Black streaks on my roof: what are they and what should I do?
Those black streaks aren't water damage. They're a hardy cyanobacteria called Gloeocapsa magma. Soft wash removes them in a single visit.
Not water damage, not asphalt failure
The dark black streaks running down Portland-area shingles, almost always pronounced on north-facing slopes, are colonies of Gloeocapsa magma. This is a cyanobacteria (often misclassified as algae) that feeds on the calcium carbonate filler in standard asphalt shingles. The streaks are not water damage and they're not a sign of shingle failure. They're a biological growth pattern that's primarily cosmetic.
Why they appear on north slopes
Cyanobacteria need moisture to germinate and grow. North-facing slopes get less direct sun, stay damp longer between rain events, and provide the conditions cyanobacteria thrive in. The same shingle on the south slope of the same house typically shows no streaks at all. This is a microclimate phenomenon, not a material defect.
Why entire blocks show streaks at the same time
Cyanobacteria spores are airborne and travel block-to-block. When one home becomes a host, the spore reservoir grows and neighboring homes get colonized within 2-3 years. This is why Portland streets often show streaks on all the north-facing slopes simultaneously. It's not coincidence; it's spore biology.
Soft wash removes them in a single visit
Sodium hypochlorite soft wash is the industry-standard treatment. The chemistry kills cyanobacteria at the cellular level, the streaks fade visibly during the application as the bacteria die. Residual chemistry suppresses regrowth for 2-4 years. Cost: $380-$1,100 for a Portland-area home depending on size and severity. Single visit, no return needed.
What to ignore
Forget the products marketed as 'spray on and rinse off' streak removers. They're chemical wash mixed with strong fragrance, and they work less well than a proper soft wash. The active chemistry is the same (or weaker); the application technique is what matters and that requires actual equipment plus access to the roof.
Does this affect home value?
Modestly. Real estate agents and home inspectors flag heavily-streaked roofs, and buyers do discount accordingly, typically $2,000-$5,000 in negotiation room for a roof with severe streaking. The $400-$800 cost of cleaning before selling pencils out in almost every scenario. If you're not selling, the streaks are aesthetic only and don't require urgent action.