Soft washing vs pressure washing, and why it matters on your building

They are different jobs, not different settings

Pressure washing uses mechanical force to remove what is sitting on a surface. It is the right tool for horizontal concrete, which is hard, thick, and can take it. Soft washing applies a cleaning solution at roughly garden-hose pressure, gives it time to work, and rinses. It is the right tool for almost everything vertical. The distinction is not marketing: they remove different things by different means.

Why pressure fails on a wall

The dark streaking on a building is a living organism rooted into the surface. Pressure knocks the visible part off and leaves the roots, so it returns within a season — while the pressure itself has driven water behind siding and into mortar joints, etched painted block, chalked prefinished metal unevenly, and on EIFS or stucco taken material off. You get a wall that looks clean for a few weeks, comes back dirty, and is now more absorbent than it was.

Which surface takes which

Soft wash: painted concrete block, brick and mortar, prefinished metal panel, EIFS, stucco, vinyl and aluminium siding, wood, and every roof without exception. Pressure wash, with a surface cleaner: concrete flatwork, walkways, dock aprons, pads, and most unglazed hardscape. The general rule is that if it is vertical or it is a roof, it wants chemistry rather than force.

Roofs are the one with no exceptions

Nothing on a roof should be pressure washed. On shingle it removes the granule layer that is the shingle's UV protection, and it can void the manufacturer warranty. On a low-slope commercial membrane it can open a seam, which turns a cleaning bill into a leak and a much larger bill. Moss and algae on a roof are soft washed, and they die off and weather away over the following weeks rather than disappearing on the day. A roof that looks patchy a week after treatment is the treatment working.

How to tell whether a contractor knows this

Ask what pressure they will use on your elevations and why, and ask what they use on a roof. Someone who answers with the substrate — "your walls are painted block so that is a soft wash, the dock apron is a surface cleaner at full pressure" — is telling you they have thought about your building. Someone who answers with equipment specifications, or who says they will "turn it down", is describing one tool for every job. And if anyone offers to pressure wash your roof, that is the end of the conversation.