I use a telescopic ladder to reach the roof with a telescopic brush to reach from the outside to the middle. I then apply a diluted solution of Fenwick's caravan cleaner and you may need this if you are parked under a tree as it will shift bird lime and algea.
when we bought ours it had spent two years under a tree collecting moss and algae and was in a very sorry state indeed .we bought a bottle of MUC-OFF from amazon ,(cheapest place) and all you need to do is pre-wet the roof with a hose pipe then spray/spread muc-off over surface ,leave for 5 -10 minutes then use long handled brush and hose pipe to wipe of the moss etc.
never be talked into using a jet wash on a caravan it will damage the seals and seek out any weak spots ,i,m speaking from experience on this one they are lethal to caravans
some good advice,but I had a simular problem,and overcome it after some elbow grease I puchased a cheep tarpauling and covered the roof completely,no more dirty roof.
A day of all things motorhome and campervan. The show is held both in the main indoor arena and across the grounds. Catering, hot drinks and food options are available. Easy access to the site and free parking included. Win one of five pairs of day tickets