Roof cleaning gets recommended for two very different reasons: appearance and moss removal. Only one of those changes how the roof performs.
Whichever reason applies, the method matters more than the result you see on day one.
Why pressure washing is the wrong tool
The granules on an asphalt shingle are held in the asphalt surface, not bonded permanently. High pressure removes them along with the moss.
A roof that looks dramatically cleaner and has a pile of granules in the gutter has been damaged, not maintained.
What gentle cleaning looks like
Low-pressure application, appropriate dwell time and a rinse that works with gravity rather than against the shingle courses.
Growth often continues to die back and weather off over the following weeks rather than disappearing immediately.
Cleaning is not rejuvenation
Cleaning addresses what is growing on the surface. Rejuvenation addresses the condition of the shingle itself.
They are sometimes sequenced together — surface growth removed first, treatment assessed afterward — but they solve different problems.
Why this is different here
Mill-town neighbourhoods in Biddeford, Westbrook and Lewiston have a lot of roofs in the 15-to-25-year range on tightly spaced lots. Those are exactly the roofs where an aggressive wash does the most avoidable harm, because the granule layer is already thinner than it was.