What kind of decking,wood,metal and is the rest of the material still solid?If so can you remove only the damaged area and tye in with new mod? Chances are the edge metal was not primed or the edge metal was not install correctly need photos to be sure.Now as far as moving water to a new location well pics would help also to make the right choice taper is expensive and if your going to go with a new system.Well you may be able to redeck with plywood to taper pic would help again now i would go with a pvc in you area or a 60 mil epdm.Will be watching for pics.,