Grout is the cement between your tiles, and unlike the tiles it is porous. It soaks up soap, skin oils, and the minerals in your water, then holds on to them. That is why the tiles in a shower can look fine while the lines between them turn grey, then brown, then nearly black.
Most dark grout can be brought back. Some cannot, and scrubbing harder will not change that. Here is how we tell the difference on a job.
Three reasons grout goes dark
Surface dirt. A film of soap scum and body oil that dirt sticks to. It sits on top, it is even in colour, and it is the most common cause by far. It comes off.
Mould. Black or green spots, usually in the corners and the lowest rows of a shower where water stands longest. Surface mould comes off. Mould that has grown into the grout comes back within weeks, whatever you use.
Worn grout. Years of scrubbing with the wrong brush or strong acid wear the grout below the edge of the tile. The groove holds water and dirt, and it looks dark even when it is clean. No cleaner fixes that; it needs regrouting.
Test before you scrub
Spend two minutes finding out which one you have.
- Wet a cloth with plain warm water and rub a 10 cm stretch of grout hard for 30 seconds.
- If the cloth comes away grey and the grout lightens, it is surface dirt.
- If spots stay put but fade with a drop of diluted bleach on a cotton bud after 10 minutes, it is surface mould.
- If the line stays dark and you can feel a step down from the tile edge with a fingernail, the grout is worn.
How we clean grout that can be saved
Rinse the wall with hot water
Hot water softens soap film. Run the shower on hot for two minutes with the door shut, then turn it off.
Apply an alkaline cleaner, not an acid
A cleaner for soap scum and grease, left for 5 to 10 minutes. Acids such as vinegar remove limescale but also dissolve cement-based grout a little each time.
Scrub along the line with a narrow nylon brush
Short strokes along the grout, not across it. A wire brush or a scouring pad cuts into the grout and makes the next clean harder.
Rinse, then dry the lines
Rinse from the top down, then wipe the grout dry with a microfibre cloth. Grout dries from the surface, and water left in it is what mould needs.
Never mix bleach with other cleaners
Bleach mixed with acid cleaners releases chlorine gas, and mixed with ammonia releases chloramine. Use one product, rinse it off completely, and open a window before you use the next.
When scrubbing stops working
If the test showed worn grout, or mould that returns within a month of a proper clean, the problem is in the grout and not on it. At that point the options are regrouting the affected rows or, for small areas, a grout pen that colours the lines. A grout pen is cosmetic: it looks good for a year, then needs redoing.
Sealing is worth it after any deep clean or regrout. A penetrating sealer fills the pores so soap and water stay on the surface where a weekly wipe removes them. Expect to reseal shower grout every one to two years.
Keeping it light
- Squeegee the walls after every shower. It takes 20 seconds and removes most of the water grout would absorb.
- Leave the shower door or curtain open until the walls are dry.
- Wipe with a mild cleaner weekly, before film builds up.
If your grout is past a weekend job, book a deep clean and note it in the booking: the crew lead will test it first and tell you which of the three you have.
Written by
Marco D.
Crew lead, cleaning
Leads a cleaning crew in Elm Park and writes the checklists the crews work from.
