Black streaks often mistaken for dirt, mildew, sap or fungus on roofs might be Gloeocapsa magma, an algae strain that thrives in warm, humid environments.
|3M asphalt shingles on a Sun City, Fla.-test house stayed algae-free for 12 years.|
3M's Scotchgard algae-resistant roofing system for asphalt shingles combines copper granules (a known algae inhibitor) with standard 3M colored granules to guard against algae. A colored layer, or seal coat, covers the granules to preserve the copper's algae-inhibiting properties long term, 3M says.
Roofing manufacturers using 3M's algae-resistant system include Malarkey and Pabco.