Last updated: August 2026
Those dark streaks running down your roof are almost always algae — a hardy organism called Gloeocapsa magma. It's mostly a cosmetic problem: it stains shingles and can slowly hold moisture, but it is not the same thing as granule loss or a roof that's drying out, and it is not what rejuvenation is designed to fix.
What are the black streaks on my roof?
The dark, dripping streaks you see on so many roofs are a blue-green algae called Gloeocapsa magma. It travels through the air as spores, lands on a roof, and takes hold — most visibly on north-facing and shaded slopes that stay damp longer. Over time the colony grows and produces a dark, protective sheath, which is why the stains look black and tend to streak downward with the flow of rainwater.
If you've noticed your roof and your neighbors' roofs streaking in similar spots, that's the algae spreading from roof to roof through the air. It's extremely common across humid climates, including here in Lancaster County.
Why does algae appear on some roofs and not others?
Algae needs moisture and something to feed on. Shaded areas, overhanging trees, and slopes that don't get much direct sun stay wet longer and give it the damp conditions it likes. The limestone filler used in many shingles can also give the organism something to feed on. None of that means your roof is defective — it mostly means the conditions were right for algae to settle in.
What does algae actually do to my roof — and what doesn't it do?
It's worth being precise here, because algae is often blamed for more than it deserves.
- It stains the surface. The most obvious effect is cosmetic — dark streaks that make a roof look old and dirty even when the shingles are otherwise fine.
- It can hold moisture. A heavy algae mat keeps the surface damp longer, which over a long time isn't ideal for the shingle.
- It does not, by itself, dry out your shingles. Algae is not what pulls the oils out of asphalt — that's sun, heat, and age.
- It is not the same as granule loss. A streaky roof can still have plenty of granules; a roof shedding granules can be streak-free. They're separate issues that sometimes show up together.
- It is not a leak. Algae staining on the surface tells you nothing about whether your flashing or shingles are keeping water out.
How is algae different from granule loss or a drying roof?
This is the distinction that trips up a lot of homeowners. Black streaks from algae are a living stain on top of the shingle. Granule loss and drying out are about the shingle itself aging — losing its protective granules and the oils that keep it flexible. You can tell them apart with a couple of quick checks: algae usually appears as long vertical streaks concentrated on shaded slopes, while granule loss shows up as thinning, bare patches and grit collecting in the gutters. If you want to dig into the aging side, our guide on how to tell if your asphalt roof is drying out walks through those signs.
Where does rejuvenation fit — is it an algae wash?
No, and this is important: rejuvenation is not an algae treatment or a roof wash. Rejuvenation restores the oils and flexibility inside an aging shingle so it can keep flexing through freeze-thaw cycles; it does not remove or kill surface algae. If your only complaint is the dark streaks, what you're looking for is a cleaning solution, not a rejuvenant — and cleaning a roof should be done carefully so the shingles aren't damaged in the process.
Where the two connect is that many roofs old enough to be badly streaked are also old enough to be drying out. If that's your roof, cleaning addresses the appearance while rejuvenation addresses the aging underneath. To see the evidence behind whether treatment actually restores an aging shingle, read does it work?
Not sure whether you're looking at algae, granule loss, or both? Book a free inspection and we'll tell you exactly what's on your roof.