If unsightly black areas have appeared on your roof, you could think an expensive new roofing job is coming your way. Contrary to what most people think, the dark streaks or splotches are not usually caused by a lack of maintenance or by normal wear and tear on the shingles. Many homeowners have replaced their roofs after just 12 or 15 years in order to get rid of the dark streaks.
They are caused by bacteria known as gloeocapsa magma (algae growth). A professional roof cleaning will take them away. Algae growth typically occurs on the north side or shaded side of the roof. Unless you have the algae cleaned off, it may start to deteriorate the shingles over time.
Reputable roof-cleaning companies use removal methods recommended by the Roof Cleaning Institute of America and the Asphalt Roofing Manufacturers Association. A proper roof cleaning will allow the homeowner to maximize shingle life for the 20 or 25 years. It’s now accepted as part of home exterior maintenance.
Cleaning costs from $250 to $1,000, depending on the size of the roof and the amount of labor required. It takes two to five hours and makes the roof look like new.
Cleaning companies spray a biodegradable cleaning solution on the roof. Because the solution adheres to the roof, there is very little runoff, and what does run off is contained. Usually, it will take two coats to get it all clean.
Never allow anyone to use a power washer on your roof. They will damage the shingles.