What are the black streaks on my roof?
The dark streaks are commonly caused by algae, often called black algae, which spreads on the wind and grows where moisture lingers. Southwest Florida's warm, wet summers suit it well. Streaks usually start on shaded slopes and spread across the roof.
- On shingles the growth can feed on the mineral filler in the shingle and leave long dark stripes.
- On concrete tile the porous surface holds moisture, so tiles turn black, gray or green, and moss can settle in the gaps.
- Rust-colored marks sometimes come from metal flashing or nearby fixtures.
The streaks are not only cosmetic. Growth holds moisture against the roof and can wear at the surface over time. Dark roofs also look older than they are, which can matter when you sell or when an association sends a letter.

