This is the one I feel strongest about. If you take one thing from anything I write, take this: a roof should never see high pressure.
What high pressure does to shingles
Asphalt shingles are protected by a layer of mineral granules bonded to the surface. Those granules are the shingle’s sunscreen — they take the UV so the asphalt underneath does not. They are also not glued on all that hard.
Put a pressure washer on them and they come off. You will see them in the gutters afterwards, and in the driveway, looking like coarse black sand. The roof will look wonderful for a season. Then the exposed asphalt starts to dry out and crack, and you have taken years off a roof to remove something cosmetic.
It also lifts shingle edges and drives water up underneath them, which is the opposite of what a roof is for.
Those black streaks are not dirt
The dark streaking running down a roof is algae. It feeds on the limestone filler in the shingle, which is why it spreads downward with the water flow. Scrubbing or blasting it treats the symptom.
A soft wash treats the cause. Low pressure — roughly what a garden hose puts out — and a cleaning solution that kills the growth outright. It rinses away and it does not come straight back, because there is nothing left alive to regrow.
The honest caveat
Not every roof is worth washing. If shingles are already curling, brittle or losing granules on their own, I will tell you that cleaning it is not the answer and you are looking at a roofer instead. I would rather say so than take the money.
If you are not sure which you have got, send me a photo. (770) 789-1606.