Why Black Roof Streaks Keep Coming Back
Short answer. Because they are a living organism, not dirt. Removing the visible layer without killing the algae leaves it to regrow, which it does within a year.
They are alive
The black streaking running down an asphalt shingle roof is almost always a cyanobacteria called Gloeocapsa magma. It is not soot, not dirt and not staining. It is a colony living on your roof, feeding partly on the limestone filler used in modern shingles.
That single fact explains everything else. You cannot wash off something that is growing, any more than you can weed a lawn by cutting it.
Why it came back
If someone cleaned your roof and the streaks returned within a year, one of two things happened.
Either they removed the visible biomass without applying anything that kills the organism, so the surviving colony simply regrew. Or they pressure washed it, which removes the streaks and a quantity of your shingles' protective granules at the same time, leaving a rougher, more porous surface that holds moisture better and is therefore easier for algae to recolonise.
The second outcome is worse than doing nothing. The roof looks better for a season and has a materially shorter remaining life.
Why it always starts under the chimney or the vents
Look at where the streaks begin. Usually below flashing, a chimney, or roof vents. Those are commonly galvanised or copper, and rainwater running over them carries a small amount of metal ions down the roof. Those ions are mildly toxic to the algae, so the strip directly below stays clean while the areas either side stain.
That is also the principle behind zinc strips: they work, in a narrow band, and only downhill of themselves.
Which slopes get it and why
North facing slopes in the northern hemisphere, and shaded slopes generally. Those surfaces stay damp longest after rain and dew, and moisture is the limiting factor for growth. A roof with a clean south slope and a filthy north slope is not unevenly dirty, it is unevenly damp.
What actually works
Soft washing. A cleaning solution applied at low pressure, given time to dwell so it kills the organism, then rinsed gently. Done properly the roof stays clear for years rather than months, and no granules are lost.
Reduce the shade and damp. Trim back branches so the roof dries faster after rain. This does more for recurrence than any treatment.
Clear the gutters. Blocked gutters keep the lower courses of shingle wet.
Zinc or copper strips. Genuinely effective but only for the area below them, so they are a supplement rather than a solution.
Algae resistant shingles. Worth specifying if you are replacing the roof anyway. Not a reason to replace one early.
What to ask before letting anyone on your roof
Ask whether they pressure wash or soft wash roofs. If they say pressure, stop there.
Ask what they apply and how long they let it dwell. A real soft wash answer involves a solution and a waiting period. "We just rinse it" means the streaks will be back.
Ask what they do to protect planting below, because runoff will reach it.
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