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Roof Discoloration Due to Algae

DSC00537a This photo  shows an  asphalt shingle roof that has discoloration due to algae.  Algae  growth on roofs is a common problem throughout the entire world and is more prevalent in areas of high humidity.  The exact name of the algae is Gloeocapse magna, which causes the dark staining which you can see  in the first photograph. This type of algae  grows and feeds on  limestone in a moist environment.  Limestone is a major ingredient in the manufacturing process of fiberglass matted shingles.

Algae growth usually begins on the north slopes and on other areas which receive little or no sunlight. These factors combined,  allow for a great environment for this algae to grow.  As algae grows on the roof, it retains even more moisture and thus speeds up the growth and  discoloration process.

DSC00539a This photo shows another type of growth found on the same asphalt shingle roof. This mold or lichen is green in color and thrives in the same conditions.

What is the best  way to eliminate the shingle  discoloration?
  • Remove the existing shingles and reroof with algae resistant shingles. Most major manufacturers now offer algae resistant shingles in most of their product lines.
  • Apply a  diluted chlorine bleach solution to the entire roof and rinse off. This works pretty well but should be left to a professional. Some companies now offer eco-friendly solutions that are less harmful to the environment.
  • Have a Z. ( zinc) strip installed at each side of the ridge. In time the chemical reaction between water and zinc will kill the algae. This solution works pretty well especially on low slope roofs  with gable ends. Higher pitched roofs and roofs with numerous hips present a challenge best left to a professional roofing contractor.

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