Wood-Safe Low-Pressure Wash Standard

Purpose

A deck or a fence sits horizontal to the weather and takes foot traffic, and it shows its wear as gray, weathered fibers and mildew driven into open grain, not as a film sitting on a painted plane. Wash it the way a wall gets washed and two things go wrong in opposite directions: too much pressure or too tight a tip fuzzes the surface fibers, a defect that does not rinse away; too weak a chemical pass leaves the mildew's root in the grain, and the gray return shows up in weeks instead of a season or more. Drift onto a fresh stain finish two sections over, or onto the flower bed lining the fence, turns a clean job into a claim on top of the callback. This standard fixes the chemical ratio, the pressure ceiling, and the direction of travel relative to the grain, so the result does not depend on which tech is holding the wand.

Scope

Covers the standard low-pressure wash of exterior wood decking, fencing, and railings on residential and light-commercial properties, using a belt-drive pump in the 4 to 8 GPM class, a sodium hypochlorite (SH) wood-wash mix applied through a skid or injector, and a wood-safe mechanical rinse. This SOP removes dirt, mildew, algae, and surface biological staining. It does not restore a UV-grayed board to its natural color; that is the Wood Brightening and Neutralizing Procedure, typically run as the next visit or phase. Species ratio and dwell leans are owned by the Wood Species and Treatment Compatibility Reference, and pressure and tip numbers by the Pressure Setting Reference for Wood Surfaces; this SOP calls both rather than re-deriving them.

Out of scope: overhead structures (Pergola and Outdoor Structure Wash Standard), siding (Soft Wash House Procedure Standard), and any wood showing active rot or structural movement, which routes to the step 2 stop rule.

Roles and the handoff between them

Before the wand comes off the truck

Sodium hypochlorite burns skin and eyes and is never staged or mixed with an acid or ammonia-based product, including the oxalic-acid brightener this crew may carry for a follow-on job. Separate carts and tanks, hose flushed between them, gloves and eye protection on while handling either concentrate. And if that does not hold, a sharp chlorine or ammonia-like odor at the mix station is a gas release: shut the pump down, move everyone upwind, do not go back to close a valve, and call it in before anyone re-enters.

Confirm the discharge point before the first drop hits the ground. Wash water carrying SH or surfactant reaching a storm drain is an illicit discharge under the federal storm sewer framework (40 CFR 122.26) in most municipalities. The accepted containment method is set locally; confirm it before the first job in a new service area, and see the Wash Water Containment and Storm Drain Protection standard for the mechanics.

The pressure ceiling below exists because wood fails differently than siding. A fan tip held too close, or a straight tip used at all, lifts individual fibers off the board face, and the damage shows only once the wood dries, after the crew is gone. There is no rinse-it-again fix for raised grain; the section gets sanded or replaced. Treat the ceiling below as a hard limit, not a starting point to push from on a stubborn spot.

The procedure

1. Confirm site protection is complete before any chemical is mixed. Get sign-off that the Plant and Landscaping Protection standard has run: adjoining beds soaked or covered, and anything isolated for the job (irrigation zone, low-voltage deck lighting) shut off and tagged. Acceptance: a checked handoff on the ticket, not a verbal "we're good." Wrong looks like a tech starting the mix while the second tech still covers the last bed. Stop rule: no chemical into a tank until the handoff is checked. Hazard: none introduced here, but skipping this turns every hazard below into a plant-kill event instead of a controlled one.

2. Walk the full run and log species and condition before deciding the ratio, including any section that changes species partway through. Identify pressure-treated pine, cedar, redwood, tropical hardwood, or a mixed species per the Wood Species and Treatment Compatibility Reference, and flag any loose board, popped fastener, active rot, or a coating film thick enough to flake. Acceptance: a species and condition note that covers the whole run, referencing the compatibility reference's category for each section. Wrong looks like guessing species at the start and assuming it holds to the end, which is exactly how a fence with cedar picket inserts in a pine frame gets washed at the wrong ceiling on the pickets. Stop rule: active rot or structural movement (a board that flexes underfoot beyond normal, a joist visibly separated) stops the job here; get owner or GC sign-off before any water load goes onto compromised wood, a fall-through risk the wash itself can cause. Hazard: splinters and popped fasteners underfoot while walking the deck; scan before you step, not after your weight is on a suspect board.

3. Mix the wash chemistry to the standard starting ratio for the species identified, and log it. Using a 12.5 percent sodium hypochlorite source, the standard start is 1 part chemical to 20 parts water plus labeled surfactant, about 0.6 percent available chlorine at the surface; the compatibility reference leans this further for bare, unsealed, or tropical hardwood sections. Acceptance: the ratio logged against the source strength and the species category chosen. Wrong looks like "the usual mix" applied without checking whether today's species calls for the leaner ratio. Stop rule: an unlabeled source concentration does not get guessed; get the label first. Hazard: concentrate splash while pouring; gloves and eye protection on before the cap comes off.

