What causes roof stains?
Short answer. Most roof stains come from biological growth (algae, moss and lichen), metal runoff (rust or copper), decaying leaves and pine straw, soot near chimneys, and water from AC condensate lines. The color and shape of the stain usually tell you which one you have.
Identify the stain by color and shape
- Black or dark streaks running downslope: algae. Cosmetic, very common in Georgia; see black streaks and algae on roofs.
- Green, fuzzy clumps along edges and shaded areas: moss, which holds moisture and lifts shingles.
- Gray green or pale crusty spots: lichen, tightly attached and slow growing; see moss and lichen on roofs.
- Orange or brown streaks below metal: rust from flashing, vent caps, nails or satellite mounts.
- Brown staining in valleys and behind debris: tannins from decaying leaves and pine straw; see pine needles and leaves on the roof.
- Dark soot near the chimney: smoke residue; check the chimney cap and flashing at the same time.
- Green or blue tint below copper: runoff from copper flashing or strips, harmless.
- White or brown trails below an AC line: condensate discharging onto shingles, which also feeds algae.
Stains that are not stains
Dark, round spots scattered over a slope can be hail hits where granules were knocked off, and large dark patches may be bald areas from age. Both deserve a closer look, especially after a storm.
What to do
- Remove debris and redirect condensate lines.
- Replace rusting metal rather than painting over it.
- Treat biological growth gently, never with a pressure washer.