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
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
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
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