How We Soft Wash House Siding to Remove Algae and Mildew

If your home's exterior has green, black, or gray streaking on the siding, what you're looking at is almost never just dirt. It's a living organism — usually algae (Gloeocapsa magma), mold, or mildew — that has taken root on your siding surface. In North Carolina's humid climate, this kind of growth is extremely common, especially on north-facing walls and areas under tree cover where moisture lingers.

The instinct is to blast it off with high pressure. The problem is that pressure alone doesn't kill the organism — it just displaces it. The growth comes back faster because the root structure is still intact. Soft washing takes a different approach: we use a chemical solution to kill the organism first, then rinse it away.

The Mix: 1–2% Sodium Hypochlorite with a Surfactant

Our base cleaning solution for house siding is sodium hypochlorite (SH) diluted to a 1–2% concentration — roughly the strength of a strong bleach solution, but applied at low pressure in a controlled, professional manner.

At this concentration, SH penetrates the cell walls of algae, mold, and mildew and oxidizes them — killing the organism at the root rather than just wiping the surface clean. The result lasts 2–4 years in most cases, compared to 6–12 months if you just pressure-wash without treatment.

We mix in a surfactant — a soap-like additive that does two important things: it helps the solution cling to vertical siding surfaces instead of running straight off, and it extends the dwell time so the SH has enough contact time to do its job. Without a surfactant, the solution slides off vinyl or brick before it can fully work. With one, it sheets onto the surface and stays put.

Plant and Landscape Protection

This is a step that separates a professional soft wash from someone renting a pressure washer and mixing their own bleach solution. Before we apply anything to the house, we thoroughly pre-wet all grass, plants, shrubs, and landscaping near the work area. This dilutes any overspray before it lands.

During the wash, we monitor the plants and rinse them again as needed — especially if the solution is blowing toward them. After the job is complete, we do a final rinse of all surrounding vegetation to flush off any residue. For particularly sensitive plants or ornamental landscaping, we can apply a neutralizing rinse to bring the pH back to neutral.

Done correctly, soft washing poses minimal risk to healthy, established plants. The approach matters: you're applying professional chemistry at low pressure, not flooding a yard with undiluted bleach.

The Application Process

We apply the solution using a soft wash pump and low-pressure nozzle — typically under 100 PSI, which is close to garden-hose pressure. The goal at this stage is coverage and dwell, not force. We work from the bottom up on application so the solution doesn't streak on dry surfaces below.

After applying, we allow the solution to dwell on the surface for 5–10 minutes. You'll often see the green or black growth visibly change color as the SH kills it — a sign the chemistry is working. For heavier growth or shaded surfaces that get less UV exposure, we may apply a second pass before rinsing.

The rinse is done top-down with clean water at low-to-moderate pressure, thoroughly flushing the dead growth and cleaning solution off the siding and away from the foundation.

What Soft Washing Works On

• Vinyl siding — the most common siding type in Fayetteville; soft washing is ideal and won't cause warping or cracking like high pressure can
• Brick and mortar — works well; soft washing cleans without eroding mortar joints the way high pressure can over time
• Wood siding and hardiplank — lower end of the 1–2% range to protect the surface; wood gets different treatment than concrete (see our wood cleaning post)
• EIFS / Dryvit stucco — soft washing only; high pressure can crack or crack these surfaces
• Painted surfaces — soft washing is gentler on painted siding than high-pressure rinsing

How Long the Results Last

In North Carolina's climate, a professional soft wash on house siding typically keeps the surface clean for 2–4 years. Homes in heavy shade or near standing water may see regrowth sooner. Homes with good sun exposure on most walls often go closer to 3–4 years between washes.

This is significantly longer than straight pressure washing without treatment, which typically needs repeating within 6–12 months because the organism was moved, not killed.