Why is my grout patchy and different shades?

Patchy grout is disappointing precisely because the tiling underneath is usually fine. It is a curing fault rather than a materials fault, and it comes from the last twenty minutes of the job — the mixing and the washing off — far more often than from the product.

What you'll need

• grout haze remover or a mild acidic tile cleaner, if the tile allows it (optional)
• clean water and two buckets
• a grout rake or oscillating tool (optional)
• grout colourant or reviver (optional)

Step by step

1. Work out whether it is haze on top or colour in the grout.Wipe a patch with a damp cloth. If the light area darkens and comes back when dry, it is residue on the surface. If the shade difference stays wet or dry, the grout itself cured differently.
2. For surface residue, clean it off properly.A grout haze remover, or a proprietary cleaner suited to the tile. Test on a spare tile or an out-of-sight area first — acidic cleaners damage natural stone and can attack grout itself.
3. For genuine colour variation, identify the cause before redoing it.Look at where the pale and dark areas are. Whole panels different means separate batches. Individual joints different means depth or sponging. Edges pale means over-washing.
4. Check for joints of different depths.Where adhesive has squeezed up into a joint, the grout is thin there and dries faster, curing lighter. Deep joints hold more water and cure darker. Raking joints out to an even depth before grouting prevents it.
5. Rake out and regrout the worst areas in a single batch.Rake to an even depth, vacuum thoroughly, and mix enough grout for the whole area in one go. Consistency between batches is nearly impossible to judge by eye.
6. Mix by measured quantities, not by feel.Same water, same powder, every time, mixed the same way, left to slake if the bag says so and remixed. This is what makes two batches match.
7. Let the grout firm up before washing off.Washing too soon pulls cement out of the surface of the joint, leaving pale, chalky and often weak grout. The product's stated waiting time is the guide.
8. Wash with a barely damp sponge and change the water constantly.A soaking sponge floods the joint with water and dilutes the surface. Two buckets — one to rinse, one clean — keeps you from putting grout slurry back on the tiles.
9. As a last resort, consider a grout colourant.Grout reviver or colourant coats the joints in a uniform colour. It is a cosmetic fix that needs the grout to be sound and clean first, and it will need renewing.

Tips

• Dark grout shows this fault far more than light grout. If you are set on a dark colour, mixing accuracy matters much more.
• In a large room, mixing enough for the whole floor at once is impossible — so mix consistently by measure and blend batches at an inconspicuous line rather than mid-wall.
• Never add extra water to grout that has started to stiffen. Throw it away and mix fresh; retempered grout cures a different shade and a lot weaker.
• Sponging in circles polishes cement to the surface. Wipe diagonally across the joints in straight passes instead.

Common mistakes

• Adding water to stiffening grout to make it workable again — The cement has already begun to hydrate. Adding water breaks that structure and gives grout that cures weaker and a visibly different shade, so the patch is both paler and more likely to crumble later.
• Washing off with a wet sponge too early — It floods the joint, dilutes the cement paste at the face and washes pigment out. This is the single commonest cause of the pale, chalky, blotchy finish people describe.
• Mixing each bucket by eye — Grout colour is extremely sensitive to the water-to-powder ratio. Two buckets that look the same on the trowel can cure two obviously different shades on the wall.

If it doesn't work

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