Black Mould on Siding: How to Identify, Treat, and Prevent It

Black mould on the exterior of your home is alarming to see — but in the vast majority of cases, exterior black growth is Gloeocapsa magma, a cyanobacteria (blue-green algae), not the toxic black mould (Stachybotrys chartarum) that grows inside homes with moisture damage. Here's how to tell the difference and what to do about it.

What's Actually Growing on Your Siding

Gloeocapsa magma (algae): The most common cause of black streaks running down from the roofline. This bacteria accumulates in a dark protective coating that gives it its characteristic black colour. It spreads via airborne spores, thrives in humid conditions, and is virtually universal on Ontario homes within a decade of construction. Not harmful to human health but accelerates surface deterioration.

Mildew: A surface fungus appearing as black, grey, or greenish spotting on shaded, moisture-retaining surfaces. Common on north-facing walls and behind vegetation. Not the same as toxic indoor mould.

How to Treat Black Mould on Siding

Soft washing is the most effective treatment. A diluted sodium hypochlorite solution kills the organism at the cellular level — including the root structure — rather than just removing the visible surface layer. This is why soft washing results typically last 3–5 years while pressure washing alone sees regrowth within 3–12 months.

For smaller affected areas, a DIY treatment of 1 part household bleach to 3 parts water with a small amount of dish soap, applied with a soft brush and rinsed after 15–20 minutes, is effective. For whole-house treatment, professional soft washing equipment delivers better, more consistent results.

Preventing Recurrence

Complete prevention in Ontario's humid climate is difficult, but you can slow recurrence: trim vegetation creating shade and moisture against siding; ensure gutters are clear so water doesn't overflow and repeatedly wet siding; improve airflow around the foundation level; and consider zinc or copper strips near the roofline, which leach trace minerals that inhibit algae growth.

Professional Treatment

D&D Home Services provides professional soft washing for black algae, mould, and mildew on all exterior siding types across Kitchener-Waterloo, Cambridge, and Guelph. 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.