Concrete would be a good floor as long as you pour a stemwall that's about 2 feet high. Local grass seed farmers do this to keep rats out of their seed. Put metal siding on the walls overlapping the concrete to prevent them from getting a "handhold" if they manage to climb the stemwall. But YOU still have to get in, and they will get in through the weakest link, which will likely be the door. But if you have metal siding on the walls and door and concrete everywhere else and put down a metal plate in front of the door so there isn't a big enough gap with exposed wood to gnaw through, you should be ok. I built my coop on 6x6 treated skids with the floor 2 ft. off the ground. Then I put down composite decking for flooring. The ramp I walk up is hinged and I hooked an old barbwire stretcher to the side to hoist it up and down. It seems to be harder for rodents to chew through the decking and it never rots. A flatnose shovel also slides over it very easily making cleaning the coop a fast job. I've had rats in the yard, but never in the coop with this setup. Also, I found by accident that if you leave a tall plastic bucket about 1/2 full of water with some way for rats to get to the edge, like a board laying over an edge, they will climb up to get the water and will fall in and drown. The water has to be deeper than the rat is long or they will jump out. I've found up to 3 dead in the same bucket! And make sure there are no piles of wood or pallets, etc. around that they can nest under. I've had a really hard time getting rats to take poison. One thing that works pretty well is a raw egg with strychnine in it. Or mix it with cat food (making sure the holes in the trap allow rats but NOT cats!) Get one of the feeders that chickens have to stand on to open (so rats and birds can't get in) and make sure all other sources of food are eliminated. It kills very fast - often the rats don't get more than 2 feet. But this poison is nasty stuff! You have to keep really tight control over it and keep it locked up. Try the buckets of water first. You might also get a rat-terrier. They LOVE to kill rats! It's what they are bred for. During rat season, I also carry a revolver loaded with snake shot. If you get close to a rat, it will put them down fast. Good luck.