It is a living organism, not staining
Gloeocapsa magma is a cyanobacteria that arrives airborne and thrives in warm, humid climates - which describes Florida perfectly. It feeds on the limestone filler used in asphalt shingles and protects itself with a dark pigmented sheath. That sheath is the black streak you see.
Why it always runs downward
The colony starts high on the roof and spreads with every rain, which is why streaking runs vertically down the slope. North-facing slopes stay damp longest and are almost always the worst affected.
What it costs you
- Shortened shingle life as the granule bond degrades
- Higher attic and roof surface temperatures from dark growth
- HOA violation notices, common across Palm Coast and Flagler communities
- Reduced curb appeal and appraisal impact at resale
The correct removal method
A low-pressure soft wash applied evenly across the roof, given time to kill the organism, then gently rinsed. This is the method asphalt shingle manufacturers endorse. Results generally hold for two to four years in this climate, with north-facing slopes returning soonest.
Do not pressure wash a roof, and be careful with anyone who offers to. Granule loss is permanent, it shortens roof life, and it usually voids the warranty.