Those dark streaks running down your shingles aren’t soot or dirt, it’s algae (Gloeocapsa magma), and it’s one of our most common calls this time of year once fall rains bring the humidity back up.
It usually shows up first on the north-facing slope, where the sun doesn’t dry things out as fast, and it spreads by windblown spores, so don’t be surprised if a neighbor’s roof starts showing it too. It’s mostly cosmetic early on, but left alone for years it can eat into the granules protecting the shingle, and that shortens the roof’s life.
A proper soft wash handles it safely, and if you’re due for a new roof anyway, algae-resistant shingles with copper or zinc granules make it a non-issue going forward. We’ll give you an honest read on whether it’s just cosmetic or something to plan for.