Best way to eliminate roof algae

Applying three-ingredient solution preferred to high-pressure wash

By Paul Bianchina

Q: We have a house that was built in 1995, in a neighborhood of other houses all built around the same time. Our house has a gray composition shingle roof that has developed large dark stains in many areas. Other homes in the neighborhood with gray or charcoal roofs also have it, but the ones with light-colored roofs don't. Is it algae? If so, would zinc strips help? --Shirley L., via e-mail

A: It's very likely that the staining is indeed some form of algae growth, and I contacted the Asphalt Roofing Manufacturers Association to confirm that this is their opinion as well.

As to how to deal with it, their recommendation is as follows: "Algae discolorations are difficult to remove from roofing surfaces, but may be lightened by applying a solution of chlorine bleach, trisodium phosphate (TSP) and water. Solutions for these ingredients may vary between shingle manufacturers and depend on the amount of discoloration. Solutions range from one cup TSP, one gallon bleach and five gallons of water, to one cup TSP and 2.5 gallons each of bleach and water.

"First, gently disperse this solution on the roofing surface. Use normal precautions for handling bleach. Be sure to apply it carefully to avoid damage to other parts of the building and its surrounding landscape. Avoid scrubbing the surface, as this friction may loosen and remove granules. If possible, always work from a ladder and/or walkboards to avoid direct contact with the roof surface. Observe all possible safety precautions when working on or near the roof. Finally, rinse the solution from the roof by gently spraying the surface with water. Be warned that this solution's application and rinse process will make the roof surface slippery and potentially hazardous to walk on during treatment.

"High-pressure washing systems for algae removal should not be used. The effectiveness of such cleaning techniques are only temporary, and discoloration will likely recur."

Opinion varies somewhat on the effectiveness of zinc strips, which release small amounts of zinc oxide onto the roof whenever it rains. At best, the zinc strips will help inhibit additional algae buildup, but will not prevent or kill it.

If you'd like to give them a try, first follow the recommended cleaning outlined above. Then, after the roof is completely dry, apply the zinc strips at the top of the roof, just under the ridge shingles, leaving about two inches of the metal exposed to the weather.