Grimy grey grout can make a spotless bathroom look dirty no matter how much you scrub the tiles. The lines between your tiles are porous, so they trap soap, moisture, and mould. This guide shows you how to lift that discolouration safely, when to whiten versus when to reseal or regrout, and the mistakes that quietly ruin grout for good.
Why grout goes grey or black
Grout is a cement-based, porous material. That porosity is the whole problem. It soaks up soap scum, body oils, and moisture, then holds them. Two different things then happen, and they need different fixes:
- Grey, dingy grout: usually built-up soap and grime sitting on and in the surface. This cleans up.
- Black or pink specks: that is mould or mildew growing in the damp grout, common in poorly ventilated bathrooms. This needs killing, not just scrubbing.
Knowing which you have decides your method. Cleaning grime off mould just spreads it; bleaching stained-but-mould-free grout is overkill.
Method 1: lifting general grey grime
Make a paste of bicarbonate of soda and a little water. Spread it along the grout lines. Spray or drizzle white vinegar over it; it will fizz. Leave it five minutes, then scrub along the lines with a stiff nylon brush or an old toothbrush. Work in short sections and rinse with clean water.
The bicarbonate gives gentle abrasion, the vinegar loosens soap scum, and the brush reaches into the recessed line. For most grey bathroom grout, one pass restores a big visible difference.
When to step up to oxygen bleach
If bicarbonate does not fully lift it, use an oxygen bleach (sodium percarbonate) powder mixed with warm water into a paste. It is gentler on grout and surrounding surfaces than chlorine bleach, penetrates the pores, and lifts deep staining. Leave it 10 to 15 minutes, then scrub and rinse.
Method 2: killing black mould
For black or pink mould spots, dilute chlorine bleach (roughly one part bleach to four parts water) applied only to the grout, with the window open or fan running, will kill and lighten it. Leave a few minutes, scrub, rinse thoroughly. Never mix it with vinegar or any acid, and wear gloves.
Important: if mould keeps returning, the real cause is trapped moisture. No amount of bleach fixes poor ventilation.
A real example
In a compact ensuite in a Regents Park flat, the grout below the shower head was going black in patches. The owner had been scrubbing with an all-purpose spray for weeks with no result, because the problem was mould, not dirt. A diluted bleach application on just those lines cleared it in one go. The lasting fix was running the extractor fan for 15 minutes after every shower, which stopped the black patches coming back.
Common mistakes and how to fix them
- Using a metal brush or heavy abrasive. It gouges grout out. Use nylon bristles only.
- Vinegar on unsealed or new grout, repeatedly. Acid slowly erodes cement-based grout. Fine for occasional cleaning, but do not make it a daily habit on bare grout.
- Mixing bleach with vinegar or bicarbonate. Bleach plus acid releases toxic gas. Use them in separate sessions, well rinsed between.
- Cleaning but never sealing. Clean grout with no sealer re-absorbs grime within weeks.
- Bleaching mould without fixing ventilation. It comes straight back. Address the damp.
The step most people skip: sealing
Once grout is clean and fully dry (give it 24 hours), apply a penetrating grout sealer with a small brush or applicator. It fills the pores so grime and moisture cannot sink in. Reseal roughly once a year in wet areas. This one step is what turns a hard scrub into an easy annual wipe.
When cleaning is not enough
If grout is crumbling, cracked, or stained so deep it will not lift, it may need regrouting rather than cleaning. A grout pen can refresh the colour of sound-but-stained lines as a cheaper cosmetic fix, though it sits on top and eventually wears.
Action checklist
- Identify the problem: grey grime or black mould.
- Grime: bicarbonate paste plus vinegar, or oxygen bleach for stubborn areas.
- Mould: diluted chlorine bleach on the grout only, well ventilated.
- Scrub with a nylon brush, never metal.
- Rinse thoroughly and let dry fully.
- Seal clean, dry grout and reseal yearly.
- Run the fan after showers to stop mould returning.
Conclusion
Grey grout is almost always fixable, but only if you match the method to the cause and then seal the result. Do a proper clean this weekend, let it dry, and seal it. Your next step is simple: check whether your discolouration is grime or mould, because that decides everything that follows.
FAQ
Can I make grout white again without regrouting?
Usually yes. General grime lifts with bicarbonate and vinegar or oxygen bleach, and mould responds to diluted chlorine bleach. Regrouting is only needed if the grout is physically damaged or stained beyond recovery.
Is bleach or oxygen bleach better for grout?
Oxygen bleach is gentler and good for general staining. Chlorine bleach is stronger and better at killing visible black mould. Never use both together, and rinse well between different products.
How often should I seal grout?
In wet areas like showers, about once a year is a sensible guide. Sealer wears down over time, and resealing keeps grime and moisture out so cleaning stays easy.
Why does my grout keep going black?
Recurring black grout is mould feeding on trapped moisture. Improve ventilation by running the extractor fan during and after showers, wiping surfaces dry, and keeping the room aired. Bleach treats the symptom; airflow treats the cause.
Can I use a steam cleaner on grout?
Yes, steam is effective and chemical-free for loosening grime and killing surface mould in grout lines. Follow with a nylon brush and let the area dry fully before sealing.
References
- NHS guidance on damp and mould in the home and its link to ventilation.
- Manufacturer instructions for grout sealers and oxygen bleach products.
