Drive through any neighborhood from Hampstead to New Bern and you’ll see them: dark streaks running down the roof like somebody spilled coffee at the ridge.
Most folks figure it’s dirt, mildew, or just age. It’s none of those. It’s a living thing, and it’s been snacking on your roof.
Meet Gloeocapsa magma
That’s the name of the culprit, a blue-green algae (technically a type of bacteria) that floats through the air and settles on roofs. It’s blue-green while it’s alive. The black you see is mostly the dead cells piling up over time.
Here’s why it loves your roof: most modern asphalt shingles use ground limestone as a filler, and this algae feeds on limestone. Add our warm, humid coastal air and you’ve basically built it a buffet. Shingle makers say that without algae protection, streaks can show up in as little as three years.
It shows up worst on the north side and in shady spots under trees, where the roof stays damp the longest.
Is it hurting anything?
Mostly it’s cosmetic, but not entirely. As the algae eats away at the surface, it can wear down the protective layer on the shingle. And a darker roof soaks up more heat. So it’s worth dealing with, not panicking over.
Don’t pressure wash it
This is the big one. Blasting a shingle roof with a pressure washer knocks off the granules, and those granules are your shingles’ sunscreen. You’ll get a clean roof that ages faster. If you clean it, use a gentle, roof-safe treatment, or have a pro do it.
The fix that lasts: copper
Algae can’t stand copper. That’s why algae-resistant shingles have copper mixed into the granules. Every time it rains, a tiny bit of copper washes across the roof and keeps the algae from taking hold. Copper or zinc strips near the ridge work on the same idea.
It’s also why we recommend algae-resistant shingles, like CertainTeed Landmark AR, on coastal roofs. The “AR” stands for algae resistant.
So the next time you see streaks on your neighbor’s roof, you’ll know what’s going on up there.
