Polycarbonate? So it'll be cracks letting water in, not just rot or holes. I can't think of anything sensible to repair it. It'll keep on deteriorating.
Best to replace it. Perhaps steel sheets... box profile..........
http://www.phoenixsteels.com/box-profile.html
Or those corrugated green sheete that Wickes etc sell.......... they're a sort of fibre/cardboard impregnated with bitumen. They're cheap, and last well on sheds