Algae discolors a roof by giving it a brown to black appearance and is particularly unsightly on a white or light-colored roof. Although it is most prevalent in coastal areas and regions subject to warm and humid weather, it occurs in all regions of the country. The algae discolorations should not be confused with those due to moss or tree droppings, which typically produce only localized discolorations. Algae discolorations are difficult to remove. They may be lightened by applying a solution of chlorine bleach, TSP( trisodium phosphate), and water as follows:
- Mix a solution of 1 part chlorine bleach, 3 parts water, and a pinch of trisodium phosphate (known as TSP).
- Apply this solution gently to the surface of the roofing, being careful to avoid damage to other parts of the building and its surroundings.
- Avoid scrubbing as well as other physical contact with the roof because the friction may loosen and remove granules that coat the surface of asphalt shingles. Work from a ladder whenever possible and tie off.
- Finally, rinse the bleach solution from the roof by gently spraying the surface with water.
- Observe all possible safety precautions when working on or near the roof. Besides giving special attention to ladder safety, be advised that the solution application and rinsing process makes the surface of the roof slippery and therefore hazardous to walk on. The effectiveness of the cleaning and bleaching techniques is temporary, and discoloration is likely to recur. A better response to the problem is to use algae-resistant roofing especially if the environment near the roof is favorable to the growth of algae.
CAUTION: High-pressure washing systems for algae removal should be used only by reliable professionals because incorrect application can result in removing granules and shortening the life of the roofing system.