Roof moss removal

My detached house and garage roof are covered in balls of moss every few inches. They aren't stuck on that hard because they often roll off of their own accord. The house floorplan is 15m x 10m and the roof is a standard up-and-over tiled roof. It's a double garage with standard up and over tiled roof. Access to both roofs is easy. How much would it cost to to have the moss removed and moss killer sprayed on to prevent regrowth? And how long would the job take? Thanks for you help.