Why shower grout turns grey so quickly

Grout is a cement-based material, which means it is porous by nature. Every shower sends soap, shampoo, body oils and water into those tiny pores, and over time that mixture sets into a film that dulls the grout lines and gives mould something to feed on. If you live in a hard water area — and much of the UK is — you can add limescale to the list, which is what gives grout that chalky, greyish-white crust.

This is why a quick wipe with bleach or a general bathroom spray rarely delivers the result you want. Bleach will lighten surface mould, but it does not break down the soap scum and mineral deposits underneath. Those need something mildly abrasive and alkaline to lift them away, which is exactly where bicarbonate of soda comes in.

What you'll need

• Bicarbonate of soda — a small tub is plenty for one cubicle
• Warm water
• An old toothbrush, ideally with soft or medium bristles
• White vinegar in a spray bottle, for stubborn areas and a final rinse
• Rubber gloves
• Two microfibre cloths — one damp, one dry
• A plastic scraper or old loyalty card for the corners

You almost certainly have most of this already. There is no need for specialist products, and the ingredients here are kinder to seals, tiles and your hands than many harsh chemical cleaners.

The bicarbonate of soda method, step by step

Mix bicarbonate of soda with a little water to make a paste. Aim for roughly three parts bicarb to one part water — you want the consistency of thick toothpaste, not a runny liquid. If it dribbles down the tiles, add more powder. If it is crumbly and won't spread, add a few drops more water.

Apply the paste along the grout lines using your fingers or the toothbrush. Push it gently into the grout rather than laying it on top, so it can get to work on the dirt trapped in the pores. Work in sections of about a square metre at a time so nothing dries out before you get to it.

Leave the paste for ten minutes. This is the part most people skip, and it is where much of the cleaning actually happens — the bicarb softens the soap scum and lifts the grease so your scrubbing does the easy work.

Now scrub gently with the old toothbrush. Use small circular movements and work along the length of each grout line rather than across it, which keeps the bristles in the channel. Keep the pressure light; the paste is doing the work, and hard scrubbing on old grout can wear it away or damage the tile edges.

Rinse with warm water and the damp microfibre cloth, then buff the tiles dry. If a faint white residue remains, spray with white vinegar, leave for a minute and wipe again — the fizzing you see is the acid neutralising the alkaline bicarb.

Tackling black staining and mould

If the grout has gone properly black, the bicarb paste will lift the surface layer, but you may need a second pass. Try spraying the grout with white vinegar first, then applying the paste on top. The reaction helps push the mixture into the pores. Leave for ten minutes and scrub again.

For mould that keeps returning, use a dedicated mould and mildew remover — but do it safely:

• Open a window or run the extractor fan, and keep the door open
• Wear gloves and avoid breathing directly over the area
• Never mix bleach-based products with vinegar or bicarbonate of soda, as the fumes can be harmful
• Rinse thoroughly afterwards and dry the area completely

If the black staining sits behind the grout line rather than on it, no cleaner will shift it. That is a sign the grout has failed and needs attention, not elbow grease.

Keeping grout cleaner for longer

• Squeegee the walls and screen after every shower — it takes twenty seconds
• Run the extractor fan for at least ten minutes after you finish, or open the window
• Spray the grout weekly with a dilute vinegar solution to keep limescale at bay
• Avoid oil-based or heavily fragranced body products, which leave a stickier film
• Leave the shower door ajar so air can circulate and the cubicle dries properly

Five minutes of prevention each week will save you an hour of scrubbing every few months.

When to reseal or regrout

Run your finger along the grout. If it crumbles, feels soft, or you can see cracks and gaps, the grout has stopped doing its job and water is getting behind the tiles. Small areas can be raked out and repointed, and the silicone sealant along the base of the cubicle is worth replacing every few years as a matter of course.

Larger patches of missing or hollow-sounding grout are a job for a tiler, and it is money well spent — water damage behind tiles is far more expensive to put right than a bit of repointing. Get into the habit of a ten-minute bicarb clean every few weeks, keep the air moving, and your shower grout will stay crisp and bright for years rather than months.