Published on September 13th, 2015 | by Henri


Removing and preventing roof algae

Q.  Ten years ago we had put fiberglass roofing shingles over the existing shingles. On one section of the roof, facing North, there now is a blackish discoloring, particularly where a downspout discharges water from an upper roof; however, this is also the case on  the same section somewhat removed from this downspout. I have noticed that other houses in our area have a similar problem. What is causing this selective discoloration, and what can I use to remove it without affecting the integrity of the shingles?

A.  The black streaks are caused by algae growing on the shingles. To remove them, spray the roof with a solution of 3-parts fresh Clorox bleach to 1-part water with a garden sprayer on a windless day. Work from a ladder, not by walking on the roof, as it can become slippery when wet. Wear goggles, old clothes and rubber gloves. Spray only enough to wet the shingles and avoid as much run-off as possible.
One gallon of the mixture covers 50 square feet of roof surface.

Before you start, thoroughly soak any vegetation below the roof and cover it with plastic. If you have metal gutters and downspouts, keep running water in them while spraying, and until all run-off stops, as the solution is very corrosive. When you are finished with the spraying, wash the plastic thoroughly with your garden hose and spray the plantings again. It will take several weeks before you see results.

Alternatively, you can buy ready-made formulas to remove algae. One example is Roof Deck Cleaner, which can be purchased through the Shingle Shield Web site at

To prevent recurrence, install special copper or galvanized strips on each side of the roof directly below the ridge vent. The strips can also be found on the Shingle Shield site, but you may also find equivalent products at your hardware, building-supply or big box stores, where zinc or copper strips come in a roll.

You can also hire a professional roof cleaner.

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