Pressure washing research

ARMA (Asphalt Roofing Manufacturers Association). The trade body for asphalt roofing manufacturers, and the closest thing to a definitive source on roof cleaning. Its guidance: never use a pressure washer on an asphalt shingle roof, because it causes granule loss and very likely premature failure. Its recommended method is a 50:50 mix of laundry-strength liquid chlorine bleach and water applied with a sprayer, dwelling 15 to 20 minutes, rinsed thoroughly with low pressure water, with landscaping protected and appropriate PPE worn. Individual manufacturers echo it - Malarkey Roofing's guidance repeats that pressure washing dislodges granules, the shingle's primary defense against damage and UV aging.

Polymeric Exteriors Association (formerly the Vinyl Siding Institute). The siding industry body. Its cleaning guidance is that a power washer can be used but with caution, that some manufacturers do not recommend power washing at all while others limit PSI and cleaners, that you must read the manufacturer's instructions for both siding and washer, that you should hold the washer straight at eye level and never aim upward because it forces water behind panels, and that you should be cautious around windows, doors, wiring and plumbing. It also lists cleaners to avoid entirely.

National Park Service, Preservation Brief 1. Written for historic masonry, but the physics apply to any brick or stone building. It warns that water at too high a pressure - "power washing" and "water blasting" - is very abrasive and can easily etch marble, other soft stones and some brick; that nozzle distance and gallons per minute matter alongside pressure; that stone grains in the runoff mean the pressure is too high; that mortar joints must be sound and the building watertight before water cleaning or water will seep inside; and that water cleaning should never be done in cold weather or when frost is likely.

Cost data. For pricing, national aggregators like Fixr publish per-square-foot and average figures by surface and material. Use them to sanity-check bids, not as a quote - local labor rates and site conditions move real prices considerably.

What is not well established publicly: standardized industry certification for exterior cleaning contractors comparable to what exists in other trades. That absence is itself worth knowing - it means your own vetting (written method, insurance verified with the insurer, before-photos) carries more weight here than a credential would.