Algae Algae on roof

Published on March 4th, 2013 | by Henri


Black streaks on roof

Q.    We have black streaks forming from bottom to top of our shingled roof.  Mostly shady areas, but sunny also. Lighter colored shingles most affected in this Pittsburgh area. I tried strong bleach, with little success. Would TSP be more effective? Is there something else I could use?

A.    The way to remove algae — the growth responsible for the black streaks — is to spray it with a solution of three parts fresh Clorox bleach (it contains some effective chemical ingredients not found in most other bleaches) and one part water. Use a plastic garden sprayer and apply the solution on a windless day at the rate of 1 gallon per 50 square feet of roof.

Wear rubber gloves and old clothing as well as eye protection. Spray the solution from a ladder as walking on the roof is dangerous and may void any warranty left on the shingles.

If you have metal gutters, run water in them while applying the solution because it is corrosive. Soak all vegetation before spraying the roof, cover it with plastic, and rinse it thoroughly when you are done and there is no longer any solution dripping from the roof. It will take several weeks for you to notice any results.

Installing special galvanized or copper strips at the ridge should prevent recurrence. You can buy rolls of zinc-coated metal in building supply stores, Home Depot, Lowe’s and some hardware stores. Or you can order 3-foot strips from, and click on Shingle Shield in the Other Products box, which are installed with nails just below the cap shingles.

An alternative is to order 2-foot zinc strips, installed with adhesive, from  There you will also find their roof cleaning solution. There are also commercial roof cleaning solutions available; one such is Wet & Forget, which you should be able to find in hardware stores.

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