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On average, cleaning a roof will take between two to five hours. This timeline depends on the size and slope of your roof and the severity of any stains. Cleaning roof stains can take additional time, especially if there are signs of moss or algae.

In addition, the age of your roof makes an enormous difference in the time it takes to clean it, as older roofs need more gentle care. If you have significant landscaping in your yard by your house, your professional roof cleaner will need additional time to protect it from cleaning chemicals.

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.

Yes, algae-resistant shingles will prevent algae and moss growth, providing double protection for your roof and roof flashing. The copper-infused granules on algae-resistant shingles release copper ions that inhibit plant growth in general, so moss will have a harder time growing on your roof, too. Keep in mind that copper ions flowing off your roof during rainfall can also inhibit plant growth below, so your gardens and grass may be affected.

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.