I have a 30' x 90' barn with gray / silver standard roof shingles. I re-shingled the roof twelve years ago and had removed the old roll roofing material. Question; I have this black staining throughout the roof starting about half way down from the peek? Prior to replacing the roof, I removed several trees that overhung the roof. With that said, the roof is in open sunlight. I do not heat the barn in the cold winter months. I do not understand what the cause is. I see this same staining on other house roofs in and outside of our area.
I would greatly appreciate tour explanation / feed back.
Much thanks, Joe C.
The dark streaks are a type of algae that lives off the minerals in some types of roof shingles. Algae 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.
While it's true that algae doesn't damage roofing, it sure looks bad. To get rid of it, 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 the algae from returning, mount zinc or copper strips near the roof ridge. Then, every time it rains, the water that washes over the strips' exposed edges will pick up ions from the metal and inhibit the algae from regaining a foothold. And 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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