Algae and black streaking
Drive any street in the Piedmont and the dark vertical streaks are there — on some roofs faintly, on others like poured ink. They are algae, feeding on the limestone filler in the shingles, and they are the Southeast’s signature roof condition: mostly cosmetic at first, never entirely innocent, and a precise map of which slopes stay damp.
What the streaks actually are
The organism is an airborne algae that colonizes wherever moisture lingers — which in this climate means north-facing slopes, tree-shaded sections, and any face the morning sun does not dry. It darkens the surface, and a darker surface runs hotter and holds damp longer, which invites more growth. The streaking itself removes little from the shingle at first; what it tells you is that the slope beneath it spends much of the year wet, and wet is how Southern roofs age.
The map matters more than the stain. One streaked slope on an otherwise clean roof is a shade story. Streaking across every face, thick at the edges, on a covering past its mid-teens is an age story — and where the growth has progressed to moss or lichen at the shaded eaves, the granule surface underneath has usually gone with it.
What to do about it, honestly
A streaked but sound roof can be soft-washed — never pressure-washed, which strips granules and takes years off the covering — and algae-resistant shingles with copper-bearing granules exist for the next replacement. But the useful move is diagnostic: have the streaked slopes looked at up close, because streaking hides the surface it covers. A covering that reads “dirty” from the street can be perfectly serviceable underneath, or brittle and granule-bare — and the ladder is the only way to know which.