Match the crack to what's underneath
The line's location is your best clue, so think about what sits below it:
- Over a concrete slab, like a basement or a slab-on-grade kitchen, a crack that wanders slightly but keeps going usually follows a crack or control joint in the concrete. Why concrete floors crack and heave explains the common slab causes.
- Over a wood-framed floor, a very straight crack, especially one parallel to a wall, often lands on a backer-board or plywood seam. USG's cement-board instructions call for staggering board joints away from subfloor joints and embedding tape in mortar over every seam. When that step is skipped, the seam is a weak line.
- A crack that runs across the middle of a room, or a floor that feels springy there, points to the floor bending between joists. USG designs its tile floor system to a minimum stiffness of L/360, meaning the floor shouldn't sag more than 1/360 of the span under load, and says large tiles and stone need a stiffer floor than that.
- Cracks near the walls or in a long, unbroken run of tile can mean the floor had no room to expand. The National Park Service's guide to ceramic tile floors names missing expansion joints as one of the major causes of cracked, broken or loose tile.
Also note whether the crack goes through the tile itself or only through the grout. A grout-only crack is a different and often smaller problem. Cracked grout versus failing caulk covers that case.
What to check and photograph
Take one photo of the whole room and close-ups with a tape measure laid next to the crack. Write down when you first saw it, and check every few weeks whether it gets longer or wider. If you know what's underneath, note it: slab, plywood, or old boards. Tap the tiles along the line with a coin; a hollow sound means those tiles have lost their bond.
Look for water, too. A crack near a toilet, tub or dishwasher plus a soft or spongy floor can mean the subfloor is wet or rotting. Soft floors next to the toilet explains what that usually means.
Don't pry up tiles or chisel along the crack to look underneath. The Park Service notes that a hammer and chisel send shock through the grout and crack the tiles around the one you're removing. Its advice is to cut out all the surrounding grout with a grout saw first, and to leave that to an experienced tile setter. Sweep up sharp tile pieces and keep people off loose ones.
Older Philadelphia-area floors
Many older houses around Philadelphia still have original ceramic or mosaic tile in vestibules, bathrooms and kitchens. The Park Service brief recommends repairing historic tile rather than replacing it where you can, saving salvaged pieces to patch more visible areas, and using flexible sealant in grout-matched colors for new movement joints so they barely show. If your floor is original, look for a contractor who has repaired old tile before, not just one who lays new tile.
Who to call, and what to ask
Start with a tile contractor. Call a structural engineer first if the floor visibly sags, slopes, or has dropped next to the crack, or if doors in the area have started to stick. Those signs mean the framing needs a look before anyone sets tile. Ask the tile contractor:
- Which layer do you think moved, and what makes you think so?
- What will you check once the broken tiles are out, and what findings would change the price?
- Will you add movement joints, and where?
- Will you send me photos of the subfloor or slab before you cover it?
If they suggest a crack-isolation membrane or uncoupling mat, ask what it's for and what its limits are. USG rates its liquid crack-isolation membrane for shrinkage and other non-structural cracks up to 1/8 inch, and its uncoupling mat for in-plane movement up to 1/8 inch. Neither fixes a floor that bends or a slab that's still moving. The written quote should list tile replacement, substrate repair and joint work as separate items. How to compare floor repair quotes when the tile is discontinued helps when matching tile is hard to find.