4. Apply the mix through the low-pressure line and confirm the path before it touches the wood. Deliver through the skid's dedicated low-pressure line or a wide-pattern nozzle, under 100 psi at the surface, moving with the grain. Acceptance: pressure confirmed at the tip, not assumed from the machine's rated output. Wrong looks like a tip swapped mid-route for a faster pass that quietly exceeds the ceiling. Stop rule: if the low-pressure path is not confirmed, do not start; fix the setup first. Hazard: chemical under pressure at the nozzle; keep the tip clear of your own face and any bystander's line of sight.

5. Hold dwell inside the wood window and do not let it dry. Give the mix 5 to 10 minutes, leaning short on bare, weathered, or tropical hardwood sections per the compatibility reference, re-wetting any section that starts drying before the window closes. Acceptance: the section still visibly wet when dwell closes. Wrong looks like a chalky white haze where the mix fully dried. Stop rule: any section drying before rinse reaches it gets re-wet on the spot, not on the scheduled pass. Hazard: continued skin and eye exposure through the re-wetting passes; PPE stays on.

6. Rinse mechanically at the pressure ceiling that matches the species in that section, moving continuously with the grain. Per the Pressure Setting Reference for Wood Surfaces, sound pressure-treated pine and sound tropical hardwood take up to 500 psi at the tip with a 25 to 40 degree fan and an 8-inch minimum standoff; cedar, redwood, and any thin or delicate member drop to 300 to 400 psi with a wider fan and a 10 to 12 inch minimum standoff. Sweep continuously along the boards; never hold the tip stationary or cross the grain. Acceptance: residue flushed clear with no fuzzing or lifted fiber anywhere the tip passed. Wrong looks like a fuzzy texture standing up off the board face. Stop rule: the moment fuzzing appears, stop, confirm you are running that section's correct ceiling and not the pine setting carried onto a cedar section, increase standoff, and widen the tip before continuing; working the same spot to finish it clean is what turns a small patch into a section that needs sanding. Hazard: wand kickback and chemical-laden runoff underfoot on a wet, sloped surface; footing squared, both hands on the wand.

7. Hand off to the Rinse Sequence and Final Inspection standard, then restore anything isolated. Confirm that standard has run and signed off, then power any irrigation zone or deck lighting back on if shut off in step 1, and verify it, not just switch it. Acceptance: a rinse worksheet attached and every isolated system confirmed live. Wrong looks like a crew leaving before rinse sign-off exists, or lighting flipped on but never checked against its timer. Stop rule: the job is not complete without both. Hazard: none from this step; the hazard it closes is a customer finding a dead irrigation zone days later with no tech there to trace it to.

The record this produces

A mix-and-application worksheet: species and condition by section, mix log (source strength, ratio, any species lean), pressure check (confirmed tip pressure by section, both passes), coverage notes (any re-wet, any fuzzing stop rule taken), isolation log, and both handoff sign-offs. This is what a callback gets checked against: an unlogged ratio cannot be defended against a damage claim, and a fuzzing stop rule that was taken and logged is the difference between a documented in-process catch and a customer finding the same defect after the crew left.

One run of this procedure, filled in

A single-level backyard deck, pressure-treated pine decking, with a fence along one side built from pine rails and posts but cedar picket inserts, moderate gray mildew, no rot or structural flags on the walkthrough.

• Step 1: plant-protection handoff signed off, bed along the fence line covered, deck lighting confirmed off and tagged.
• Step 2: decking identified as pressure-treated pine. Fence identified as mixed: pine rails and posts (standard category) with cedar pickets (leaner, lower-pressure category), noted by section.
• Step 3: 12.5 percent SH source. Decking and pine members mixed at the standard 1 to 20 ratio, about 0.6 percent. A leaner batch mixed for the cedar pickets per the compatibility reference.
• Step 4: skid's low-pressure line confirmed under 100 psi; decking and pine members sprayed with the standard mix, pickets with the leaner batch.
• Step 5: decking dwelled 8 minutes with two re-wets on a sunnier section; cedar pickets dwelled 6 minutes, toward the short end.
• Step 6: this step failed on the cedar pickets. The tech rinsed the whole fence run at the 500 psi pine setting used on the rails and posts, never dropping to the cedar ceiling, and a fuzzed patch appeared across three pickets near a knot. Stop rule taken: work stopped, pressure dropped to the cedar ceiling and standoff increased, the rest finished at the corrected setting with no further fuzzing. The three pickets were flagged for a sanding pass rather than re-washed, since re-wetting fuzzed fibers does not lay them back down.
• Step 7: rinse and final inspection standard signed off. Irrigation zone confirmed cycling; deck lighting confirmed live at dusk before the crew left.

The fuzzed pickets are why step 6's stop rule is a live, mid-pass check rather than an end-of-job read: caught after three pickets, it cost one flagged section; caught at the end of the run, it would have cost most of the picket line.

References

• 40 CFR 122.26, the federal storm sewer discharge framework, which most local stormwater ordinances enforce for wash-water containment; confirm the accepted method with your municipal authority.
• See related: Wood Species and Treatment Compatibility Reference, Pressure Setting Reference for Wood Surfaces, Wood Brightening and Neutralizing Procedure, Plant and Landscaping Protection Before a Wash, Rinse Sequence and Final Inspection Standard, Chlorine Gas Risk From Incompatible Chemical Mixing.