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. The algae can attack all roof types and take over within a few years of construction.

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.