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It is not advisable to use salt on your roof to prevent or remove ice dams. It’s not only mostly ineffective but can also cause serious damage to your roof itself. Salt can rust metal, damage masonry, pit shingles, and cause runoff that’s damaging to plants and animals. 

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.

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.

No, you should not use a pressure washer or power washer on roof shingles, including asphalt shingles. The high water pressure is too strong and can damage the shingles by loosening their adhesive, forcing water underneath them, or causing them to fall off. The safer and better option is soft washing with a pump sprayer. For best results and to avoid damage, consider hiring a local roof cleaner to clean your shingles safely.