Dark streaks running down a roof, or patches of green moss in shaded areas, are common enough that a lot of homeowners assume they’re purely cosmetic. They’re not entirely — but the urgency depends on which one you’re seeing.
Those black streaks are usually a type of algae that feeds on the limestone filler in asphalt shingles. It’s mostly a cosmetic issue and doesn’t significantly shorten shingle life on its own, though it can affect resale curb appeal. Moss is a different story: it holds moisture against the shingle surface, and over time that trapped moisture can lift shingle edges and accelerate granule loss, especially in shaded, north-facing sections of a roof that don’t dry out quickly.
Pressure washing is not the answer — it can strip protective granules and do more damage than the moss itself. If moss has taken hold, it’s worth having it professionally treated and removed, and having the affected area inspected for any underlying damage once it’s gone.