Halmyre presented the following explanation :
On a similar subject, I spent a couple of hours last weekend perched on the
very top step of an aluminium ladder with a long handled brush in one hand
and a hose in the other, cleaning the mossy crud off the top of a caravan.
T'was a bit wobbly...

I park the caravan tight up to the garage and work from it's flat roof.
Then for the far out of reach bits, I put board over the caravan's
roof.