Grout is porous, so it soaks up dirt, grease and mop water that the tile around it sheds. That's why a floor can look clean while the lines between the tiles keep getting darker. The good news is that most dirty grout comes back with baking soda, oxygen bleach and a stiff brush.
Mix baking soda with just enough water to make a thick paste. Spread it along the grout lines, mist it with water (or vinegar, if your grout is sealed and the tile isn't natural stone), and let it sit for 10 minutes. Scrub with a grout brush or an old toothbrush, working along the line rather than across it. Wipe up the residue with a damp cloth and rinse. This handles everyday grime in kitchens and bathrooms.
If the baking soda didn't do it, use oxygen bleach (sodium percarbonate, sold as a powder). Dissolve it in hot water following the label, pour or brush it onto the grout, and give it 10 to 15 minutes to work. Scrub, then rinse with clean water. Oxygen bleach lifts organic stains like food, mildew and dirt, and it's generally safe on colored grout, which chlorine bleach is not.
For white grout with mildew stains in a shower, a solution of no more than 1 cup of bleach per gallon of water will whiten it. Ventilate the room, wear gloves, and rinse thoroughly. Don't use it on colored grout; it will leave pale patches.
A steam cleaner with a narrow nozzle is one of the best grout tools available. The heat loosens grime without chemicals, and you can wipe the dirt away right behind the nozzle. It's worth borrowing or renting one for a big tiled floor.
Once the grout is clean and completely dry (give it 24 to 72 hours), apply a penetrating grout sealer with a small brush or applicator bottle. Sealed grout resists stains and wipes clean far more easily. Most cement grout benefits from resealing every one to two years; a quick test is to drip a little water on it, and if it darkens and soaks in, it's time. Epoxy grout doesn't need sealing.
Use a pH-neutral floor cleaner, change the mop water when it looks dirty, and don't over-wet the floor. Dirty mop water is the number one cause of dark grout. For more on caring for tile and other floors, see our flooring guide.
Start with a thick paste of baking soda and water, let it sit 10 minutes and scrub with a grout brush. For stains, use oxygen bleach dissolved in hot water, leave it 10 to 15 minutes, scrub and rinse.
No. Chlorine bleach will lighten colored grout unevenly. Oxygen bleach is generally safe on colored grout and works well on stains.
Most cement grout should be resealed every one to two years. If a few drops of water darken the grout and soak in, it needs sealing. Epoxy grout does not need sealing.
Dirty mop water is the most common cause. Grout is porous and soaks up the dirt. Change the water often, use a pH-neutral cleaner, avoid over-wetting the floor, and seal the grout.
