Roof mold or moss can stain your shingles green or black. While this is unattractive, it also harms the roof by allowing moisture to develop beneath shingles. Roof wood can rot, and asphalt shingles can degrade. Learn how to kill roof moss and mold.
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Website ThisOldHouse.com suggests a 1:1 chlorine bleach and water solution to remove moss. The website advises spraying the bleach solution onto the roof shingles with a sprayer. If you don't have a sprayer, you can use a stiff-bristled brush to scrub the bleach solution onto your roof. Chlorine bleach may kill other plants, so you'll want to spread a tarp to protect shrubs that lie below your roof line.
An oxygen bleach solution won't harm other garden plants, but kills both moss and roof algae (which comes in green or black, and can be mistaken for mold). Mix oxygen bleach powder with water following the manufacturer's suggested ratio, then apply the solution to your roof, and leave it on for 20 minutes. Scrub the roof, and rinse it to remove algae and moss.
If neither bleach solution solves your problem, you can find fungicides designed to remove moss and mold from your roof. Mix the fungicide with water following the manufacturer's directions. Wear rubber gloves and long pants to protect your skin. Coat the roof with the fungicide solution, and leave it on the recommended time, then rinse off the fungicide with water.