Guide
What the Black Streaks on Your Roof Actually Are
Black streaks on asphalt shingles are a living algae colony, not dirt. What Gloeocapsa magma does to a roof and how it is removed safely.
It is alive, and it is eating your roof
The dark streaks running down the north-facing slope of almost every roof in the Ozarks are a blue-green algae called Gloeocapsa magma. It is not dirt, it is not soot, and it is not shingle discolouration. It is a colony, and it is feeding.
What it feeds on is limestone. Asphalt shingle manufacturers blend crushed limestone into shingles as a filler, and the algae has adapted to live on it. The streaks run downward because that is the direction rain carries the spores and the nutrients.
Why it starts on one side
You will almost always see it worst on the north-facing slope, and on slopes shaded by trees. Those surfaces stay damp longest after rain and dew, and moisture is the one thing the colony cannot do without. South-facing slopes in full sun dry too fast to support much growth.
This is also why it spreads along a street. The spores are airborne. One heavily colonised roof seeds the ones downwind of it.
What it costs you
- Heat. The dark colony absorbs sunlight the reflective granules were meant to bounce away, so the attic beneath runs hotter.
- Moisture retention. The colony holds water against the shingle surface rather than letting it run off.
- Granule loss. As the algae feeds, it loosens the bond holding granules to the mat. Granules are the shingle's UV protection, and they do not grow back.
- Resale. A streaked roof reads as an old roof. Plenty of roofs get replaced years early because they look finished rather than because they are.
The wrong way to remove it
A pressure washer will take the streaks off a roof. It will also take the granules, and that is not recoverable. Every major shingle manufacturer specifically warns against pressure washing, and doing it can affect warranty coverage.
If a contractor proposes pressure washing your shingle roof, that is your answer about whether to hire them.
The right way
No-pressure soft washing. A shingle-safe cleaning solution is applied at low volume, left to dwell long enough to kill the colony at the root, and rinsed gently. The algae dies rather than being knocked loose, which is why the roof stays clear for years instead of weeks.
Done properly the streaks lift during the treatment or over the following days. Moss is slower — it dies on contact but weathers off over several weeks.
How often
In this climate, every two to three years for most roofs. Roofs under heavy tree cover or with permanent shade on one slope come back faster. If you are seeing streaks again within a year, the issue is usually overhanging limbs holding moisture on the roof.