· by Michael Blair, Nashville's Tile and Stone
Tile cracks because the floor beneath it moves. Deflection standards for tile are stricter than for other floor coverings, and older Nashville joist spans frequently do not meet them without help.
Stiffness first
Tile assemblies have deflection limits, and natural stone is stricter than ceramic. A subfloor that feels springy underfoot is already outside them.
Adding a second layer of subfloor, or sistering joists, is what brings a floor into range. It cannot be corrected with a thicker mortar bed.
The right underlayment
Cement backer board or an uncoupling membrane over a sound subfloor. Backer board adds stiffness; an uncoupling membrane allows small substrate movement without transmitting it to the tile.
Neither is optional. Tile bonded directly to plywood is a failure waiting to happen, because plywood moves with moisture.
- Check deflection before anything else
- Cement backer board or uncoupling membrane, never bare plywood
- Movement joints at perimeters and at intervals
- Waterproof membrane in wet areas, over the backer
- Flat is different from level — tile needs flat
Flat versus level
Tile needs a flat plane; it does not need to be level. A floor with a consistent slope tiles fine, and a floor with dips and humps does not, particularly with large-format tile.
Large tiles are far less forgiving of unevenness, which is why they often need leveling compound where a small mosaic would not.
Wet areas
A waterproof membrane over the backer, not the backer alone. Cement board is not waterproof — it is a stable substrate that tolerates water passing through it.
That distinction is the cause of most failed showers, and it is worth confirming explicitly with any tiler.