A grout joint does three jobs: it absorbs dimensional variation between tiles, it accommodates movement, and it hides the fact that neither the tile nor the substrate is perfect.
Below a certain width it stops being able to do any of them.
Rectified or not
Non-rectified tiles come out of the kiln at whatever size firing gives them, with a slightly rounded edge. Variation of 1/16 inch or more between tiles in the same box is normal.
They need a joint of at least 1/8 inch, and often 3/16, because the joint is what absorbs that variation. Force a 1/16 inch joint and every size difference shows as a wandering line.
Rectified tiles are mechanically ground to exact size with a square edge after firing. They can take a 1/16 inch joint, and that is the look people are usually after when they ask for minimal grout lines.
Check the box. Rectified tiles cost more, and it is the only way to get a very tight joint that stays straight.
Typical widths
| Tile | Joint |
|---|---|
| Rectified porcelain, large format | 1/16–1/8 in |
| Non-rectified ceramic | 1/8–3/16 in |
| Natural stone, gauged | 1/16–1/8 in |
| Natural stone, tumbled | 1/4–1/2 in |
| Mosaic sheets | as set on the sheet |
| Exterior, freeze-thaw | 3/16 in minimum |
Sanded or unsanded
The crossover is 1/8 inch.
Unsanded for joints 1/8 in and narrower. It shrinks more, so it cracks in wider joints.
Sanded for 1/8 in and wider. The sand resists shrinkage. It will also scratch polished stone, glass and some glazes — use unsanded or an epoxy grout there, and test on an offcut.
Epoxy grout works across the range, resists stains and needs no sealing, and it is considerably less forgiving to install: short working time, and residue that must be removed before it cures.
Never grout a movement joint
Tile expands and contracts, and the substrate moves independently. TCNA EJ171 requires soft joints — silicone or a flexible sealant — at:
- Every change of plane: wall-to-floor, wall-to-wall, inside corners
- Every perimeter where tile meets a wall or fixed object
- Every 20 to 25 ft in interior fields, and every 8 to 12 ft in exterior or sun-exposed fields
- Over every structural joint in the substrate
Filling those with cement grout guarantees a crack, and the crack will run through the tile rather than the joint about half the time.
This is the requirement most often ignored on residential work, and the one that produces the cracked line across an otherwise perfect floor two summers later.
Colour
A grout close to the tile colour makes a floor read as one surface and hides layout imperfections. A contrasting grout draws a grid and shows every deviation in your spacing.
Choose contrast only if the layout is genuinely accurate. Work quantities on the grout calculator — joint width and depth drive the volume far more than area does.