Drive through Wallingford, Ravenna, or West Seattle and look up. A large share of the roofs carry dark streaks running vertically down the slope, usually heaviest on the north side. Most homeowners assume it is dirt, mildew, or simply age. It is none of those.
They are a living organism — a cyanobacterium, most commonly Gloeocapsa magma. Cyanobacteria are photosynthetic bacteria, and this one forms a dark protective sheath that gives the streaking its colour. It arrives as airborne spores, which is why it spreads across whole neighbourhoods and why a clean roof next to a streaked one does not stay clean indefinitely.
It grows where moisture persists, which in Seattle means most roof surfaces for most of the year, and it is heaviest on north-facing slopes for the obvious reason that they receive the least drying sun.
The pattern is a clue to the mechanism. Colonies establish higher up the slope, and rainwater running down carries cells and nutrients downhill, seeding the path below. The streaking therefore radiates downward from the point of establishment.
You will often notice clean vertical stripes beneath any galvanised or copper component — a vent flashing, a chimney cap, a satellite mount. That is not a coincidence, and it is the entire basis of the solution described below. Trace metal ions washing off that component suppress growth on the strip of roof directly beneath it.
Asphalt shingles are manufactured with limestone filler, and the calcium carbonate in that filler is a nutrient source this organism can use. Cheaper shingles with higher filler content tend to streak more readily. This is worth knowing because it means the streaking is not merely sitting on your roof, it is consuming part of it.
Less than moss, and it is important not to overstate it. Unlike moss, algae forms a thin biofilm rather than a mat, so it does not hold standing water against the surface and it does not anchor with rhizoids that lift shingle edges. It will not cause a leak.
What it does do is darken the surface, which raises shingle temperature under sun and modestly accelerates the loss of volatile compounds from the asphalt. It also degrades the granule bond over long periods, and it is unattractive enough to affect how a house shows when it is listed. So: a genuine but secondary problem, and primarily an aesthetic and marginal-longevity issue rather than a structural one.
The reason to care is that the conditions producing algae are the same conditions producing moss, and moss is genuinely destructive. A heavily streaked roof is telling you the surface stays damp and shaded, which is the same message.
Algae-resistant shingles. The real solution, available at re-roof time. AR shingles incorporate copper-infused granules distributed through the surface. Rainfall releases trace copper ions continuously, suppressing colonisation for many years. The cost premium is small — a modest percentage of material cost, which is itself a minority of project cost. In a market where nearly every roof eventually streaks, this is one of the highest-return specification decisions available, and it is frequently not mentioned during a sales conversation unless you raise it.
Zinc or copper strips at the ridge. The retrofit version of the same chemistry, and effective. Installed near the ridge, they protect the slope below via runoff. Protection weakens further down a long slope, so additional strips partway down help on tall roofs. Copper outperforms zinc and costs more.
Cleaning, carefully. Existing streaking can be treated with appropriate products applied at low pressure and allowed to work, followed by weathering. What must not happen is pressure washing, which strips the granules that are the shingle's only UV protection and does more damage in an afternoon than the algae does in a decade.
Reducing shade and debris. Trimming overhanging limbs increases light and airflow and shortens drying time after rain, which addresses the underlying condition rather than the symptom.
If your roof is streaked and has years of life left, treat it gently and add metal strips — do not let anyone pressure wash it. If your roof is approaching replacement, the streaking is largely cosmetic at this point, and the decision that matters is specifying AR shingles on the new roof so the problem does not recur for the next two decades.
This post is part of our in-depth coverage of roof replacement in Seattle.
Roofing Materials for the Puget Sound Climate → Roof Replacement →
Less than moss. Algae forms a thin biofilm rather than a mat, so it does not hold water against the surface or lift shingle edges, and it will not cause a leak. It darkens the surface, modestly accelerating ageing, and it signals that conditions favour moss, which is genuinely destructive.
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