Here's a video I made of the inauguration of my miniramp two summers ago:
See if you recognize the song.
The surface is masonite and I would religiously put the tarp on when it rains. Tarps aren't completely waterproof but I'd put a beam across, tent style, so that the water doesn't pool up.
The masonite lasted 1.5 summers but next spring I'll redo the bottom probably with 1/4" plywood. The transitions are still in decent shape because the water doesn't accumulate there.
I was riding it one night thinking "this is so much fun, that's all I'd ever do" when my neighbour came to tell me the noise was driving he and his wife nuts so I had to cut down my riding time - I also can't ride it in the winter, obviously. I don't have a garage nor room to build one large enough to put a miniramp in so I've been trying to figure out how I can ride more...
Was thinking of renting a space to build one in and have it be some private club to spread the cost, but then the logistics of it seem like a little bit too much trouble.
We do have bigger pools and ramps around town, but I'm a bit of a chicken little so I like working out tricks in the miniramp at home before trying them on the bigger stuff (if I ever do).