I was walking round our small village on my lockdown-permiitted exercise a few weeks back and saw someone standing on the apex of a pretty high bungalow roof with some kind of hose and gun arrangement surrounded by loads of spray. Saw the name on his van and looked it up. "Steam Cleaning your roof is a speciality...." Honestly, don't people have more important things to worry about than paying to have a steam cleaned roof. I'm a bit lax about vacuuming the carpet in the living room or cleaning the cooker.
I don't think of it as a 'big thing', except perhaps on a flat roof where it might make water back up round the flashing. We get a fair bit, only nuisance is that it occasionally blocks a gutter, seems to wash away most times, but it normally just falls over it. Can be useful for birds, bugs to eat and moss to make nests.
If it troubles you, suggest first thing to do is nothing, then a bit more focussed nothing, then maybe 2 or 3 weeks of advanced nothing, until we get a 2 week hot dry spell in summer then knock off what you can reach with something long and light, 8ft garden cane does it. The first proper heavy rain, like a thunderstorm, after a dry spell might do it for you. If it still bothers you, then try suggestions above.
I wonder how humanity coped for the few centuries of stone, slate and clay roof coverings and 2 centuries of concrete tiles before steam cleaning and pressure washers came along.
