Tile can sometimes be installed over cracked concrete, but the crack cannot simply be hidden. Concrete movement can reflect through the tile layer, so the crack must be classified and the installation method selected for the actual movement expected.
Why Concrete Cracks Reappear in Tile
The Tile Council of North America explains that movement in a concrete slab can transfer into bonded tile, producing reflective cracking. Even small concrete shrinkage cracks can remain dimensionally active. A tile finish does not stop the slab underneath from shrinking, moving or settling.
Crack Isolation Is Not Structural Repair
Crack-isolation or anti-fracture membranes are designed to reduce transfer of limited in-plane movement from the substrate into the tile. Products are sold as sheet-applied and liquid-applied systems, and performance limits vary. Custom Building Products, for example, publishes an ANSI A118.12 crack-prevention membrane intended for non-structural cracks due to minor surface movement.
| Concrete condition | Tile-installation question |
|---|---|
| Dormant hairline/shrinkage crack | Is a crack-isolation method specified and compatible with the setting system? |
| Control/saw-cut joint | How will the tile assembly honor or accommodate the joint rather than rigidly bridging it? |
| Active widening crack | Why is it moving, and is cosmetic tile work premature? |
| Vertical displacement | Does the slab need stabilization, lifting, replacement or structural evaluation first? |
A Membrane Has a Published Limit
Do not accept “we use an anti-crack membrane” as a complete answer. Ask for the exact product and whether the crack type, width and anticipated movement fall within its published use. A membrane designed for minor in-plane movement is not a cure for differential vertical movement or unsupported concrete.
When Tile Should Wait
If one side of a crack is higher than the other, sections of slab have settled, the crack is changing noticeably, or there are signs of voids or erosion, diagnose the concrete first. Installing expensive tile over a moving substrate can merely conceal the evidence until the tile fails.