Green and black streaks creeping across a roofline rarely show up overnight, and by the time they're visible from the street, the algae or mildew responsible has usually been growing for months underneath.
A roof collects more than dirt over time. In warm, humid climates, algae, moss, and mildew take hold on shingles and slowly work their way into the surface rather than sitting on top of it.
Dirt, algae, and mildew build up on driveways, walkways, and patios a little at a time, which makes the problem easy to overlook until the surface looks neglected.
