You'll find people who'll clean a roof for a fraction of a professional price. Here's usually where the difference goes.
Access
A mobile scaffold tower costs time and money. Walking on the roof is cheaper, and it cracks tiles.
Method
A jet washer is fast. It's also how surfaces get stripped and water gets driven under tiles.
Biocide
Skip the treatment and the moss is back within a year or two, so you pay again.
The rest of the house
Moss ends up in the gutters and on the paths. A proper job clears and washes all of it.
Insurance
Working at height carries risk. Ask any roof cleaner for their public liability insurance; ours is £1m.
