Sorry I haven't replied to your question about an oval helix. The difficulty with most helixes is to get as much track in them as possible to gain height with reasonable grades. 2% is considered a max if you want reasonable length trains. What I was suggesting is that you extend it across the width of the shed i.e. a half circle at each end with straight sections between, This gains you more height between turns, fewer turns to achieve that height and relieves the other bugaboo of helixes, the constant inward pressure against the couplings between rolling stock of a continuous circle. As far as weatherproofing for rain, sheeting over the top (with a slope) will take care of that.