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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.

If you’re painting your home or monitoring a job as it’s being completed, squash the issue ASAP if you notice paint is getting onto the roof. Keeping it clean is one of the keys to extending your roof’s life.

Removing latex paint before it dries is much easier and lowers the risk of damaging your shingles (more on that below). Simply spray the spot with a hose for one to two minutes to clear away the drying paint. In general, this is a good way to remove ugly stains from your roof without using chemicals.

Roof cleaning can damage shingles if it’s done incorrectly—like with a high-power power washer—but it likely won’t damage your roof if you pay to have an expert do the work. A professional roof cleaner will use a gentle cleaning solution and a cleaning method that matches your roof material to ensure the process doesn’t damage the roof. Some homeowners worry that roof cleaning will shorten the life of their roof by removing shingle particles or protective coatings, but it often extends roof life by removing plants that cause damage and improves your home’s curb appeal simultaneously.

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.