These two get used interchangeably and they should not be. They solve different problems, and picking the wrong one is the most common way a cleaning job turns into a repair bill.
Pressure washing
High pressure, lower volume, mostly water doing the work. It physically blasts material off a hard surface. Right for concrete, brick, pavers, heavy equipment and anything where the dirt is stuck on rather than alive.
Soft washing
Low pressure, high volume, chemical doing the work. Usually around 300 PSI at 8 to 12 GPM. You apply a cleaning solution, let it dwell, then rinse. Right for roofs, siding, render, screens, fences and anything that growth is living on.
The key difference: soft washing kills the organism. Pressure washing only removes what you can see, so algae and lichen come back faster because the roots were never touched.
How to choose, by surface
- Roof. Soft wash, always. High pressure strips the granules off asphalt shingles and voids most roof warranties.
- Siding, render, painted timber. Soft wash. Pressure drives water behind the cladding.
- Driveway, patio, footpath. Pressure, through a surface cleaner for an even finish.
- Fencing and decking. Usually soft wash, or low pressure at most. Timber marks easily.
- Commercial concrete with oil. Pressure and heat.
What a soft wash setup needs
A soft wash rig is a different machine, not a setting on your pressure washer. You need a diaphragm pump built for volume at low pressure, a way to mix chemical accurately, and hose and a gun rated for the solution.
- A pump. Either 12 volt for lighter work, like the 12V soft wash pumps, or a gas powered skid such as the AR45 soft wash skid for production work.
- A proportioner or mixing valve, so you can change your mix on the fly instead of batch mixing in a tank.
- Soft wash hose, which is a different hose from pressure hose.
- Tanks for water and solution. See tanks.
Most crews end up running both
A house wash and a driveway on the same property need both methods, which is why most rigs carry a pressure washer and a soft wash system side by side. That is exactly what the rig builder is for: pick the platform, add both, and see the price as you go.