A roof should never be pressure washed. That's not coming from us, it's coming from the roof tile manufacturers. Many companies ignore this,  or try to fool the customer by telling them that they steam clean. Don't be fooled by this, steam cleaning is hot water pressure washing. 

At Gutter CleanAyr we hand scrape each individual tile to remove the moss. We then clear the gutters of all debris before applying a chemical treatment to the roof. The treatment kills any remaining moss spores and leaves a residual protection to stop moss returning for around 5 years. The treatment also cleans the biofilm from the tiles allowing them to gradually return to their natural colour.