Jet washing of roofs is not the best thing to do. In fact it can cause extensive damage and there is a much better product than any of those mentioned so far in this thread.
Moss Away from Everbuild available in sachets or 5 litre containers from many hardware stores and garden centres and is a biocide / algicide for which no permit is required and which can be applied using a watering can to most surfaces including roofs patios and decking. The only caution is to make sure pets are out of the way before work starts.
You need a dry day, put the pressure washer away, you won't need it to rinse off afterwards or at all later on, this is a process which works over time and keeps working too, as opposed to an instant fix which will just need doing again and again and again.
You will also need to accept / trust that this although slower way of doing it, is safer and more thorough and the chemical used acts as an inhibitor to ****** future recurrence.
All you need to do to prepare the surface to be treated is to remove the surplus moss growth and the other loose things which you would obviously brush away first anyway, apply the Moss Away diluted 9 parts water to 1 part Moss away, more concentrated if the area is badly affected, use a soft brush on a long pole if doing a roof, and the fluid needs spreading about to ensure a good coverage and then get the deck chair out and put the kettle on.
For badly affected areas a second application may be necessary a few months down the road. Normally you will notice a real difference) as the chemical goes to work over a period of time) in a few weeks.
The chemical provides a retardant element which slows down any recurrence. However, in heavily affected areas, several applications a few weeks / months apart may be needed to kill off the root causes of the problem.
For less than the outlay involved in a pressure washer, you can invest in a battery pumped horticultural sprayer, a long window cleaner's pole and a window cleaning brush and some microbore pipe and apply the solution to a roof without having to climb ladders. Moreover, you can use the same kit on paths, paviors and decking, it cuts the deadly slime out as well as getting rid of the algae so makes what were once slippery surfaces safe once again, even in the autumn and winter.
No I don't work for Everbuild, I operate a cleaning business which used to provide many of its cleaning services from the end of a pressure washer using loads of water and even more testosterone to clean everything, until I came across this stuff.
