How to Remove Green Algae and Mould from Your Home's Siding

Green streaks creeping down your siding. Black patches in the corners and under the eaves. That persistent dull film that never quite goes away no matter how hard you scrub. If any of this sounds familiar, your home has an algae or mould problem — and it's one of the most common exterior issues for Ontario homeowners. Here's what's causing it and how to eliminate it properly.

What Causes Algae and Mould Growth on Siding

Algae and mould thrive wherever moisture and organic material meet. On home siding, this happens more readily than most homeowners expect. Ontario's climate is particularly conducive to biological growth: humid summers, wet falls, significant shade from mature trees, and cold winters that keep moisture on north-facing surfaces for months.

The primary factors that cause siding to grow algae and mould:

• Shade: Surfaces that don't receive direct sunlight stay damp longer after rain. North-facing walls and areas under large tree canopies are the most common problem spots in Kitchener-Waterloo neighbourhoods.
• Proximity to vegetation: Trees, shrubs, and gardens release spores, pollen, and organic matter that settles on siding and provides nutrients for growth. Overgrown foundation plantings create a perpetually shaded, moist microenvironment at the base of your walls.
• Gutter overflow: Clogged or damaged gutters that overflow deposit moisture down the exterior walls, creating streaks of organic growth. This is a sign that your gutters need attention alongside the siding.
• Roof algae runoff: If your roof has algae or moss growing on it, rain carries spores and organic material down onto the siding below. Roof treatment should accompany siding treatment for a complete solution.
• Age of the siding: Older vinyl and wood siding develops microscopic surface scratches and oxidation that creates a textured surface where spores can grip and take hold. New siding is much more resistant.

Green growth is typically algae — single-celled organisms that photosynthesize. Black growth is usually mould or mildew (fungal growth). Both are living organisms that reproduce via airborne spores, which is why simply scrubbing them off returns in weeks — you haven't killed the source, just removed visible growth.

Health and Property Risks of Siding Mould

Beyond the obvious curb appeal problem, mould and algae growth on exterior siding presents real risks worth understanding.

Property damage: Algae and mould retain moisture against the siding surface. This continuous moisture contact accelerates the deterioration of wood siding, can infiltrate behind vinyl siding panels, and contributes to rot at the base of siding where it meets trim, window casings, and foundation. What appears as surface staining can be covering deeper moisture damage if left untreated for years.

Health concerns: Exterior mould spore concentrations can be drawn into the home through air intake vents, windows, and gaps in the building envelope. While outdoor mould exposure is generally less concerning than indoor contamination, heavy exterior growth near ventilation points can contribute to indoor air quality issues — particularly for households with members who have asthma or mould sensitivities.

Curb appeal and property value: Stained siding is one of the most visually prominent signs of deferred maintenance. Real estate agents consistently report that exterior cleanliness is one of the most impactful factors in first impressions for potential buyers.

The Soft Washing Solution: Why Chemical Treatment Beats Scrubbing

Many homeowners reach for a scrub brush when they see algae on siding. This physically removes the visible growth but does nothing to kill the root structures embedded in the siding surface — the spores and mycelium that will regrow within weeks. It also does nothing to kill dormant spores in the area that will re-colonize the surface immediately.

Soft washing kills algae and mould chemically using sodium hypochlorite (bleach) as the primary biocide. At the concentrations used on siding, it is lethal to all common types of algae, mould, mildew, and lichen. The results are dramatically longer-lasting than mechanical cleaning because the biology causing the staining is eliminated rather than rearranged.

The standard soft washing solution for siding consists of:

• Sodium hypochlorite (bleach): The biocide. Concentration varies by surface and contamination level (see next section for specifics by siding type).
• Surfactant: A specialized soap that does two things — it helps the solution cling to the vertical siding surface rather than running off immediately, and it helps penetrate the cell walls of algae and mould for more effective kill. Standard dish soap can work as an emergency substitute, but professional surfactants are significantly more effective.
• Water: Dilutes the bleach to the appropriate concentration and carries the solution to the surface.

Some professionals add sodium hydroxide (lye) or TSP (trisodium phosphate) as additional cleaning boosters for heavily contaminated surfaces, but these require careful handling and thorough rinsing to prevent surface damage.

Treatment Differences: Vinyl, Wood, and Stucco Siding

Not all siding types respond the same way to soft washing solutions. Using the wrong concentration on the wrong surface can cause bleaching of colour, damage to wood fibres, or surface etching on stucco.

