You can wash your shingles with several shingle-safe solutions that remove algae, moss, and dirt without damaging protective granules or voiding warranties. The most recommended by shingle manufacturers like GAF, Owens Corning, and Atlas is oxygen bleach, also called sodium percarbonate, mixed with water and a small amount of mild dish soap. A typical mixture is one gallon of water, one quart of oxygen bleach, and a quarter cup of dish soap, applied with a low-pressure garden sprayer from the ridge downward on a cool, overcast day, allowed to dwell for 15 to 20 minutes, and then gently rinsed with a garden hose on low pressure, never a pressure washer which strips granules. Commercial products specifically labeled safe for asphalt shingles are also excellent, including Spray and Forget, Wet and Forget Outdoor, Roof Shampoo, and similar products that kill algae and moss gradually with rain and require no rinsing. For moss, products containing potassium salts of fatty acids or zinc sulfate like Moss B Ware are effective. Avoid using chlorine bleach in concentrated form, as it can damage shingles, corrode metal flashing and gutters, and kill landscaping. Also avoid harsh detergents, pressure washers, and wire brushes. Always protect plants by soaking them with water before and after washing, and wear non-slip shoes with safety precautions.
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