What the streaks are
Most black streaks on asphalt roofs come from a type of algae called Gloeocapsa magma. It's common across the humid Southeast, and the NC Piedmont suits it fine.
The algae itself is blue-green. It builds a dark outer coating to protect itself from the sun, and that coating is what you see. As it spreads, rain carries it down the slope. That's why the stains run in streaks instead of spots.
You'll usually see it first on north-facing slopes and under trees. Those spots stay shaded and damp the longest. A big pine or oak hanging over the roof makes it worse.
Our treatment restores flexibility to dry asphalt. It does not kill algae, moss or lichen, and it won't wash streaks away. Spraying it over algae only puts a good product on a dirty roof.
Is algae hurting my roof?
The Asphalt Roofing Manufacturers Association, the shingle industry's trade group, describes algae staining as mostly a looks problem. Some in the trade say the damp film holds moisture against the shingles and ages them faster. That debate isn't settled.
What isn't up for debate is how it looks. Dark streaks make a roof look older than it is. They can also get noticed by people other than you. As our page on roof age and insurance explains, some carriers judge roofs from aerial photos, and streaking has been flagged that way in North Carolina. Buyers' inspectors note it too, which matters if you're selling a home.
Algae streaks or something else?
Not every dark area is algae. Here's how to tell.
| Item | Algae streaks | Something else |
|---|---|---|
| Pattern | Long streaks running downhill | Patches or spots in one area |
| Where | North faces and shaded areas | Anywhere, often after a storm or under a rubbing branch |
| Surface | A film on top, granules still there | Granules missing and asphalt showing |
| Likely cause | Algae growth | Wear, hail bruising or damage |
| Next step | Have it cleaned | Have the roof looked at |
The right order
- 01Get it cleaned
Algae should come off with a low-pressure wash made for roofs. ARMA advises against pressure washing, since it can strip granules off the shingles.
- 02Let it dry
Our treatment only goes on a dry roof in mild weather, so the roof needs to dry out fully after cleaning.
- 03Check the shingles
With the streaks gone, it's easier to see the real condition of the shingles underneath.
- 04Tune-up and treat if it fits
If the shingles are dry but sound, the tune-up and treatment can go on.
- 05Slow the return
Trim branches that shade the roof and keep debris cleared off. More sun and air means less algae.
Who does the cleaning
If you're hiring a roof cleaner, ask what they spray and how they protect your plants. Ask whether they use low pressure. Once the roof is clean, book a free roof assessment and we'll tell you whether the shingles are worth treating. If the roof looks washed out under the stains, read about faded shingles too.
Common questions
What causes black streaks on roofs in North Carolina?
Usually an algae called Gloeocapsa magma. It grows well in warm, humid climates like ours, and it shows up first on shaded, north-facing slopes. The dark color comes from a protective coating the algae forms against the sun.
Does roof rejuvenation remove black streaks?
No. Rejuvenation restores flexibility to dry asphalt shingles. It doesn't kill algae and won't clean streaks off. The roof should be cleaned first. Then, if the shingles are a good candidate, the treatment can go on a clean, dry roof.
Should I pressure wash my roof to get rid of black streaks?
No. The Asphalt Roofing Manufacturers Association advises against pressure washing or scrubbing shingles, because it can strip off granules. Granules are the shingles' sun shield, and losing them ages the roof faster. A low-pressure wash made for roofs is the safer route.
Will the black streaks come back after cleaning?
They can, because the shade and humidity that grew them are still there. Trimming overhanging branches and keeping debris off the roof helps. Some newer shingles are made with algae-resistant granules, which is worth knowing when you eventually replace.