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.
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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.
Exterior vs. Interior Mould: Exterior biological growth does not indicate toxic mould inside your home. Interior toxic mould requires interior moisture problems — plumbing leaks, high humidity, poor ventilation. If you have concerns about mould inside your home, that's a separate assessment from exterior cleaning.
Working at height? Whole-house treatment means ladders, cleaning solutions, and wet surfaces. If growth extends beyond what you can safely reach from the ground, calling an insured professional is the safer call.
Black streaks creeping across your siding?
Our soft wash treatment kills algae and mildew at the root, so results typically last 3–5 years — not one season. Free, no-obligation quotes across Kitchener, Waterloo, Cambridge & Guelph.
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.
Key Takeaways
- ✓ Professional service saves time and delivers better results than DIY
- ✓ Regular maintenance protects your home's value and curb appeal
- ✓ D&D Home Services proudly serves Kitchener-Waterloo and surrounding areas
- ✓ Get a free no-obligation quote — call or book online anytime
Sources & References
- City of Kitchener — Property Maintenance Standards
- Ontario Building Code — Exterior Maintenance Guidelines
- D&D Home Services field experience across 100+ homes in KW Region
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