Moss and algae
Around the Puget Sound, the question is not whether a composition roof will grow moss — it is which slopes, and how soon. Eight damp months a year, shade from mature conifers, and granule surfaces that hold moisture make this the defining roof problem of the region, and it is structural, not cosmetic.
What the growth actually does
Moss works like a wet sponge stapled to the roof: it holds water against the shingle surface long after the rain has stopped, so the covering underneath spends weeks wet that it was designed to spend dry. As the cushion thickens, it physically lifts shingle edges and pries at the joints between courses — and a lifted edge is an opening, in a climate that will find it. The black streaking of algae is milder, a surface condition more than a structural one, but it marks the same thing: a slope that stays damp.
The geography of it is readable from the driveway. North-facing slopes and anything under canopy go first; south-facing, open slopes can stay clean for years on the same house. That difference is why one roof can carry two ages at once — and why an evaluation grades slopes separately.
What treatment saves, and what it cannot
Caught early, moss is manageable: a fall treatment that the winter rains carry through the growth, gentle removal once it releases, and zinc or copper at the ridge to slow the return. What treatment cannot do is undo the damage under an established mat — edges that have been lifted and wet for years are done, whatever the field looks like. A covering that has carried thick moss through several winters usually reads fine from the street and gone at the eaves, which is exactly the case where a proper look pays for itself.