Low pressure cleaning, usually called soft washing, applies a treatment that kills algae, moss and mould at the root and rinses it away gently, which suits render, roofs, fascias and other delicate surfaces. Pressure washing uses force instead and suits hard surfaces such as block paving, concrete and patios.
Soft washing and pressure washing solve different problems, and using the wrong one on the wrong surface is how driveways get etched, render gets cracked, and roof tiles get chipped. The short answer: soft washing is a low-pressure, chemical-led clean best for delicate or biologically-grown-on surfaces like render, roofs and soft brick, while pressure washing is a high-pressure water-only clean best for hard, resilient surfaces like concrete, block paving and tarmac.
The trouble is most people, and quite a few contractors, use whichever equipment they own on every job. Here is a straightforward breakdown by surface so you know what to ask for before booking anyone.
The core difference in one sentence
Pressure washing relies on mechanical force, water at high PSI physically blasting dirt off a hard surface, while soft washing relies on chemistry, a biocide-based solution that kills algae, moss and lichen at the root and is then rinsed off at low pressure. One scrubs, the other treats.
This is why soft washing is the standard approach for roofs, render, pebbledash, cladding and softer brickwork, while pressure washing remains the right tool for driveways, patios, block paving and tarmac, surfaces tough enough to take the mechanical force without damage.
Decision table by surface
Roof tiles and slate: soft wash. High pressure lifts the granules off concrete tiles and can crack slate or dislodge tiles entirely, plus it does nothing to stop moss regrowth. Render, pebbledash and K-rend: soft wash. Pressure blasts through the surface texture and can crack the keying, and once water gets behind render it is a genuinely expensive repair.
Soft brick and older pointing, common in Victorian terraces: soft wash. High pressure erodes soft lime mortar joints and can blow the face off older bricks. Driveways, patios and block paving: pressure wash. These surfaces are built to take mechanical wear and pressure washing is the efficient way to lift ingrained dirt, moss and oil stains. Decking and fencing: usually a lighter pressure wash or soft wash depending on the wood's condition, since aggressive pressure can splinter and gouge softer timber.
What goes wrong when the wrong method is used
Pressure washing a roof is the single most common cause of expensive callouts we see from other companies' work: cracked tiles, stripped granule coating that then accelerates weathering, and moss that returns within months because nothing was done to kill the spores. On render, the damage is often worse, since water forced behind the surface can lead to damp patches appearing indoors months later.
Soft washing a driveway, on the other hand, is usually just underwhelming rather than damaging, since chemistry alone struggles to shift ingrained tyre marks and moss gripped into rough concrete without the mechanical scrubbing action pressure provides.
What it costs and how to ask the right question
Roof cleaning quotes depend on roof area, pitch, tile type, moss coverage, the cleaning method and safe access. Rather than asking for either method by name, tell a contractor what surface you have and ask them which method and equipment they will use and why, a competent professional should be able to explain the reasoning without hesitation.
If a company quotes the same price and method for your roof and your driveway, or turns up with only a pressure washer regardless of the job, that is worth treating as a red flag.
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