, and covering those floors is how a cosmetic project turns into a repeat repair. When those signs are present, demolition is not the expensive option; it is the option that only has to be paid for once, because it lets the real cause be fixed before the new tile installation goes down.
