Only take this advice if you are confident in the safe use of ladders.If they are set back on a sloping tiled roof,using a good,strong set of pointers,position them so they are sitting on the tiles,using your van to support the bottoms,or someone footing the ladder.If using this technique,use your common sense and be careful.If you dont feel confident,DONT DO IT.Failing that,clean them from inside the house.If they are just too awkward,dont bother.Also,are you insured?