Pressure washing

The Interlocking Concrete Pavement Institute describes professional paver cleaning with a high pressure sprayer applying cleaner and water between 1,000 and 4,000 psi (6.9 and 27.6 MPa), at a rate between 2 and 6 gallons per minute. ICPI says the pressure loosens dirt and pushes water from the surface without the need for scrub brushes.

That range is a concrete paver figure and never a roof figure. What the psi and the flow rate in ICPI's sentence each measure is set out at PSI and GPM.

Duro-Last, a single-ply membrane manufacturer, caps a power washer at 1000 psi on its own membrane: “If using a power washer, it is imperative that the maximum pressure is set to 1000 psi with a fan tip, and the nozzle is kept at least 12 inches from the roof to prevent damage to the membrane.” That figure belongs to Duro-Last's membrane and to no other roof.

The Brick Industry Association, writing about clay brickwork, defines where high pressure begins: “High pressure: Greater than 800 psi (5600 kPa). High-pressure water cleaning is not appropriate for brick and may damage the brickwork.” BIA adds that pressures are measured at the tip of the nozzle. That is a threshold for brick, not a roof figure, and not a cap in the way Duro-Last's number is.

Asphalt shingle and metal carry no pressure figure at all; their own bodies rule the pressure washer out. ARMA writes that high-pressure washing systems are likely to damage asphalt roofing and should not be used to remove algae or for any other purpose, and its flagship line ties a pressure washer on an asphalt shingle roof to granule loss and very likely premature failure of the roof system; the Metal Roofing Alliance says never to use a power washer on metal. The case document by document is set out at how soft washing works.

Concrete tile sits between the two. Eagle Roofing Products recommends only water, with the water pressure never exceeding 1200 psi, and a cleaner only in the most diluted solution possible when the roof calls for one; whether that makes pressure washing a tile roof a reasonable idea has its own answer.

SoftWash Systems draws the contrast from the other side: pressure washing “uses high-pressure water to blast away dirt and grime,” where soft washing uses a low-pressure spray and specialized cleaning solutions.

The vocabulary of the job is its own question: roof cleaning versus roof washing takes up whether the two words name one method or two. No maker or association document read for this page publishes a cleaning interval for either method; that question is taken up at how often a roof can be cleaned.

Florida's water rules add a related term that is not defined by pressure at all. The South Florida Water Management District defines low-volume pressure cleaning by equipment designed to reduce the inflow volume, and that definition is set out at low-volume pressure cleaning.