Roof cleaning is not one job, it is several different methods that get lumped together under the same name, and the differences between them matter a lot more than most people realise until something goes wrong. Scraping, soft washing and pressure washing all remove moss, but they do it in very different ways, with very different risks and very different results afterwards.
Here is a straightforward comparison of the three main methods, what each one is actually good for, and what a sensible combined approach looks like in practice.
Physical scraping: fast but incomplete on its own
Scraping means physically removing moss cushions and lichen by hand or with a soft brush, working across the roof and clearing debris as you go. It is quick, gives an immediate visual improvement, and is a necessary first step on heavily mossed roofs where thick growth needs removing before any chemical treatment can work properly.
Its weakness is that scraping alone does nothing to kill spores left behind in the tile texture, so without a follow-up biocide treatment, regrowth is typically visible again within six to twelve months. Done carelessly, scraping can also dislodge or crack tiles, so it needs a competent, gentle hand.
Soft washing: the standard for treated, lasting results
Soft washing combines a low-pressure rinse with a biocide-based chemical treatment, killing moss, algae and lichen at the root rather than just removing what is visible. It is now the industry-standard approach for roof cleaning because it treats the underlying cause rather than only the symptom, and it carries essentially no risk of tile damage since no high-pressure water is involved.
The main trade-off is that results are not instantly dramatic on the day, since dead moss and lichen often take a few weeks to fully die off and wash away naturally with rain, rather than being stripped off immediately.
Pressure washing: fast and dramatic, but the highest-risk method
High-pressure washing strips moss and dirt off a roof quickly and gives an immediate, dramatic before-and-after. It is also the method most likely to cause real damage: stripping the protective granule coating off concrete tiles, cracking slate, forcing water under tiles and into the roof space, and loosening tiles that were otherwise sound.
It has essentially been phased out by reputable roofers and cleaners in favour of soft washing for this reason, and most manufacturers of concrete and slate roof tiles explicitly advise against high-pressure cleaning in their maintenance guidance.
The combined approach and typical pricing
A properly done roof clean on a heavily mossed roof typically combines scraping to remove thick growth, followed by a soft wash and biocide treatment to kill remaining spores and prevent fast regrowth. This combined approach typically costs £400-£1,200+ depending on roof size, pitch and access, more than a quick pressure wash but with results that last years rather than months.
When getting quotes, ask specifically which of these methods will be used and in what order, a vague answer or a quote that only mentions pressure washing is worth treating with caution given the damage risk involved.
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