Dark streaks on a roof are not exclusively indicative of water damage but often signal the presence of algae or moss growth. These organisms thrive in moist environments and can cause dark discolorations on roofing materials. While moss and algae are not direct signs of water damage, their growth suggests excess moisture, which can eventually lead to structural issues like wood rot. It's essential to address these growths promptly to prevent potential damage and maintain the roof's integrity. Regular roof inspections can help identify and manage such issues effectively.