What PSI Should You Use for Different Surfaces When Pressure Washing in Boston?

Why PSI Is Only Part of the Story

PSI (pounds per square inch) tells you the pressure at the pump, not what actually hits the surface. Nozzle type, distance from the surface, flow rate (GPM), and dwell time all affect cleaning action. A 3,000 PSI machine with a 40-degree nozzle at 18 inches applies very different force than the same PSI with a 0-degree nozzle at 6 inches. That said, PSI ranges provide useful starting guidance for surface categories — and exceeding them is how contractors cause damage. PWNA pressure washing standards →

Siding: 500–1,200 PSI Maximum

Vinyl siding in Boston is most commonly damaged by contractors using 2,000+ PSI at close range — the pressure forces water behind panels, damages locking channels, and can crack vinyl that's brittle in cold weather. Wood siding on older Boston homes is similarly vulnerable: high pressure raises grain, damages paint adhesion, and can force water under clapboards creating moisture problems for years afterward. We use soft wash technique for all siding — 500–800 PSI maximum with biodegradable cleaning solutions that do the actual cleaning work. The detergent cleans; the low pressure rinses. This Old House siding cleaning guide →

Concrete and Pavers: 2,000–3,000 PSI

Driveways, walkways, and concrete patios handle significantly higher pressure than wood or siding. 2,500–3,000 PSI with a surface cleaner attachment — a rotating dual-nozzle tool that maintains consistent distance — produces uniform results without the striping that results from a single wand nozzle. Brick and mortar require more care: mortar is softer than brick face, and high pressure erodes joint mortar, particularly in older Boston buildings where mortar is already weathered. We use 1,500–2,000 PSI maximum on masonry surfaces and increase dwell time with appropriate cleaners rather than relying on pressure alone. ICRI concrete surface preparation standards →

Wood Decks: 1,000–1,500 PSI Maximum

Wood decks require enough pressure to remove mildew and gray oxidation from the wood surface without raising grain. 1,000–1,500 PSI with a fan nozzle (25–40 degree) at 12–18 inches from the surface working with the grain direction produces good results. Higher pressure damages the wood surface by tearing and raising soft grain between growth rings — the resulting fuzzy surface absorbs more stain unevenly and wears faster. After washing, decks require 48–72 hours of drying before any stain application. TWP deck preparation guide →

Roofing: Soft Wash Only — No Pressure Washing

Asphalt shingle roofs should never be pressure washed. The granules that provide UV and impact protection strip off under pressure, voiding manufacturer warranties and dramatically shortening roof life. The correct approach is soft washing: 200–400 PSI application of a sodium hypochlorite-based solution that kills algae and moss at the root. Results appear over days as dead biological growth washes away with rain. No exception to this rule exists for well-built roofing work. Asphalt Roofing Manufacturers Association cleaning guidelines →