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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. 

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.

Yes, moss can be dangerous for your roof if left in place to grow without treatment. The moss soaks up and holds moisture. Having a constantly damp roof could lead to problems with rot for the parts of the roof that consist of wood. Additionally, a patch of thick moss may draw insects and birds, which could damage the roof. Thick moss could block the roof’s drainage system. Water then may back up after a rainstorm and cause further damage that could eventually lead to roof repair expenses or the cost of roof replacement.

If you’re asking yourself, should you pressure wash your roof, the answer almost always is no. The water pressure that the power washing machine generates could damage the shingles by forcing water underneath or causing them to loosen. Soft washing with a pump sprayer is the better and safer option.