Ours is about 1 1/2' off the ground so critters like rats can't move in and live underneath. Had that problem once. Wooden floor, coop on the ground. Rats moved in underneath, chewed through the floor and stole eggs and ate feed all winter. I couldn't figure out why the feed consumption went up and egg production went down until I cleaned the coop in the spring. The little buggers were popping their heads up in the holes watching me! I looked down one of the holes and it was just crawling with rats under there. *shudder* So, we put out lots of bait stations. We had cats at the time, but they never caught or ate rats so I didn't worry about secondary poisoning. I'm sure a concrete floor would be effective in keeping burrowing critters out, too.