Why pressure is the wrong tool
Vinyl siding is lapped: each course hangs over the one below with an air gap behind it. Spray upward at pressure and water goes straight up into that gap, past the weather barrier if there is a gap in it, and into the sheathing. Nothing visible happens that day. The staining, the swelling and the smell arrive months later, long after the invoice cleared, and they are yours.
Meanwhile pressure does almost nothing useful. What is on the wall is biological — algae, mildew, pollen bound in a film — and it dies with chemistry, not force. Soft washing is not a gentler version of pressure washing; it is a different job that happens to use the same machine as a pump.
Surface damage risk
The mix
Vinyl and fibre-cement want roughly 0.5–1.5% sodium hypochlorite at the wall, plus a surfactant so the mix clings long enough to work instead of running to the ground. From 12.5% stock that is about one part to eight or twelve parts water, or a downstream injector drawing at around 1:10 with stock in the bucket.
Stronger is not faster. Above that band you start taking the sheen off painted trim, stressing shrubs no amount of pre-wetting will save, and giving the customer a smell that lingers on the porch for two days.
| Symptom | Cause |
|---|---|
| Green comes back in weeks | Mix too weak, or rinsed before it had dwelled |
| Trim looks dull afterwards | Mix too strong for painted surfaces |
| Streaks running down clean siding | Applied top-down onto dry wall |
| Mix dried on the wall | Hot day, no re-wetting; now it is etched in |
Bottom up, then top down
Apply from the bottom of the wall upward. Mix running down over already-wet siding does nothing; mix running down over dry siding leaves a permanent-looking streak that you will spend twenty minutes chasing. Rinse in the opposite order, top down, so the dirt has somewhere to go.
- Wet the planting and the ground below the elevation.
- Apply the mix bottom-up, one elevation at a time.
- Let it dwell — five to ten minutes is normal. Watch it, do not time it blindly.
- Re-wet if the day is hot enough that it is flashing off.
- Rinse top-down at low pressure with a 40-degree tip well back.
- Rinse the planting again.
Chalking, oxidation and stains that are not dirt
Run a dark cloth down the wall before you quote. If it comes back white, the paint or the vinyl surface is chalking, and washing will remove the chalk but leave a patchy, faded wall the customer will read as damage. Say so before you start, in writing.
Other things that will not wash off: artillery fungus (the tiny black dots that come from mulch and are mechanically embedded), rust bleed from a fixing, and oxidation on old vinyl that has lost its surface. All three are conversations to have at the quote, not at the invoice.
PSI & nozzle quick reference
- 0 degRed
- Spot jet. Etches most surfaces.4000 PSI
- 15 degYellow
- Stripping bare concrete.4000 PSI
- 25 degGreen
- General concrete and walkways.3500 PSI
- 40 degWhite
- Siding, wood, vehicles.1500 PSI
- ~65 degBlack
- Detergent draw, low pressure.500 PSI
Protecting everything else
A house wash touches more than the walls. Work the walk-round before the pump starts:
- Windows and storm windows shut. One ajar window is one wet carpet.
- Exterior outlets, doorbells, low-voltage lighting covered.
- Beds wetted, delicate planting tarped. Wet again halfway through.
- Furniture, grills and cushions moved well clear — not to the far end of the same patio.
- Cars moved off the drive. Hypochlorite overspray on a dark car is a detailing bill.
Gutters are the exception on a house wash