Vinyl siding is the most forgiving siding type for soft washing. It's chemically resistant to bleach at concentrations up to 3% and shows no surface damage with proper technique. The main caution is for darker vinyl colours — always test a hidden section first, as concentrated bleach can lighten some pigmented vinyls.

Wood siding needs more careful treatment. Lower bleach concentrations prevent excessive greying or bleaching of the wood fibres. Very thorough rinsing is essential to remove all bleach residue that would otherwise continue acting on the wood surface after treatment.

Stucco is the most delicate. Stucco is a rigid plaster material that cracks from vibration or high-pressure water. Rinse pressure must stay under 500 PSI — many professionals use a garden hose for the final rinse on stucco rather than any pressure washer at all.

Application Process: Step by Step

1. Protect the surroundings: Pre-wet all surrounding vegetation (grass, shrubs, garden beds) thoroughly. Close all windows and doors. Cover any outdoor furniture, light fixtures, or surfaces you don't want treated.
2. Mix the solution: Prepare your soft washing solution at the appropriate concentration for your siding type. Use sodium hypochlorite from a pool supply store for the most consistent concentration — household bleach varies in strength between brands.
3. Apply bottom to top: Apply the solution from the bottom of the siding up, not top to bottom. This prevents clean solution from running down over dirty sections and creating uneven treatment. Use a garden pump sprayer or low-pressure applicator.
4. Dwell time: Let the solution sit on the siding for 5–10 minutes. You'll often see the colour of the growth change during this time as the organisms die. Don't let the solution dry on the surface — if it's a hot or windy day, mist the area lightly to keep it wet.
5. Rinse top to bottom: Rinse from top to bottom, flushing the dead organic material down and off the siding. Keep rinse pressure appropriate to your siding type.
6. Post-rinse vegetation: After rinsing the house, thoroughly rinse all surrounding vegetation with clean water to dilute any bleach that contacted them.
7. Inspect and repeat if needed: Heavy infestations or lichen may require a second application. Lichen, in particular, may take several days to fully release from the siding surface even after treatment — re-inspect 48–72 hours after cleaning.

"Scrubbing algae off siding is like cutting dandelions at the stem — satisfying but temporary. Soft washing kills the root, and you won't see regrowth for a couple of years."

— David, D&D Home Services Co-Founder

Prevention Tips: Keeping It Clean Longer

After a professional soft wash, your siding will look significantly better. Here's how to extend those results as long as possible:

• Trim back vegetation: Shrubs and trees touching or overhanging the siding transfer moisture, shade, and organic material directly to the surface. Keep plants at least 18–24 inches from siding. This single step makes the most consistent difference in how quickly algae returns.
• Clean your gutters: Overflowing gutters are a primary cause of moisture-driven algae growth down siding faces. Clean gutters at least twice per year in Ontario — spring and fall. Professional gutter cleaning ensures complete cleaning including downspout flushing.
• Improve drainage away from the house: Water pooling near the foundation creates a perpetually moist environment at the base of your siding. Ensure the grade slopes away from the house and that downspout extensions direct water at least 2 metres from the foundation.
• Address roof algae: If your roof has algae or moss, treat it — rain will continue washing spores down onto your siding indefinitely otherwise. Zinc strips near the ridge cap can help prevent roof algae regrowth.
• Annual spring rinse: A quick rinse with a garden hose in spring before spores have a chance to establish removes the winter's accumulation and significantly slows regrowth.

When to Call Professionals

DIY soft washing is feasible for single-storey sections of siding if you're comfortable mixing and handling diluted bleach solution and working with a garden pump sprayer. However, professional soft washing is the better choice for:

• Two-storey homes and anything requiring ladder work above one storey
• Stucco, wood, or EIFS siding where wrong concentrations cause damage
• Heavy mould infestations (black mould covering large areas)
• Homes where surrounding landscaping needs careful protection
• Any situation where you're uncertain about chemical mixing or application

Our professional soft washing service covers all siding types throughout Kitchener, Waterloo, Cambridge, and Guelph. We use the appropriate solution concentration for your specific siding and level of contamination, protect your landscaping throughout the process, and rinse thoroughly to prevent any chemical residue.

D&D Home Services TeamExterior Cleaning Experts

D&D Home Services is Kitchener-Waterloo's trusted exterior cleaning company serving Kitchener, Waterloo, Cambridge, and Guelph.