Mould on Concrete: How to Remove It Safely and Prevent It Returning

Green algae, black mould, and grey-brown mildew on concrete surfaces are widespread on Ontario properties — patios, driveways, walkways, and garage floors are all susceptible. Concrete's porous structure holds moisture and provides a surface for biological growth, particularly in shaded areas. Removing it properly involves more than pressure washing: effective treatment kills the organism rather than just displacing it.

Identifying What's on Your Concrete

Green, slimy growth is typically algae — most common on shaded, moist concrete. Black spots or streaks are usually mould or Gloeocapsa magma algae. A grey-white powdery coating is often efflorescence (mineral deposits) — not biological, and treated differently with acidic cleaners. Orange staining is usually mineral contamination — also not biological.

Biological growth needs to be killed, not just washed off — otherwise it regrows from the remaining root structure within weeks.

How to Remove Mould and Algae from Concrete

Mix 1/2 cup household bleach per gallon of water with a small amount of dish soap. Apply to the affected surface, scrub with a stiff-bristle brush, and allow to dwell for 15–20 minutes. Rinse thoroughly. Wear appropriate eye and skin protection during application.

For larger areas or heavy growth, pressure washing with a surface cleaner attachment after chemical treatment provides consistent results. A surface cleaner attachment cleans concrete evenly without the streaked "zebra stripe" pattern that a wand nozzle creates on flat surfaces.

Preventing Regrowth on Concrete

The most effective long-term prevention is applying a penetrating concrete sealer after cleaning. A quality silane-siloxane sealer fills the concrete pores that hold moisture, creating a surface environment significantly less hospitable to algae and mould growth. Sealed concrete also resists Ontario freeze-thaw damage and is easier to clean in future seasons.

Additional measures: improve drainage so water doesn't stand on the surface, trim overhead vegetation that keeps the surface permanently shaded, and keep the surface clear of leaves and organic debris.

Professional Concrete Cleaning

D&D Home Services provides professional concrete and driveway cleaning across Kitchener-Waterloo, Cambridge, and Guelph. We treat and remove biological growth properly — not just wash the surface — and can combine cleaning with driveway sealing for lasting protection. Contact us for a free estimate.

D&D Home Services Team Exterior Cleaning Experts — Kitchener-Waterloo

With years of hands-on experience cleaning hundreds of homes across Kitchener, Waterloo, Cambridge, and Guelph, our team shares practical, locally-relevant advice you can trust.