Why pressure washing is risky here
The granules on an asphalt shingle are the armour over the asphalt, and they are what takes the weather. A pressure jet aimed at a shingle roof strips them off, along with the surface the treatment was applied to.
The damage does not show immediately. It shows as faster fading, more granule loss and a roof that dries out ahead of its time.
What the treatment changed about cleaning
A treated surface sheds water and dries faster, so it holds less dust and less growth than it did before. In most cases there is less to clean, not more.
Where growth has established on a shaded slope, removing it gently is the right approach rather than blasting the surface.
What to use instead
A soft wash at low pressure, applied from the ground where possible, and a rinse that does not drive water under the shingle courses. Gentle removal of growth and debris is all the surface needs.
If you are not sure, ask us before anyone starts. Cleaning a roof is not a warranty matter, but it can become a repair if the surface is damaged while doing it.
Where the line is
General wear and surface damage caused by cleaning or by work on the roof sits outside the cover, because it is not a failure the treatment caused. That is worth knowing before a contractor offers to "blast the roof clean".
If a roof genuinely needs cleaning that a soft wash cannot handle, that is a conversation about the roof rather than about the treatment.
What cleaning a roof is actually for
Cleaning is cosmetic unless the surface is holding growth or debris that keeps it damp. Where that is the case, gentle removal solves it; where the roof is simply old, cleaning changes the look and nothing else.
That distinction is worth keeping in mind when a contractor offers to restore a roof by washing it. It is a different service from a treatment, and it can do harm if it is done hard.
If a contractor suggests it
Ask what pressure they intend to use and whether they will work from the roof or the ground. A soft wash from the ground is a different job from a jet at close range, and only one of them belongs on asphalt shingle.