What is the best way to clean mould off concrete in New Brunswick?

Diluted bleach solution is the most effective and widely available treatment for mould on concrete in New Brunswick -- but killing the mould is only half the job; addressing the moisture that enables it to grow is what prevents it from returning.

Mould on concrete is extremely common in NB, particularly on shaded driveways, north-facing walkways, basement walls, and patio surfaces near tree canopy. Our Maritime climate -- wet springs, humid summers, and high annual precipitation -- creates ideal conditions for mould and algae colonization on porous concrete surfaces. Beyond appearance, surface mould can make concrete walkways and steps dangerously slippery in wet weather.

The standard cleaning approach for NB concrete mould:

Mix a solution of 1 part household bleach to 3 parts water. For heavy mould infestations, up to 1 part bleach to 1 part water can be used. Wet the concrete surface with water first, apply the bleach solution, and allow it to dwell for 10-15 minutes. Scrub vigorously with a stiff-bristled brush, then rinse thoroughly with a garden hose or pressure washer. On vertical surfaces like basement or retaining walls, apply from the bottom up to prevent streaking.

Protect nearby plants and grass before applying bleach -- it will kill vegetation on contact. Cover shrubs and garden areas with plastic sheeting before treatment, and rinse the surrounding soil and plants thoroughly with clean water after the treatment is complete.

Trisodium phosphate (TSP) is an effective alternative to bleach, particularly for heavily soiled concrete. Mix 1/2 cup TSP per gallon of warm water, scrub, and rinse. TSP is available at NB hardware stores and is particularly effective on outdoor concrete that has accumulated years of grime along with the mould.

Commercial concrete and masonry cleaners formulated for mould and mildew are available at NB building supply retailers. Products containing quaternary ammonium compounds or sodium hypochlorite kill mould on contact and some provide residual protection against regrowth.

After cleaning, apply a penetrating sealer to reduce the moisture absorption that allows mould to take hold. Sealing is particularly important on shaded, moisture-prone surfaces like a north-facing walkway or a patio under tree cover.

For basement walls with mould, cleaning the surface is necessary, but if mould returns repeatedly it indicates ongoing moisture infiltration through the concrete. That is a waterproofing problem -- a professional assessment of the drainage, weeping tile, and exterior grade around the foundation may be needed. New Brunswick Concrete can help you find qualified contractors for concrete cleaning, sealing, and foundation waterproofing services across all of NB.