Undoubtedly you have noticed a lot of roofs in our neighborhoods that have black streaks. And judging from the number of customers who call to find about what can be done about the stains, we want to identify what those streaks are and how to best treat them.
Those stains are algae, an airborne spore that can travel from roof to roof. While the only preventative measure is to buy AR (algae resistant) shingles when replacing your roof, the manufacturers’ warranties are only good for 10 years, and that warranty is restrictive. Your roofing contractor should introduce you to the manufacturers’ rep and work with you on the warranty issue.
There are several schools of thoughts on whether the algae itself is harmful to your roof, and what can be done about it once it arrives. Unfortunately, since algae is airborne, the wind can carry it from roof to roof. In 1994 a survey was taken that identified algae as the number one aesthetic problem associated with asphalt shingles (Tackling the Algae Problem, “Roofer”, August 1994, p. 10). If considering a new roof, it is wise to limit your selection to darker colors.
The guidelines we recommend, and that are also endorsed by the NRCA (National Roofing Contractors Association) is to mix 4 gallons of water with one gallon of commercial bleach and 1 cup of TSP (Tri-Sodium Phosphate, found at many home centers). First wet the surface of the roof with plain water. Then apply the mixture and let it set for 5 minutes before gently rinsing it off with a garden hose. This procedure is not permanent and will have to be repeated when algae shows up again. Under no circumstance should you allow anyone to power wash your roof. The granules on the shingles protect the asphalt from ultraviolet radiation, and power washing will cause the granules to loosen and come off.
There are many products on the market and the internet that claim to get rid of algae, but read the product information carefully because they could invalidate your warranty and you could apply a harmful solution. Proceed with caution on who, how and what you allow on your roof.