Roof cleaning

To clean or not to clean?  All our roofs, whether they are Roman tiles (the big curvy ones) or normal tiles (the small flat ones) collect moss and lichen like a small child collects head lice.  Unlike head lice, moss and lichen can be cleared with a power washer.  Actually, I can't say for sure that head lice can't be cleared with a power washer but I can't get either of my kids to volunteer.

Three houses in our close decided to try having the roof cleared and the results are mixed.  The two houses with flat tiles have come up quite well but the one roof with Roman tiles has come up quite pale.  I'll publish accounts of all three shortly plus prices and photos.