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The best way to clean a roof without damaging it is to opt for soft washing, or better yet, leave it to a professional roof cleaner. Soft washing involves a biodegradable cleaning solution being sprayed on the roof and gently rinsed away after it dissolves built-up residue on your shingles. 

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.

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 may become so frustrated with the look of moss on your roof that you contemplate whether to clean or replace your roof. Fortunately, you don’t have to go as far as replacing it. After cleaning the moss, you can use a chemical solution to keep it away. For further protection, trim branches that overhang the roof, as fallen seed pods and leaves on the roof allow moisture to collect, encouraging moss growth. Consider adding zinc-coated flashing to the roof, too. As rain washes across the zinc, it leaves zinc particles behind that inhibit moss growth.