A lot of them are sealed up with that rubberized undercoat / roofing stuff. My friend seals all of his up with the rubberized paint on undercoat, and first makes sure to seal all the seams where the steel is near the plywood base with Sika caulking (paintable). The overcoats it with a couple of coats... and it is pretty impervious to water. Make sure you seal up all the seams however or water could get in there and sit there.
Looking good Andy!