Expert Answers: Cleaning Roof Stains

Q: How do I clean my shingles? The shingles on the north side of my house have ugly black stains that run in streaks. Could this be algae?
John Dale, Albion, NY

A: That sounds like the black algae known as Gloeocapsa Magma. It typically grows on the north side or on shaded areas of roofs because these areas stay wet longer, and it thrives on the elements that compose shingles, such as ground limestone. Many new shingles contain algae killing ingredients such as copper granules, but that doesn’t help your existing shingles.

The only way to improve the appearance of your roof is to kill the algae and try to limit future growth. Apply an exterior cleaner that contains oxygenated bleach (not chlo-rine) or a roof cleaner such as Spray and Forget, Shingle Renew or Defy. Algae is stubborn, and you’ll likely have to re-apply the cleaner.

Another strategy to help prevent future growth is to install copper or zinc strip flashing under the top course of shingles, leaving a couple of inches of the flashing exposed. The rain will wash tiny amounts of the metal flashing over the shingles below and poison the algae.

A more aggressive approach is to wash off the algae and then apply an algae inhibiting product. There are two main concerns with washing shingles. First, a wet roof is dangerous to walk on, especially when it’s covered with wet algae. Second, scrubbing or spraying can damage shingles.

The safest way to clean a roof is to hire a cleaning contractor. However, if the roof has a low pitch (4-12 or less),you can consider washing it yourself with a low-pressure hose attachment designed for cleaning shingles. Always spray the water down the roof, and never scrub the shingles. You can use a pressure washer, but you must use a wide sprayer tip (25 or 40 degrees) and low pressure. Adjust the pressure and distance to remove only the algae. Keep in mind that most shingle warranties do not cover damage caused by pressure washing.




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