Green algae on conny roofs facia's etc is no problem what I use a 1.5 litre hand sprayer . Spray some onto magic sponge /pad wipe over item to be cleaned then over with dry cloth job done ( if algae is really thick wipe twice ) . With a stiff brush and a bit of scrubbing will take off litchen from conny roofs . If plastic is in a reasonable condition it will shine like new . If its been in the sun for 20 years so much of a shine but still cleans . Have a customer that's ocd about cleaning and she is very happy with results .
Did a small bungalow sat prob only used half a bottle so only cost about 20p of Nuclean.
When trad on windows cost about 2p per gallon of water, wfp cost about 2p per 100 litres .