The green on your roof is eating it

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Portland grows moss the way other cities grow grass. Roughly seven months of drizzle, big tree canopy, and north-facing slopes that never fully dry give you the green cushions in the valleys and the black streaks running down the shingles. Both are doing damage while they sit there.

What is it doing to the shingles?

Moss anchors with root-like threads that work under shingle edges and pry them up, and the colony holds water against the surface through every storm. The black streaks are a different organism, an algae feeding on the limestone filler in the shingle itself, stripping the granule layer as it spreads. One shortens the roof's life from outside; the other eats it from within.

Why not just pressure wash it?

Because pressure washing takes the granules off with the moss and can drive water up under the laps. It is the fastest way to turn a dirty roof into a ruined one. The method that preserves the shingles is gentle removal plus a treatment that kills what is left, and that is what a roofer who works this climate will describe.

Can you keep it from coming back?

You can slow it a lot. Zinc or copper strip near the ridge leaches a rinse that growth does not like, debris cleared out of valleys removes its footing, and trimming back overhanging limbs gets sun on the roof. Shaded, needle-dropping trees overhead mean it returns on a schedule regardless; treatment on a schedule is cheaper than the roof it saves.

When does cleaning stop making sense?

When the roof under the moss is already at the end. Part of an honest assessment is telling you that cleaning a dead roof is money spent twice.