you can get a extended pole with brush on from maplins, boyes, wilkinsons, around £10-£15 connect this to an outside hose from tap., after spraying virosol all over roof,then use this extension brush pole to get all the poop off.
then if you do wfp cleaning, rinse with your wfp and pure water. keeps this brush clean.
if you dont wfp just use the other extention brush you cleaned it with to rinse after cleaning all the green algae off