You may have the most attractive landscaping, beautiful windows, and charming mailbox, but black streaks running down your roof can ruin your home’s curb appeal.
What causes those dark marks, and what can be done to eradicate them?
The source is an algae called Gloeocapsa Magma. And depending on the age and condition of your roof, cleaning may be the most cost-effective solution, since it’s about 5 to 10 percent of the price of a roof replacement, which can be as much as $10,000.
Algae-caused marking isn’t preventable but can be removed, though not always permanently. The algae survive through photosynthesis and by feeding on limestone filler used in asphalt shingles.
Black marks became a problem about 20 years ago when manufacturers began adding limestone granules to add weight to the material used to coat shingles.
While the dark streaks are unsightly, the greatest danger to the roof is from moisture retention or root damage that algae and other life forms can cause. Also, algae and fungus can grow together to form lichen, the roots of which can wrap around and feed on the granules covering the shingles. Once established, lichen is not easily removed. Even if it dries out, it can come back to life with the next rain. Scrubbing or power washing lichen will only cause more damage.
Before determining whether having your roof cleaned is the right option, be sure you have a sense of your roof’s age and condition, and compare costs accordingly.
Article from https://www.angieslist.com/articles/whats-causing-those-black-streaks-your-roof.htm