We have a large shed turned Zoo.
Plywood painted flooring, we covered with linoleum.
Our floor ended up about 8'x12', Home Depot had a $50 solid roll of linoleum in exactly that size. Yippee! They do have rolls that they will custom cut from.
I had access to an unusual material to put up along the walls, for deep litter method. Ever seen interior of a refrigerated tractor trailer? FIL had one at work getting stripped, so he cut out the material for me. Called Chem-Lite or something to that effect. It's a non porous solid material that comes in sheets about 2' high. Tacked onto walls over the sheetrock, then caulked at floor seam, perfect moisture barrier.
I've been using pine shavings and dry grass clippings on floor. I'm not sure if I'm doing a deep litter method correctly. My chickens poop EVERYWHERE in the coop so there isn't just cleanup under the roosts to do. The poop just kind of sticks to shavings and gets crushed in...
I considered sand but dont think it's a viable option for the size of our coop. Trouble with sand is moisture can potentially build up then cause frostbite. A quick daily scooping like a cat box is the benefit though.