How to Remove Algae From Exterior Siding

That green film on the side of your house is almost always algae, sometimes mixed with mildew and dirt. It’s especially common in Northern Virginia, where shade, humidity and tree cover keep siding damp.

Here’s how to remove it safely — and how to keep it from coming back.

What causes algae on siding?

Algae grows where siding stays damp and shaded:

• North-facing walls that rarely get direct sun
• Walls near trees, shrubs or wooded lots
• Areas under decks, near sprinklers or by downspouts
• Narrow side yards between townhouses

Algae isn’t just cosmetic. It traps moisture against the surface and makes siding look older than it is.

Option 1: Remove light algae yourself

For a small area of light green growth on vinyl siding, a careful DIY approach can work.

You’ll need: a garden hose, a soft-bristle brush with an extension pole, a pump sprayer, and a siding cleaner labeled for algae and mildew (or a product recommended by your siding manufacturer).

Steps:

1. Protect plants. Wet nearby plants with plain water first, and rinse them again afterward.
2. Wet the siding from the bottom up with the hose.
3. Apply the cleaner according to the product label, working in small sections.
4. Let it dwell for the time recommended on the label — don’t let it dry on the surface.
5. Gently scrub stubborn spots with a soft brush.
6. Rinse thoroughly from the top down.

Safety first: wear eye protection and gloves, follow the product label, and never mix bleach-based cleaners with ammonia or acidic cleaners. Avoid working from a ladder with a hose and brush if you aren’t comfortable — that’s how most DIY injuries happen.

What not to do

• Don’t blast siding with a high-pressure washer. High pressure can force water behind vinyl panels, crack brittle siding, strip paint and damage window seals.
• Don’t use a wire brush or abrasive pads that scratch siding.
• Don’t clean in direct afternoon sun if you can avoid it — cleaners dry too fast.
• Don’t ignore the label. Some cleaners aren’t safe for certain siding types or for landscaping.

Option 2: Professional house washing

For a whole house, a two-story home, heavy growth or anything that requires a ladder, a professional soft wash is faster and safer. Professionals use low pressure with appropriate cleaning solutions that kill and remove algae and mildew, then rinse thoroughly — reaching second stories without ladders against your siding.

It’s also the better choice for delicate surfaces like older painted siding, stucco or fiber cement, where the wrong product or technique can cause damage.

How to keep algae from coming back

• Trim back trees and shrubs to let in more sun and airflow.
• Fix drainage issues — overflowing gutters and downspouts that soak walls feed growth.
• Adjust sprinklers so they don’t spray siding.
• Wash on a schedule — every one to two years for most NOVA homes, yearly for heavily shaded homes. See how often to pressure wash your house.

The bottom line

Light algae on a single wall can be a weekend DIY job if you go gently. For the whole house, second stories or heavy growth, a professional low-pressure house wash gives a more complete result without risking your siding.

Get a house washing quote from NovaBlastWash in about five seconds.