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The average cost to clean a roof is around $500, but prices can fall between $150 to $1,000 or more. The size and complexity of your roof are the most significant cost factors, as larger roofs and those with various bump-outs and gables will increase your cost. Your roof material will also make a difference. An asphalt roof will require more labor, as your professional will need to be careful not to damage the shingles. A metal roof can stand up to more intense scrubbing and shouldn’t take as long to clean.

It’s possible that installing solar panels can lead to mold on the roof. The panels themselves are not the issue. Problems caused by improper installation—such as roof leaks and mounts that aren’t sealed properly—can cause mold. Another potential cause of mold is debris accumulation under panels, which can trap moisture. If you live somewhere with extreme snow, rain, or hail, your home could be more prone to mold.

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.

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.