What are the algae streaks on my roof, and how can I tell them from other dark marks?
Short answer. Algae streaks are dark gray to black vertical stains that follow the path water takes down the roof, usually heaviest on shaded north slopes. If a dark area instead looks like bare, smooth patches, sits only around a chimney, or appears suddenly after a storm, it may be granule loss, soot or damage rather than algae.
Why algae makes streaks
Rainwater carries algae cells and nutrients down the slope, so growth stretches in lines from the ridge or from a feature like a chimney toward the gutter. The dark color comes from a pigment the organism produces to protect itself from sunlight.
Telling algae from other dark marks
- Algae: soft edged streaks running downhill, darkest on shaded slopes, with granules still in place underneath. A small test spot with diluted bleach usually lightens in minutes.
- Granule loss: patches where the surface looks smooth or shiny, granules collecting in gutters, sometimes the black asphalt showing. That is wear or damage, not staining.
- Hail bruising: random dark spots of missing granules, often with a soft feel, appearing after a storm. See Georgia hail season.
- Soot: darkening concentrated near a chimney or fireplace vent.
- Tree sap and debris stains: spots under specific branches.
- Moisture shadows: a darker band that disappears as the roof dries.
What the streaks mean
Algae streaks are mostly cosmetic and do not by themselves mean the roof is failing. But they show where your roof stays damp longest, which is also where moss and lichen tend to appear next.
What to do
Clean gently or leave it; never pressure wash. For cleaning methods and prevention, read black streaks and algae on roofs.