Mould grows down inside the pores of grout, and in a coastal bathroom it has moisture to feed on for most of the year. The dark line along the bottom of your shower wall is usually a living colony sitting below the surface. That matters, because most advice on how to clean grout that has turned black is written for surface dirt. You scrub for an hour, the tile looks better, and the colour creeps back within a month.
Grout is cement with a bit of sand in it. Water goes in. Whatever is in the water goes in with it.
Why grout goes black
There are four usual causes, and in older Island bathrooms they tend to arrive together.
Mould and mildew are the most common. A shower in a Fairfield or James Bay character house with no exhaust fan stays damp for hours after anyone uses it, and the grout lines never get a chance to dry out. Basement suites have the same problem with worse airflow.
Ground-in dirt is the other big one, and it shows up on floors rather than walls. Foot traffic pushes soil, beach sand and shoe grit into grout that sits lower than the tile. Kitchen grout picks up cooking grease on top of that, which holds the dirt in place.
Failed or missing sealer lets both of those problems in. Sealer is not permanent. On a bathroom floor it wears through in a few years, and on a shower floor faster than that.
Bleach damage is the one nobody expects. Repeated chlorine bleach eats at the cement binder, which leaves the grout more porous than it started. The grout soaks up soil faster every time you clean it.
How to clean grout that has turned black: methods ranked
Start at the gentlest method that could work. Working up is easier than repairing grout you have chewed through.
Baking soda paste
Mix baking soda with enough water to make a thick paste, spread it along the grout line, wait ten minutes, then scrub. This handles light soil on kitchen and hallway floors. Against mould it does almost nothing, because it cleans the surface and stops there. Cheap, safe on coloured grout, worth trying first on a floor.
Hydrogen peroxide with baking soda
Three percent hydrogen peroxide from the pharmacy, mixed into baking soda until it makes a paste. Leave it on the grout for fifteen minutes so the peroxide has time to soak in, then scrub and rinse. This is the best home method for black grout in a shower, because peroxide penetrates rather than sitting on top. Test a hidden corner first if your grout is dark grey or charcoal.
Oxygen bleach
Sodium percarbonate, sold as a laundry booster, dissolved in warm water. Pour or brush it over the grout, give it fifteen to twenty minutes, then agitate and rinse. It covers a large floor faster than paste does and will not strip colour out of the grout the way chlorine does. For a whole bathroom floor this is where I would put my money.
Steam
A rented steam cleaner with a narrow nylon brush head loosens grease and soil with heat instead of chemistry. It works well on kitchen floors. Two cautions. Steam will not restore grout that is stained through the body, and in a bathroom with no fan you are pushing a lot of moisture into a room that already struggles to dry. Open a window and run a fan afterward.
Skip chlorine bleach. It whitens the mould stain without removing what is underneath, and the damage it does to the cement makes the next cleaning harder.
The tools that actually matter
An old toothbrush is fine for the corners of a shower niche and nothing larger. For anything past a few tiles, buy a proper grout brush with stiff nylon bristles and a narrow head, so the bristles sit down in the joint instead of riding over it.
Keep metal away from grout. A wire brush gouges the cement and leaves grey streaks that look worse than the mould did.
Kneepads and a rinse bucket round it out. Rinse as you go, because dirty water left to dry on the tile puts the soil straight back into the grout.
When the grout is past DIY
Some jobs will not respond to a brush and a Saturday afternoon, and it helps to know that before you spend the afternoon.
Grout that has been black for years has mould growth deep in the cement, below where any household paste reaches. Shower floors are the worst case, since the grout sits wet several times a day and the joints are wider. Large tile areas are a matter of arithmetic: scrubbing a kitchen and hall by hand on your hands and knees takes most of a weekend, and the result is rarely even.
Crumbling, cracked or missing grout is a repair rather than a cleaning problem. So is grout that stays dark when wet and dries lighter in patches, which usually means water is getting behind the tile.
Our tile and grout cleaning uses commercial equipment and non-toxic solutions to pull soil out of the pores instead of polishing the top of it, and we can seal the grout afterward so it holds its colour longer. Sealing is the step that decides whether you are doing this again in eighteen months.
Cleaning versus regrouting
Homeowners often assume black grout means the whole bathroom needs redoing. Sometimes it does. Usually it does not.
Professional cleaning is priced by area. Our residential tile and grout rate is $0.90 per square foot, with a $280 job minimum, and the full list is on our pricing page. A bathroom floor and a shower surround together is a morning’s work.
Regrouting is a different trade. A tile setter grinds the old grout out of every joint and refills it, which takes days on a full bathroom, costs considerably more than cleaning, and puts your shower out of use while it cures. Worth it when the grout is failing. Wasted money when the grout is sound and filthy.
The honest test: if you can press a fingernail into the grout and it powders, call a tile setter. If it is hard and just dark, clean it and seal it.
Get a quote on your tile
Call Joanne at (250) 710-5244 or send a note through the contact page. She will ask what rooms you are dealing with, the rough square footage and how long the grout has been dark, and give you a number over the phone. We work from Sidney and Oak Bay through Victoria and Langford, and up over the Malahat to Mill Bay, Duncan, Ladysmith and Nanaimo. Every job carries a fifteen-day re-clean if you are not happy with it.