· by Michael Blair, Nashville's Tile and Stone
Grout is a rigid cement product with no meaningful flexibility. Where an assembly moves, grout cracks — so the location of a crack tells you whether it is a detailing error or something structural.
Corners and changes of plane
Wall to wall, wall to floor, and around a bath or tray are all changes of plane where the two surfaces move relative to each other. Those joints should be silicone, not grout.
Grout there cracks within a year and is not a defect in the material — it is the wrong material in that joint.
Cracks across the field
Grout cracking in a straight line across a floor usually means movement in the substrate beneath — deflection in the subfloor, a slab crack telegraphing up, or a missing movement joint.
Movement joints are required at intervals on larger areas and at perimeters, and they are frequently omitted in residential work.
- Inside corners and changes of plane: silicone, never grout
- Straight cracks across a field: substrate movement
- Crumbling grout: bad mix or premature washing
- Cracks with loose tiles: bond failure beneath
- Movement joints at perimeters and at intervals
Substrate deflection
Tile needs a stiff floor. A subfloor that flexes underfoot cracks grout and eventually tiles, and no grout product compensates for it.
In older Nashville houses with dimensional joists at wide spacing, adding a layer of subfloor before tiling is frequently necessary.
Repair
Rake out the failed grout and replace, and where the joint is a change of plane, replace it with silicone rather than grout.
If tiles sound hollow when tapped, the problem is bond failure beneath and regrouting only postpones it.