iv actually done a few of these jobs cleaning the roof, treat with a antifungal biocide then coat with special roof coating paint that acts like a gortex coating where the rain beads off the roof, you target roofs with tiles that are 25years old that have lost there coating.there is a good profit margin to be made if you price well.aquashield quoted £3500 for just one bungalow we did the 2 for £1700 and still made approx £1000 but we had actually priced really low just to get our first ones done.the work is quite hard work lots of bending but once you get upto the top of the roof on a cat ladder and clean a portion of the tiles they are actually quite good for grip once you have removed the moss lichen ad algae and dirt.when we setup this business properly we are hoping to setup some form of scaffold that we can be harnessed from as someone said it only takes one fall.id suggest you invest in proper pressure washing equipment i.e honda 11-13hp and even go for a diesal engine as they do use a good amount of petrol with the use of a turbo nozzle or you will be there all day.there are courses to go on run by kingfisher coating and nutech they are quite pricey tho but worth it to give you a good insight into this sort of business.hope this helpsrichy