Exterior cleaning uses two fundamentally different approaches, and most disputes in the trade come down to one being used where the other was needed. They are not grades of the same thing, and one is not the gentle setting of the other.
The actual difference
Pressure washing removes growth and dirt mechanically. Water at high pressure knocks material off a surface, and the cleaning happens in the moment the jet hits it. It is fast, it is immediately visible, and the force that removes the moss is the same force that can remove the surface underneath.
Soft washing removes growth chemically. A biocide is applied at very low pressure — closer to a garden sprayer than a jet — and left to work. It kills algae, moss and lichen at the root, and the dead growth weathers off over the following weeks. The surface keeps improving after the work is finished, which is disconcerting the first time you have it done and is the reason it lasts.
The practical distinction is what each one is doing to the material. Pressure acts on everything it touches equally: the growth, the coating, the mortar, the sealer, the timber fibres. A biocide acts only on the living organism and leaves the substrate alone.
Which surface takes which
Pressure is usually right for: block paving, plain concrete, most natural stone that is genuinely hard, and brick paving. These are dense, unsealed, and structurally indifferent to being hit with water.
Soft washing is required for: all roofs regardless of tile type, all render including K-Rend and pebbledash, painted masonry, uPVC and rooflines, timber decking and cladding, and anything glazed. Every one of these is either a coating over something else, or a material whose surface is the part that matters.
Two surfaces are worth calling out because they get this wrong most often. Tarmac looks robust and is not: it is stone held in bitumen, and pressure lifts the stone out and leaves permanent pitting. Imprinted concrete is a pattern under a sealer, and aggressive cleaning strips the sealer, which turns a cleaning job into a resealing job.
What damage looks like
The difficulty for a homeowner is that over-pressured work looks excellent on the day. The surface is visibly cleaner than everything around it, and the damage is a change in texture that nobody notices until later. Signs, in the order they usually appear:
- Stripes or a banded pattern across a surface, matching the width of the lance passes. Most visible on render and softer stone, and permanent.
- Exposed aggregate on concrete or tarmac — the surface looks rougher and more granular than the parts that were not cleaned.
- Furred or raised timber on decking, which feels rough underfoot and traps dirt faster than before.
- Eroded mortar joints on brickwork, sometimes to the point of needing repointing.
- Faster regrowth, the season after. This is the one people notice, and it is the tell: a roughened, more porous surface holds water better and gives moss more to grip. A roof pressure washed once tends to need doing more often forever.
On a roof there is a further problem that is not cosmetic. Water driven up under the tile laps goes into the roof space, and a lance held at the wrong angle does exactly that. That damage shows up as damp in the loft rather than as anything visible outside, so it is rarely connected back to the cleaning.
What to ask a quoting company
Three questions separate a considered quote from a machine and a van. None of them requires you to know anything technical.
- “What method are you using on this surface, and why that one?” A good answer names the surface and the reason. A bad answer is about how powerful the equipment is.
- “Does the price include a biocide treatment?” On a roof or render this is most of the difference between quotes that look far apart. Cleaning without treating is a cheaper job that does not last, which is a legitimate thing to buy as long as you know that is what you are buying.
- “What happens to my planting and where does the runoff go?” Anyone who does this properly has a ready answer, because it is ten minutes of work they do on every job.
For what this means on specific jobs, see roof cleaning, render and house washing, and driveways and patios — the one page on this site where high pressure is often the correct answer.