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For the best chemicals to clean a metal roof, stick to gentle detergents such as dish soap, laundry detergent, or car-wash soap mixed in a diluted solution of ¼ cup of soap to 1 gallon of water. These mild cleansers are safe for most roof finishes but are still strong enough to remove remaining dirt particles. Chemicals like bleach or acids can damage your roof, poison landscaping and grass, and pose a hazard to you and your family. If your roof is very dirty or you think it requires chemical cleaning, call a roof cleaning professional.

Ideally, you will tackle removing algae from your roof during spring or summer when the weather is drier and warmer. Because algae tends to grow during the colder and wetter months, you want to tackle this project before the temperature drops to hopefully keep it from coming back at all.

It may require stronger cleansers or chemicals, higher-pressure spray, or scrubbing to completely remove algae from a metal roof. Start with a mild cleanser; if that doesn’t work and you're comfortable doing so, use a stronger mix of ⅓ cup of laundry detergent, 1 quart of bleach, and 3 quarts of water to gently scrub the algae with a soft-bristled brush. If algae is present across large portions of your roof, doesn’t come off with gentle scrubbing and cleansers, or is in hard-to-reach areas, this may be a job better left to a pro.

You could void your roof warranty using a power washer if you have a newer roof. With an older roof, you could knock loose older shingles. On the other hand, some people safely use a pressure washer to remove moss from a metal roof. Although you may want to use a pressure washer, the truth is that you probably don’t need it to knock moss loose. A garden hose and brush will do the job just fine and in a safer manner than a pressure washer.