I have an existing temporary type building (720 sq ft) with a wood floor. The wood floor keeps rotting out. I would like to pour a cement slab to replace all the floor joists and floor. It sits on top of gravel and appears to get very damp (no standing water). I really do not want to raise the building and add a footer (which was one suggestion $$$). I have thought about using marine plywood, plastic board or Armoroc(cement fiber board)instead in hopes that it will last longer. Any ideas??