Moss on a British roof is normal and mostly harmless, and roof cleaning is one of the few trades where the honest answer is often that you do not need it. There are real cases where it matters, and there is a method that damages roofs and is still widely sold, so the useful thing is knowing which situation you are in before anybody quotes.
When moss actually matters
Moss holds water against the covering, which accelerates frost damage on porous tiles and can shorten the life of an already weathered roof. It sheds into gutters and blocks them, which is the commonest practical harm and the cheapest to fix by clearing the gutters rather than the roof. On a shallow pitch it can bridge the laps and let water track sideways. On a sound roof at a normal pitch with clear gutters, moss is cosmetic.
The methods, and the one to refuse
Scraping and brushing by hand removes the growth without touching the covering and is slow, which is why it costs more. A soft wash with a biocide kills the growth and lets it weather off over months, which looks worse for a while and is gentle. High pressure jet washing strips the surface off concrete tiles along with the moss, opens the pores and leaves the roof holding more water than before, and it is still routinely sold as roof cleaning. On any covering with a surface finish, it takes years off the roof.
What the price is made of
Access is most of it, exactly as with any other roof work: a two storey house needs a scaffold or a tower and that dominates the quote. Then area, then method, then whether the gutters are being cleared and the moss taken away rather than left in the borders. A quote that includes a biocide treatment will usually offer a return visit, because the growth dies slowly. A quote that includes re-bedding ridge tiles has become a repair quote and should be read as one.
Questions people ask about roof cleaning cost
Does moss damage a roof?
It can, on a weathered or porous covering and on a shallow pitch, mostly by holding moisture. On a sound roof its practical effect is usually blocked gutters, which is cheaper to solve at the gutter.
Is pressure washing a roof safe?
On concrete tiles it is not. It removes the surface layer along with the moss, which makes the tile more absorbent and shortens its life. It is one of the few roofing services worth declining outright.
How often does a roof need cleaning?
Most British roofs never need it. Where it is being done for a reason, treatment every several years is normal and annual cleaning is a subscription rather than a need.