While a pressure wash relies on the high-pressure water alone to clean properly, soft washing systems use a combination of soaps, bleach, and water to ensure your exterior stays clean for the long-term without damage from the high water pressure.
Pressure washing alone, without using some sort of bleach/soap mixture is and can be very harmful to siding and other surfaces. Many people think that a pressure washer along can remove all the dirt and mildew. The truth is, while you may remove some dirt, you will also etch and ruin your surface with to much pressure. The other fact is when using only water to remove mildew, the mildew/algae will come back very quickly.
Soft washing however uses low pressure and a cleaning solution to gently wash siding. This cleaning solution removes dirt, mold, and mildew from the surface. A Soft wash solution contains mostly water and high chlorination. The sodium hypochlorite found in the mix is what removes mold, mildew, algae, and other organic growths from the siding. Cleaning solutions often contain soap additives as well, such as surfactants and foaming agents to help the solution “cling” to the surface and work more efficiently.
There are problems with both ways of cleaning if you ask me and I’ve seen them both in action. Pressure washing alone is very harmful to whatever surface your cleaning without putting a soap on the surface to loosen the dirt first. And while soft washing uses low pressure the bleach content is way to high and can result in to many surfaces/plants getting destroyed. So what is the middle ground? The middle ground is lower water pressure with a good soap and a lesser percentage of bleach. With these things in play it will be less likely to ruin any surface while at the same time effectively cleaning and removing algae and mildew from you house.