Just an opinion, but what you don't want to do is build in anything that traps water. Carpet is bad enough, if you cover the floor with a sheet of metal and water finds it's way underneath you are going to have a rusty rotting mess on you hands in short order.
I was at Home Depot the other day and they have construction (rubber/plastic) matts four feet by eight feet by 3/4" inch thick. They are used to protect the floors doing a remodel and are made out of old plastic bottles and tires. They are somewhat porous and breath. You really want something back there that will breath and let moisture evaporate.