Yeah, sheet metal, or do a good job of water sealing your entire ramp (actually, do that if you use sheet metal, too), make sure to seal up all of the edges of the ramp (besides water pooling in the flat, water seeping into the edges of your plys is what tends to rot and disintegrate first), then cover your ramp with a vinyl tarp when you're not using it.
If you can, make a post that you can put in the flat (something very movable that will stand up straight) and make a tepee with your tarp, then you can skate it right after a rain... if wind up needing more than one tarp, you'll want to make sure they overlap in such a way that the tarp itself doesn't create pockets, so you can peel 'em back without spilling rainwater on everything.