Why is the grout in my Ottawa bathroom turning dark brown and how do I fix it?

Grout turning dark brown in an Ottawa bathroom is almost always caused by one of two things: moisture penetration and mould growth within the porous grout, or accumulated dirt and body oils that have soaked into unsealed or poorly sealed grout over time. In most cases, it is a combination of both.

Cementitious grout is porous by nature. When the sealer wears off — or if the grout was never properly sealed in the first place — water, soap residue, body oils, and airborne mould spores penetrate the surface and become trapped within the grout matrix. Ottawa bathrooms are particularly susceptible because of the city's extreme humidity swings. During the humid summer months and after every hot shower, moisture saturates the grout and feeds mould colonies that grow within the pores. The resulting discolouration appears as a gradual darkening from the original light colour to tan, then brown, then dark brown or even black in severe cases.

Fixing this depends on how deep the discolouration goes. Start with the least aggressive approach and escalate if needed. For surface-level staining, a paste of baking soda and water applied to the grout lines with a stiff grout brush, left for 15 minutes, then scrubbed and rinsed can lift significant discolouration. For more stubborn stains, a 50/50 mixture of white vinegar and water works well — but do not use vinegar if you have natural stone tile, as the acid will etch the stone surface. Oxygen bleach (sodium percarbonate) mixed with warm water is another effective option that is safe for most tile types.

For deep mould penetration that does not respond to surface cleaning, you may need a professional grout cleaning with steam extraction equipment, which reaches contaminants that household products cannot. This typically costs $200 to $500 for a bathroom in Ottawa. After cleaning, apply a quality penetrating grout sealer to prevent the problem from recurring.

For grout that is permanently discoloured beyond what cleaning can restore, you have two options. Grout colourant (sometimes called grout paint or grout stain) is a topical coating that covers the existing grout with a fresh, uniform colour — it works well as a cosmetic fix and typically lasts two to four years. The more permanent solution is regrouting — removing the old grout and replacing it with new. For a standard bathroom, regrouting costs $300 to $800 in Ottawa, and this is an opportunity to upgrade to epoxy grout, which will never develop the mould and staining problems that caused the darkening in the first place.

Going forward, the best prevention is proper ventilation — run your bathroom exhaust fan for at least 20 minutes after every shower — and reseal cementitious grout every 12 to 18 months.

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• The Fixer