What the streaks actually are
Roof algae feeds on the limestone filler in asphalt shingles. As the colony spreads it holds moisture against the shingle surface, which accelerates granule loss — and granules are the only thing protecting the asphalt underneath from UV.
It spreads by air, which is why it usually shows up on the shaded north-facing slope first, and why a house with heavy tree cover gets it worse than the identical house two doors down in full sun.
Left alone it doesn't stop. It just gets darker, and it takes years off a roof you paid a lot of money for.
Why nobody should pressure wash a roof
High pressure will absolutely remove the black. It will also remove your granules, and once those are gone the shingle is finished.
Beyond the damage, most shingle manufacturers specify low-pressure cleaning. Pressure washing a roof can void the warranty on it — which means a roof you might have gotten fifteen more years out of is now both aged and uncovered.
The correct method is a low-pressure application that kills the organism and lets rain carry the dead colony off over the following weeks. No walking the roof more than necessary, no pressure, no granule loss.
What to expect
- Landscaping pre-wet and protected before anything is applied
- Low-pressure application from ladder or ground where the pitch allows
- Immediate visible change on most roofs; full clearing continues over the next few rain cycles
- Before and after photos from the same position, sent to you
Roof washing vs. roof replacement
A roof wash is a fraction of the cost of a replacement, and on a roof that still has life left it buys back years. But it isn't magic. If your shingles are curling, cupping, or losing granules by the handful in the gutter, cleaning them won't fix that — and we'll tell you so rather than take the job.
If we look at your roof and think you need a roofer instead, that's what we'll tell you.
