Removing algae from a rooftop requires a gentle, shingle-safe approach that kills algae without damaging protective granules. First, prioritize safety by working on a cool, overcast day, wearing non-slip shoes, and using a roof harness if needed, and never work on a wet roof. Never use a pressure washer, as high pressure strips granules and voids warranties from manufacturers like GAF and Owens Corning. For black algae streaks caused by Gloeocapsa magma, mix a solution of oxygen bleach, sodium percarbonate, with water and mild dish soap, typically one gallon water to one quart oxygen bleach. Apply with a low-pressure garden sprayer from the ridge downward to avoid driving water under shingles, allow to dwell 15 to 20 minutes, and rinse gently with a garden hose on low pressure. Commercial products like Spray and Forget, Wet and Forget Outdoor, and Roof Shampoo are highly effective and require no rinsing, as they work gradually with rain over weeks to months to remove dead algae. For heavy growth, you may need two applications. Protect landscaping by soaking plants with water before and after application. After cleaning, install zinc or copper strips near the ridge to prevent regrowth, as rain washes metal ions down the roof inhibiting algae. Trim overhanging branches to increase sunlight and reduce damp shade where algae thrives.
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