Cleaning green mold, which is often actually moss, algae, or mold growth, off shingles requires a gentle, shingle-safe approach to avoid damaging protective granules. First, ensure safety by working on a dry, cool day, wearing non-slip shoes, and using a roof harness if needed. Never use a pressure washer, as high pressure strips granules and voids manufacturer warranties. Start by removing loose debris with a soft broom, sweeping from ridge downward. For green growth, the most effective method is to apply a shingle-safe cleaner. Oxygen bleach, sodium percarbonate, mixed with water and mild soap is recommended by manufacturers like GAF and Owens Corning. Mix one gallon water with one quart oxygen bleach, apply with a garden sprayer, let dwell 15 to 20 minutes, and rinse gently with low-pressure garden hose. Commercial products like Spray and Forget, Wet and Forget, or Moss B Ware containing zinc sulfate or potassium salts of fatty acids are highly effective for moss and green algae, as they kill growth and allow rain to wash it away over weeks without scrubbing. For thick moss mats, apply killer first, wait until moss dies and dries, then gently sweep dead moss off with a soft broom from ridge downward, taking care not to lift shingles. Protect landscaping by soaking plants with water before and after. After cleaning, install zinc or copper strips near ridge for long-term prevention and trim overhanging branches to increase sunlight.
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