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Green algae and other organic growth can make otherwise clean siding look neglected, especially on walls that receive limited sunlight or stay damp after rain. The good news is that you usually don’t need aggressive scrubbing to remove light algae growth. The key is to loosen the buildup with an appropriate cleaning solution and let it do most of the work.
Why Does Green Algae Grow on Siding?
Algae thrive where there is moisture, shade, and limited airflow. Siding on the north-facing side of a home is often particularly vulnerable because it may remain damp for longer periods.
Other factors can contribute, including:
- Poor drainage around the house
- Overhanging trees
- Constantly wet surfaces
- Dirt and organic debris
- Limited sunlight
- Clogged or overflowing gutters
Removing the visible algae helps, but reducing the moisture that allows it to return is just as important.
The Low-Effort Cleaning Method
For many types of exterior siding, a practical approach is to use a commercial siding or exterior-house cleaner labeled for algae and mildew, following the manufacturer’s dilution and application instructions.
A typical process is:
- Remove loose dirt first.
Gently rinse the siding with a garden hose. - Apply the appropriate cleaner.
Work in manageable sections and follow the product’s instructions carefully. - Let the solution work.
Don’t immediately scrub everything. Many exterior cleaners are designed to loosen organic growth so that much of it can be rinsed away. - Rinse thoroughly.
Use clean water and work from the top downward when appropriate for the siding and cleaner. - Repeat stubborn areas if necessary.
Heavy growth may require a second application rather than aggressive scrubbing.
The exact method depends on whether the siding is vinyl, painted wood, fiber cement, brick, or another material.
Why You Shouldn’t Just Blast It With a Pressure Washer
Pressure washing can be effective, but more pressure isn’t necessarily better.
A powerful stream can force water behind siding, damage finishes, strip paint, or even cause physical damage to certain materials.
If you use a pressure washer, follow the siding manufacturer’s recommendations and use an appropriate pressure and nozzle distance.
For delicate or painted surfaces, a garden hose and suitable cleaner may be the safer option.
A Crucial Safety Rule: Never Mix Cleaning Chemicals
This is especially important when cleaning exterior surfaces.
Never mix bleach with ammonia, acids, vinegar, or other household cleaners. Some combinations can release dangerous gases.
If you choose a commercial algae or mildew cleaner, use it exactly as directed and don’t add other chemicals to the mixture.
Protect nearby plants and landscaping according to the product instructions, and wear appropriate protective equipment when recommended.
How to Keep Algae From Coming Back
Cleaning removes existing growth, but prevention can make the biggest difference.
Improve Sunlight and Airflow
If practical, trim vegetation that keeps the siding constantly shaded and damp.