PSI

Eagle Roofing Products, a concrete tile manufacturer, spells the abbreviation out in its own cleaning advice, recommending only water “with the water pressure never exceeding 1200 pounds per square inch (psi).” That figure is Eagle's, for its concrete tile, and how to read it is taken up at a safe pressure for a concrete tile roof.

The Interlocking Concrete Pavement Institute adds the metric side, giving its paver cleaning range as “between 1,000 and 4,000 psi (6.9 and 27.6 MPa).” That is a concrete paver figure, never a roof figure, and it sits with the other paving materials in the paver cleaning page's cleaning limits by paver material.

The Brick Industry Association uses psi to mark a line of its own: for clay brickwork it defines high pressure as greater than 800 psi, which it also gives as 5600 kPa, and says high-pressure water cleaning is not appropriate for brick. That is a brick figure, not a roof one. Psi describes how hard the water pushes; how much water moves is a separate unit, GPM.

No trade association or roofing maker document read for this page publishes a psi threshold that defines soft washing. The documents describe the method by what it does, low pressure paired with a cleaning solution, and print pressure figures only for single materials.

A figure from one material cannot fill that gap. Eagle's 1200 psi is a concrete tile ceiling on water pressure, not a soft wash setting and not a target, and why no figure travels from one roof to another is set out at how pressure numbers stay with their material. The reasoning for naming soft washing by mechanism instead of a number is at how soft washing works.

Nor does a psi figure settle the vocabulary of the job; whether roof cleaning is the same as roof washing is a separate answer.