My Swift TA is always covered when in storage as it’s under trees. The amount of debris (flora and fauna) that collects on the roof is staggering. Once a year, usually about now, I deep clean the caravan prior to its annual service and our main annual holiday in France for June and July. I leave the cover on the caravan and wash it off with a soft brush and clean water. Park it in the sun for an hour or so to dry then remove it and roll it up. Then I can give the caravan it’s initial shampooing before setting off to home.
there’s usually some fairly substantial bits of tree on the roof together with a carpet of pine needles but never any damage to the roof under the cover.