The black streaks running down your shingles are Gloeocapsa magma — a cyanobacteria, not dirt — which is why rain never washes them away and why roof cleaning is a chemistry job rather than a scrubbing job.
A living thing, eating your roof
Gloeocapsa magma feeds on the crushed limestone that shingle manufacturers use as filler. It arrives as airborne spores, establishes on the slopes that hold moisture longest — north-facing and shaded first — and spreads downhill as rain carries new spores with the runoff. That is why the streaks run vertically and widen over time.
The dark colour is the organism’s UV shield, a protective pigment it produces as the colony matures. By the time a roof looks streaked from the street, the colonisation is well advanced.
Why it matters beyond appearance
The colony holds moisture against the shingle surface, and it consumes the limestone filler as it feeds. Both accelerate granule loss — and granules are the sacrificial layer protecting the asphalt underneath from sunlight. A colonised roof ages measurably faster than a clean one.
There is also a heat effect: darkened shingles absorb more summer sun, which works against your cooling bills on the worst months of the year.
The fix, and the method that makes it worse
A low-pressure chemical treatment kills the organism outright — you can watch the staining lift within minutes on most roofs — and because the roots are dead rather than trimmed, the roof stays clean for years.
A pressure washer also removes the streaks, along with the granule layer, several years of roof life, and quite possibly your shingle warranty. The correct method is also the gentler one, which makes choosing it easy.
Send a photo of your worst slope and we will tell you what stage it is at and what treating it costs. Read more about roof cleaning →