Learning All About Roofing Materials

Dark Streaks On The Roof: What To Do About This Common Problem

27 May 2015

Beverley Hall

Dark streaks and spots that develop on asphalt shingles are usually caused by algae, even if the discoloration looks more like mildew. Not only is this aesthetically unattractive, but the plant-like organisms can gradually deteriorate the roof as they feed on limestone granules in the shingles. 

Roof Algae Facts

Gloeocapsa magma is the species that causes dark stains on roofs. The algae have most likely appeared on parts of your roof situated under trees or on the north side of your home. Like other kinds of algae, these organisms like shade and humidity.

They also don't survive in contact with zinc or copper. If you look at your roof or roofs on other houses, you'll see that the streaks don't occur below components such as copper chimney flashing or galvanized steel -- which is coated with zinc -- for attic vents and exhaust pipes. 

Removing the Algae

You can remove the algae as a do-it-yourself project or hire a roofer to complete the task for you. To do it yourself, spray a blend of one part chlorine bleach to four parts water onto the problem areas; add one part trisodium phosphate (TSP) to boost the effectiveness if the streaking problem is particularly bad. An easy formula includes 1/4 cup of TSP, 1 quart of bleach and 1 gallon of water.

Commercial removal solutions also are available at stores that carry home improvement products if you don't want to make your own. 

Allow the solution to soak into the dark streaks and spots for 15 or 20 minutes and then rinse with a hose. Don't use a power washer, because that equipment is too hard on asphalt shingles. If dark streaks are still evident, apply your bleach solution with a soft brush; a telescopic pole is helpful if you don't want to climb onto the roof. 

Since bleach is harmful to plants, cover any plants underneath the spray area. 

A roofing contractor can complete this project if you prefer not to do it yourself.

Preventing Algae Return

A roofing contractor also can install metal in strategic parts of the roof to prevent algae from coming back and to eliminate any that does start growing. This involves placing a narrow piece of copper or galvanized steel under the roof ridge just above problem areas. Rainwater and melting snow wash metal molecules down the shingles that eradicate the algae. 

When your home eventually needs a new roof, consider having a contractor install shingles that contain copper granules. This makes the shingles resistant to algae. To learn more, speak with a company like Di Roma Roofing.

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