How to clean grout, get it white again, and when to seal it

Why grout goes dark

Most grout is cement based. It is porous, it sits slightly below the face of the tiles, and everything that runs across the surface ends up in it: soap, body oils and limescale in a shower, cooking grease on a splashback, and on a kitchen floor the grey water from every mop that has crossed it.

That last one is the usual reason floor grout darkens. A mop spreads dirty water evenly over the tiles and it settles in the joints, so a floor can have clean tiles and dark lines. Black spots in the corners of a shower are a different thing, mould, and they need a mould spray rather than a scrub.

What to use on which tiles

Oxygen bleach is sold as a powder, usually sodium percarbonate, and it is gentler on grout colour than household chlorine bleach.

Step by step

• Vacuum or sweep a floor first, so you are not scrubbing grit into the joints.
• Wet the grout lightly with warm water.
• Apply the cleaner along the lines. For a paste, mix bicarbonate of soda with a little water to the thickness of toothpaste.
• Leave it for ten minutes or so, or the time on the product label.
• Scrub along the joint with a stiff nylon brush or a grout brush. An old toothbrush works for a small area.
• Rinse well with clean water and a cloth, changing the water often, then wipe dry.
• Look again when dry. Wet grout always looks darker than it is.

Getting grout white again

If a thorough clean still leaves grout grey or patchy, the stain is in the cement itself, or the surface has worn away and exposed darker grout underneath. Cleaning harder will not change that.

The two real options are recolouring, where a pigmented coating is worked into clean grout and seals it at the same time, and regrouting, where the old grout is raked out and replaced. Recolouring is far quicker and suits grout that is sound. Grout that is cracked, crumbling or missing needs replacing, because a coating on failing grout will fail with it.

Steam cleaners do a good job on ceramic and porcelain floors. Check the manufacturer's advice before using one on natural stone, sealed or not.

Does grout need sealing?

Cement grout benefits from it. A penetrating sealer slows down how fast water, grease and dirt soak in, which keeps it lighter for longer and makes cleaning easier. Seal only once the grout is clean and completely dry, and on new grout only after the curing time the grout manufacturer gives.

Epoxy grout does not need sealing. It is non-porous once cured, which is the reason it is often chosen for showers and kitchen worktops.

To see whether existing sealer is still working, drip some water on the grout. If it beads, the seal is doing its job. If it soaks in and darkens the joint within a minute or so, it is time to reseal. Sealing slows staining in a shower, but it does not stop mould where the room stays damp.

Mistakes to avoid

• Acid on cement grout. Vinegar and descalers slowly dissolve it, and on limestone or marble they etch the stone as well.
• Wire brushes. They scratch glazed tile edges and gouge soft grout.
• Mixing products. Bleach with an acid cleaner or with ammonia gives off harmful gas.
• Sealing dirty grout. It locks the stain in under a layer that then has to be stripped.
• Leaving cleaner behind. Residue attracts dirt, so rinse until the water runs clear.

When a professional is worth it

Specialist stone and tile cleaning costs about £8 to £20 a square metre cleaned and sealed, over a minimum charge of £200 to £300. A small kitchen floor typically comes to £250 to £450, and grout recolouring is £10 to £25 a square metre.

It earns its fee on natural stone, where the wrong chemical does permanent damage, on large floors where hand scrubbing takes days, and on grout that needs recolouring evenly across a whole room rather than one patch.