Those dark streaks on your roof are a type of algae that live off the minerals in the roofing shingles. Algae doesn’t damage your roof but it sure looks bad. It thrives in moisture, so it’s seen most often on shaded or north-facing roof slopes that don’t get a lot of direct sun.
To Get Rid of Algae, use a pump sprayer to wet down the roof with a mix of one part bleach to two parts water. Bleach will damage foundation plantings, so rinse them well with fresh water before you start and after you’re done. Then, scrub the surface gently with a soft window-washing brush mounted on a telescoping extension pole. Don’t use a pressure washer, which could damage the shingles. Also, do this work from the safety of a ladder or from the ground, not by climbing on the roof.
To Stop Algae from Returning, mount zinc or copper strips near the roof ridge. Then, every time it rains, water that washes over the strips’ exposed edges will pick up ions from the metal that inhibit the algae from regaining a foothold. When it’s time for a new roof, look for shingles with algae-resistant granules and the best warranty you can find.
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