put the plastic on the top side... trust me on this one. Plastic on the underside means STINKY PLYWOOD in about 2 weeks. Plastic on top.. We use the plastic sheeting that comes in a huge roll from RONA...vapour barrier!O my that poo removal system what hubby has in mind for my cages. His plan is to build it using playwood with plastic stapled to it on the underside. And a plastic gutting to catch it all and get it out of the barn.