You could put a whirlybird on top if they would let you, or you could just cover the entire shed with a tarp which tucks in under the edge of the slab. The only problem is that you could still end up with humidity in there with the tarp idea. Tough call. You could always explain the issue to the owner and ask if they'll let you grind a groove around the outside edge just before the sheet metal that runs to the edge, so any rain that falls will go into that channel and run off before entering the shed. Either way, its a bit sucky, sorry mate.
