What are those blacks streaks on a roof? Though often attributed to an accumulation of dirt, mold or mildew; the most common culprit is actually a blue-green algae known as Gloeocapsa Magma that is spread by airborne spores. While algae growth does little to damage the roofing materials (unlike moss which will damage the roof coverings), the stains do not help the appearance of a home. You may have noticed that the algae stains are absent directly below the metal flashing around chimneys or roof vents. This is due to fact that copper and the zinc coating on galvanized sheet metal are toxic to algae. Every time it rains, trace amounts of metals are washed down the roof, inhibiting algae growth.
To help keep your roof clean, install a strip of copper or zinc coated sheet metal along each side of the roof just below the ridge. To be effective, 2”- 4” of metal should be visible on the roof. While copper is more toxic to algae, galvanized sheet metal is much less expensive. Both copper and galvanized metal are available in rolls of various widths and thickness both online and from local metal suppliers. You can also purchase zinc strips specifically made to eliminate roof algae from Z-Stop – http://www.z-stop.com/