Short answer
Black streaks are almost always an algae called Gloeocapsa magma, which thrives on humid, shaded roofs and is mostly cosmetic. Moss is different: it holds water and lifts shingles. Soft washing with a roof-safe cleaner removes both, and zinc or copper strips and algae-resistant shingles slow their return.
Where it grows
Algae streaks are most common in the humid Southeast and Gulf states, while moss thrives in the damp Pacific Northwest and on heavily shaded roofs anywhere. North-facing slopes stay damp longest, so growth shows up there first.
Is it damaging the roof?
Algae mostly darkens the roof and can raise heat absorption slightly. Moss and lichen hold moisture against shingles and can lift them, so they should be removed.
How to remove it safely
- Use a low-pressure soft wash with a cleaner made for asphalt roofs.
- Never pressure wash: it strips granules and can void the warranty.
- Protect plants below with water before and after cleaning.
- Leave steep or two-story roofs to a professional.
Keeping it away
Trim branches to let in more sun. Zinc or copper strips near the ridge release metal ions when it rains, slowing regrowth. When you replace the roof, choose shingles with algae-resistant granules.
When to call a roofer
- Moss or lichen is growing on the roof.
- You want the roof cleaned without risking the shingles.
- Growth came back within a year.
What the roofer does: see roof maintenance & cleaning.
Common questions
Does insurance cover algae or moss?
No. It's considered maintenance, not damage.
How long does a roof cleaning last?
Typically a few years, longer with zinc or copper strips and more sun on the roof.
Editorial note: this guide is general information for homeowners, not a diagnosis or insurance advice. Conditions vary by roof, so a roofer should inspect and confirm the cause before any repair.