Guide
What Causes Those Black Streaks on a Roof?
It's not dirt, soot, or mold in most cases. It's algae, and it's eating your shingles while it sits there.
What it actually is
Algae, not dirt
Those dark streaks are a type of algae that feeds on the limestone filler in asphalt shingles. It travels by airborne spores, which is why it usually shows up first on the north-facing slope of a roof, the side that stays shaded and damp the longest. Humid coastal air, like Sussex County gets most of the year, is exactly the environment it spreads in.
Left alone, it doesn't just look bad. It holds moisture against the shingle surface, which shortens the shingle's life.
Getting rid of it
Why it has to be soft washed
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No pressure washer
High pressure strips the protective granules right off the shingle along with the algae.
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A cleaning solution
Applied at low pressure, it kills the algae at the root instead of just knocking the surface layer loose, so the roof stays cleaner longer than a pressure-only wash would leave it.
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It comes back
Cleaning doesn't stop new spores from landing. Expect streaking to return over time, same as anywhere humid.
See roof cleaning for how we do it, or soft washing vs. pressure washing for the full explanation of why pressure isn't used on a roof at all.