Why has my grout sunk below the tile surface?

A properly filled joint should finish just slightly below the tile face — enough to read as a joint, not enough to form a channel. When it sinks further than that, the joint traps water and dirt, is harder to clean, and the exposed edges of the tile chip more easily.

Step by step

1. Work out which of the three causes applies.Too much water in the mix shrinks everywhere fairly evenly. Underfilling is patchy and worst in deep joints. Washing out shows as joints that are lower where you sponged hardest or earliest.
2. Mix to the stated ratio and let it stand.Grout is mixed to a specific consistency and then usually left to slake for a few minutes before a final stir. Adding water to make it easier to work is the single most common cause of shrinkage.
3. Pack the joints, do not just wipe across them.Work the float diagonally across the joints and press firmly so the grout is compacted into the full depth. Wiping across the surface fills the top and leaves a void underneath.
4. Wait before sponging, and use a barely damp sponge.Let the grout firm up first. A wet sponge used too early washes cement out of the joint, which both weakens it and lowers it.
5. Rinse the sponge constantly and change the water.A sponge loaded with grout slurry smears rather than cleans, and encourages more passes, which is what causes the washing out.
6. Regrout over sunken joints once fully cured.Provided the existing grout is sound and clean, a second application over the top fills the joint properly. Scrape out anything loose or crumbling first.
7. Rake out and start again where the grout is weak as well as low.Sunken and powdery together means the mix was wrong throughout. Adding more grout on top of weak grout gives a joint that fails at the interface.

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