When to use soft washing instead of pressure washing
Pressure washing and soft washing are not two settings on the same machine. They are two different jobs. Pressure cleans by force. Soft washing cleans by chemistry and dwell time, at roughly garden-hose pressure. Choosing the wrong one is how people permanently damage render, paint, stone and roof tiles, and the damage does not weather out.
The rule in one line
If the surface is hard, dense and replaceable, pressure is fine. If the surface is soft, porous, painted or coated, soft wash it.
Surfaces that take pressure
- Concrete block paving, at a controlled distance with a rotary head
- Poured concrete drives and hardstandings
- Dense natural stone paving such as Pennant sandstone flags
- Commercial forecourts and car parks
Even here, the tool matters. A rotary surface cleaner at a set height cleans evenly. A wand held close cuts stripes into the surface, which is what most damaged driveways in Bristol actually are.
Surfaces that must be soft washed
- Bath stone and other soft limestone. Pressure opens the face of the stone. Once open it soils faster and weathers faster than before you cleaned it.
- Painted render and painted masonry. The paint film lifts and you have bought a repaint of the whole elevation.
- K-Rend and other silicone renders. They mark permanently under pressure, and the marks look worse than the algae did.
- Pebbledash. The aggregate comes off and does not go back.
- Roof tiles of every kind. Pressure drives water under the laps and strips the tile surface.
- Lime render and lime mortar. Softer than the stone around it, and the first thing to go.
How soft washing actually works
A biocide is applied at low pressure and left to dwell. It kills the algae, lichen and moss at the root rather than knocking the visible growth off. The dead growth then weathers away over the following weeks, and because the organism is dead rather than displaced, regrowth is slower. On a roof that is the difference between a three-year and a six-year interval.
Why the result looks slower
Because it is. A pressure wash on render looks finished when the operator packs up. A soft wash looks partly done on the day and finished a month later. That gap is where most bad decisions get made, and it is why we say up front what the timeline is rather than letting you wonder.
What about hot washing?
Hot water is a third option, and it earns its place on grease, chewing gum, forecourt oil and heavy commercial soiling where cold pressure would need excessive force. It is not a substitute for soft washing on a delicate surface. Heat does not make pressure safe on limestone.
Frequently asked questions
Can you soft wash in winter?
Yes. The biocide works at low temperatures, though it works more slowly, so the visible result takes longer to arrive. What stops us is wind, because treatment drifts, and hard frost.
Will a soft wash actually remove heavy black staining?
Yes, on organic staining, which is what almost all black staining on render and roofs is. Non-organic marking such as iron staining or carbon deposit on a city frontage is a different problem and a different treatment.
Is soft washing safe around planting?
With preparation. We sheet the working line and pre-wet the beds so anything that carries is diluted on contact. On specimen planting we would rather work around it and say so.
My neighbour's cleaner used a pressure washer on their render and it looks fine.
It often does, for a season. What you are looking for is the tide line where the paint film has started to lift at the edges, and how quickly the elevation goes green again compared with a soft-washed one.
Related
Soft washing · Hot washing · Render & wall cleaning · DOFF steam cleaning for stone