How to clean grout (and when cleaning will not save it)

Why grout goes black in the first place

Cement-based grout is porous. It absorbs soap film, body oils, cooking grease and cleaning residue, and in a bathroom that damp film feeds mould. The dark colour in a shower is almost always mould growing in the surface of the joint, not dirt sitting on top of it.

Floor grout goes grey for a different reason: mop water. Mopping pushes dirty water into the joints and leaves it there to dry, so the joints darken faster than the tiles ever will.

The routine clean that actually works

Start with the mildest thing that will do the job. Warm water, a pH-neutral tile cleaner and a stiff nylon brush — worked along the joint, not across it — lifts most everyday soiling. Rinse with clean water and dry the area, because leaving cleaner in the joint is what causes the next stain.

For mould in a shower, a dilute household bleach solution (roughly one part bleach to four parts water) left on the joint for ten minutes and then rinsed will kill the growth. Ventilate the room, wear gloves, and never mix bleach with any acidic cleaner.

Steam cleaners are excellent on floor grout and use no chemicals at all. Keep the head moving and do not park it on one joint on a heated floor.

What not to use

Acidic descalers, vinegar and lemon-based cleaners dissolve cement grout. They will brighten a joint once and then leave it soft and open, so it stains again within weeks. The same acids etch marble, limestone and travertine permanently.

Wire brushes and abrasive pads scratch glazed porcelain and tear the surface off the grout. Angle grinders and rotary wire wheels in untrained hands take the tile edge with the grout — we repair that job more often than we repair the grout itself.

Sealing after cleaning

Once the grout is clean and fully dry, an impregnating grout sealer soaks in and repels water and oil rather than sitting on top as a film. On a bathroom floor or shower we would reapply roughly every 12 to 18 months.

Sealer will not lighten grout that is already stained. Seal it clean or you seal the stain in.

When cleaning is the wrong answer

If the joint is crumbling, powdery, cracked along its length, or missing in places, no cleaner will fix it — the grout has failed and water is getting behind the tiles. The same goes for grout that is repeatedly black within days of cleaning, which usually means moisture is coming through from behind rather than sitting on the surface.

Silicone in the internal corners and around the bath or tray is a wear item. If it is peeling, blackened underneath, or has been painted over with more silicone, it gets cut out and replaced, not cleaned.

In those cases the fix is raking the joints out to depth, re-grouting with a suitable grout and renewing the silicone. It is a day's work in a typical bathroom and it looks like a new room afterwards.

Getting it done properly

We do tile and grout repair, re-grouting and re-siliconing across London, Essex and Cambridge, usually as a single-day visit. Send a photo of the worst corner on WhatsApp and we will tell you honestly whether it needs a clean or a re-grout.