My 18 year old van has to live in the garden under some trees when I'm not using it, and so the roof (which will bear practically no weight at all) is a little difficult to clean and gets a good layer of green algae building up on it during the winter.
There is no access from inside the caravan to the roof, no sunroof or Heiki etc just a tiny roof vent), so unless one has the physical dimensions of a meerkat) that approach is a non starter, so I have worked out a strategy over the years that works well for me...
I use a stepladder and a long handled telescopic car brush ( £10 from the market) and a bucket of very hot water with Asda's car shampoo to clean it off when I lay up the caravan at the end of September. I use double the recommended amount of the shampoo and the whole van takes me about 45 minutes to do.
By Christmas the caravan has a lovely green carpet of algae all over the roof so its time for its mid winter clean...
The same cleaning procedure is adopted, but the secret is to do it when it's raining! I get a few strange looks from passers by, but the rain helps enormously and the green gunge just seems to float away with very little effort. I use a hosepipe on a gentle spray setting to rinse off, and the van comes up looking like new.
This midwinter clean is repeated again at the end of March/ beginning of April so my van is clean and shiny for our first use around Easter time..... this later clean onty takes half an hour or so as the Asda car shampoo seems to have a high wax content and this seems to seal the surface so the green gunge tends to sit on top rather than become engrained in the plastic roof.
My only reason for using the Asda car shampoo is that its the cheapest I could find, A 2.5 litre can cost me about £2.50 and using it at the rate of 2 or 3 capsful in a bucket of very hot water 3 times a year, I reckon it will probably see me out!
During the summer the rain cleans the roof satisfactorily, supplemented by a gentle spray hosing if necessary, so no shampoo is needed at all.
As I have not seen the use of very hot water being recommended, or Asda's cheapest car wax shampoo being recommended for caravan use, I am not recommending that anyone folows my strategy.....I have, however been doing it this way for years and with no adverse effects and my 18 year old van is shiny and looks like new!
Regards to all
paws