Those Black Streaks on Your Roof: What They Are and What to Do
It's algae, not dirt or age — and how you remove it matters as much as whether you do.
What's actually causing the streaks
The dark streaking that shows up on asphalt shingle roofs, usually running down from the ridge, is caused by a specific algae species that feeds on the limestone filler used in many asphalt shingles. It spreads via airborne spores and thrives in humid conditions, which is why it's common across much of the Midwest, not unique to any one property.
Why it's worse on some roofs than others
North-facing slopes, shaded areas under tree cover, and roofs in humid microclimates show the heaviest streaking, since algae favors moisture and reduced direct sun exposure. Roofs in open, sunny locations tend to show noticeably less staining even with the same shingle product.
Is it actually a problem, or just appearance?
In the short term, it's primarily cosmetic. Left untreated for years, however, algae and any accompanying moss growth can hold moisture against the shingle surface longer than normal, which contributes to accelerated granule loss over time — a real, if slow-developing, factor in roof lifespan.
The right way to remove it
A low-pressure soft-wash treatment removes algae without stripping protective granules, unlike pressure washing, which can significantly shorten remaining shingle life even as it clears the staining faster. Algae-resistant shingle products, which include copper or zinc granules, are also worth considering on your next reroof if staining has been a recurring issue.