Why is my roof stained?
Short answer. Most roof stains are black or dark green streaks from algae, which is mainly cosmetic. Other common causes are moss or lichen, rust running from steel vents or flashing, tannins and pollen from overhanging trees, mineral deposits below chimneys, and bare patches where granules are gone, and a few of those signal a real problem.
Match the stain to the cause
- Black or dark streaks running down the slope: algae. Cosmetic, but it can be cleaned gently. See black streaks and algae.
- Green, fuzzy clumps: moss. More serious, because it holds water and lifts shingle edges.
- Crusty gray green or pale circles: lichen, tightly bonded to the granules.
- Reddish brown streaks below vents, flashing or a satellite mount: rust from corroding steel. Harmless to the shingles, but the corroding part may be on its way to leaking.
- Brown or yellow patches under trees: tannins from wet leaves, sap, or heavy spring pine pollen. Usually washes away over time.
- White chalky streaks below a chimney: mineral salts leaching from the masonry, a sign water is soaking into the chimney.
- Dark, flat patches with no streak pattern: often granule loss exposing the asphalt, from age or hail. Worth a close look.
- Green or blue streaks below copper: runoff from copper flashing or strips, harmless.
If the stain is on your ceiling
A brown ring on a ceiling or wall is not a roof surface stain; it is water getting in. Check the attic above it with a flashlight during or right after rain, and see common causes of roof leaks for where to look first.
What to do next
Algae, pollen and copper runoff are cleaning or patience problems. Moss, rust from flashing, chimney salts and bare patches deserve an inspection, because each can lead to a leak or show damage an insurance claim might cover. Never pressure wash a stained roof; it removes granules and can void the shingle warranty.