The definition that matters
Soft washing is cleaning an exterior surface with a dilute chemical solution applied at roughly 100 to 500 PSI — garden-hose territory — and left to dwell, rather than with mechanical force. The chemistry kills the biological growth; the rinse carries the dead material away. Pressure plays almost no part.
That is the whole idea, and it is genuinely different from pressure washing rather than a gentler version of it. A pressure wash removes what is on a surface. A soft wash kills what is growing on it, which is why a soft-washed wall stays clean for years and a pressure-rinsed one is green again by the following spring.
What the kit looks like
Two setups dominate, and either works.
| Setup | How it works | Best for |
|---|---|---|
| Downstream injector | Chemical drawn into the stream after the pump, black soap tip fitted | Most residential work; cheapest to start |
| Dedicated soft-wash system | 12V pump, tank, hose reel; no high-pressure machine involved | Roofs, tall work, high volume |
| Pump-up sprayer | Manual, small batches | Spot work and touch-ups only |
The injector route matters because it is how most operators start: the machine you already own becomes a soft-wash rig for the price of a brass fitting and a soap tip.
Which surfaces need it
- Asphalt shingle roofs. Not optional — the manufacturers’ own association rules out pressure entirely.
- Vinyl and fibre-cement siding. Pressure drives water behind the courses and into the wall.
- Stucco and EIFS. EIFS in particular is foam behind a thin skin; pressure punches through it.
- Painted wood siding. Test for chalking first, then soft wash gently or not at all.
- Screens, awnings and soffits. Anything thin, fixed at the edges and easy to deform.
Concrete flatwork is the clear exception: it is dense, unglazed and horizontal, and a surface cleaner at full pressure is both faster and better. A post-treat with dilute hypochlorite afterwards is a soft-wash step bolted onto a pressure job, and it is why well-cleaned driveways stay clean.
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
The mistake that defines the category
The most common error in this trade is fitting a 40-degree white tip, leaving the machine at 3,000 PSI, and calling it soft washing. It is not. The tip spreads the energy but the pressure at the surface is still far above what a soft wash means, and every risk the method exists to avoid is still present.
Soft washing means the pressure is genuinely low — either because a soap tip has dropped it, or because a 12V system is doing the work and the high-pressure machine is switched off entirely. If you cannot comfortably put your hand in the stream at working distance, it is not a soft wash.
Surface damage risk