There's no hope it works out for me - it is working out!!!
What is your experience?
I'll give you one of my experiences - a supermarket roof - some 20 000sqm, small amount of ingress, a lot of mess and one very happy customer, who happens to be one of the top three supermarkets in the country.
Where does the figure taking 15 years off the life of the roof come from?? and if it comes from your mate where does he get that nfo from.
There is an amount of surface that comes down with the algae & detritus and the work is like anything else, if done properly by someone who knows what they are doing it is like looking at a new roof!!!
I would say I have more experience than you and I know that the impact on the roof does not take 15 years off the life of it.
Rob