Pressure washing guides

Chemistry does the cleaning; water removes it. On anything organic — algae, mildew, moss — the solution and its dwell time do the work. Today's Homeowner's mixing guidance uses a quarter gallon of chlorine to a gallon of water at roughly 1:9 with a 25 to 30 minute dwell, or a quarter gallon of bleach to a gallon of water with a 15 to 30 minute dwell. ARMA's roof method is a 50:50 bleach and water mix with a 15 to 20 minute dwell, rinsed with low pressure water. If a crew is not letting anything dwell, they are relying on pressure instead.

Pressure by surface. Fixr's fence figures give a useful ladder: softwoods like pine and cedar 500 to 600 PSI, hardwoods 1,200 to 1,500, composite under 1,500, aluminum under 1,800, steel 1,800 to 2,000, wrought iron under 2,300. Its house data notes the range runs from around 500 PSI for brick to 3,500 for concrete, and its deck guidance sets wood and composite at 1,500 PSI. Today's Homeowner treats anything above 2,000 PSI as excessive for wood.

Technique. Distance and angle matter as much as the dial. Today's Homeowner recommends a wide-angle nozzle about two feet from cedar siding at a 45-degree angle. Polymeric Exteriors guidance says to hold the washer straight at eye level on vinyl siding and not aim upward, because that forces water behind the panels. NPS guidance on masonry notes that nozzle distance and flow rate significantly affect the outcome, and gives the field test: stone grains or sand in the runoff means the pressure is too high.

Preparation. ARMA advises protecting landscaping from the bleach solution. NPS advises confirming mortar joints are sound and the building is watertight before any water cleaning, because otherwise water seeps through the wall and damages interior finishes. Both of those checks happen before anyone turns a machine on.

Steam and hot water. NPS describes steam cleaning as effectively low-pressure hot water washing — its example shows under 100 psi — and calls it a gentle, effective method for stone, particularly acid-sensitive stone. Heat is a tool for grease and stubborn deposits, not a shortcut for using less care.

How to choose

Judge a crew by whether they let solution dwell

Apply, wait, rinse. If they're spraying and immediately blasting, they're cleaning with pressure and that's what damages surfaces.

Ask for the PSI they'll use, surface by surface

There should be a different number for concrete, brick, siding, wood and composite. One number for the whole job is a warning sign.

Watch the nozzle angle and distance

Wide-angle tip, appropriate distance, never angled up under siding or shingles. Close-in narrow tips are where the damage happens.

Insist on plant and opening protection first

Landscaping covered or pre-wet, windows and doors accounted for, electrical fixtures avoided. This should happen before mixing, not after.

Start low and step up

Test in a hidden area at the low end of the range. You can always add pressure; you cannot take etching back out of brick.