There are some people earning substantial amounts of money from cleaning roofs.
Myself personally i clean roofs now and again ...if the job pays well,is safe to do (not from a health and safety aspect but from a damage to property etc aspect ...some jobs should be left well alone).
Some firms are claiming 33% saving in heating bills, 25year guarantees on the coatings ...i for one am not/do not want to be any part of this.
Seems to me that a lot of the roof coating firms are "branches" of double glazing firms or part of the travelling community.Take from this what you will .
My advice to you would be try it see if you can make it pay,pick your jobs carefully,understand the risk to persons and property,never underestimate the time it takes to clean the area around where you have been working.
I believe there are several firms on ebay that run roof/driveway cleaning coarses. These firms spend a lot of money advertising ( more than an individual would like/could spend!!) They produce a LOT of good quality leads for contractors who have done there training (training days cost about £100 )