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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.

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.

Soft washing chemicals typically consist of a mix of sodium hypochlorite (or a chemical component of bleach) and water. While sodium hypochlorite and bleach are often thought to be the same thing, there is one small but key difference between the two: sodium hypochlorite comes in powdered rather than liquid form, which means that it’s also more concentrated and more effective from less product overall.

Chemical herbicides will kill moss permanently. The same goes with bleach water. However, moss can start growing again once a new layer of dirt and debris is caught in your sand in paver joints. Therefore, the problem is not about killing moss permanently, but ensuring effective prevention of moss regrowth.