Q: What are all the black streaks you see on people’s roofs?
— Helen R. Butler
Cuyahoga Falls
A: I get calls and emails periodically asking about those streaks, which are caused by an algae-like bacteria called Gloeocapsa magma. It spreads via airborne spores and feeds on the calcium carbonate in the crushed limestone that’s used in some asphalt shingles.
If your shingles are in good condition, you can scrub the roof lightly with a solution of chlorine bleach and water or a product made for cleaning the streaks. However, you need to be very careful not to damage the shingles. Protect your skin and eyes if you use bleach, and of course, take proper precautions for working on a roof. You should also protect the plants that may be affected by watering them well beforehand and rinsing them afterward.
You may be better off hiring a professional to clean the roof. You can find roof cleaners in the Yellow Pages, or ask a roofing contractor you trust if he can do the work or suggest someone who can.
To prevent the streaks from returning, you can install copper or zinc strips under the shingles at the top of the roof, leaving part of the metal exposed. Rainwater will carry tiny amounts of the metal down the roof and kill the bacteria.
Have a question about home maintenance, decorating or gardening? Akron Beacon Journal home writer Mary Beth Breckenridge will find answers for the queries that are chosen to appear in the paper. To submit a question, call her at 330-996-3756, or send email to mbrecken@thebeaconjournal.com. Be sure to include your full name, your town and your phone number or email address.