mdf:

 

If there's a substantial amount actually on the roof, it might be better sorting it out from the roof? Can then scrub it off or treat it directly, rather than with a tonne of overspray that will end up in the stormwater. If any paint etc. has lifted under the moss, can also investigate and remedy.

 

I'm okay generally with heights, but we've got at least a three story drop from some points on our house. I got the absolute heebie jeebies the first time I was anywhere near it. I've now got a roofers harness for when I need to get up there, and feel pretty safe wearing that and strapped directly into a solid house pile. But otherwise would very much suggest getting in a professional with the proper safety equipment. Moss is pretty slippery at the best of times, and usually the treatment makes it even slicker.

 

You could simply disconnect the storm water down pipes to minamise this. In my view, spraying is way more safer than physically getting on the roof. Water and scrubbing won't stop the moss coming back when it is really bad. I spray my roof standing on it working backwards on dry surface, but that stuff is really slippery! Don't wear gumboots! Rain does the work for you.

Also worth mentioning you should also remove the causes such as large trees / leaf litter etc. In my case, leaf litter was allowed to build up. Birch trees! PITA.