The black streaks running down an Ohio Valley roof are not dirt. They are an airborne algae that colonizes the limestone filler in modern asphalt shingles and spreads down the slope with rain. Low-pressure roof washing treats the organism at the root.
Why pressure washing is the wrong tool
High pressure can strip the protective granules off the shingles and may void some manufacturer warranties. It also does not kill the algae — it just removes the visible streak, so it returns. Low-pressure application of a roof-safe solution kills the organism and rinses away with garden-hose pressure.
Moss and lichen are worse
Moss and lichen grow into the shingle, lift the edges, and trap moisture that works the seal strip loose. In freeze-thaw, lifted edges can crack. Heavy moss may need a follow-up as it dies and releases over a few weeks.
When streaks signal something else
If you also see lifted, curling, or missing shingles, or interior water stains, that is damage — a roofer's repair, not a wash. A clean will not fix structural or seal problems.
Five-minute self-check
- From the ground with binoculars, look for streaks from the ridges down.
- Check north and shaded slopes first.
- Look for green moss clumps or gray lichen patches.
- Note any lifted or curling shingle edges.
- Check for interior water stains that mean a leak.
Related reading
See our roof washing page and our post on moss and manufacturer instructions.