We were overrun with rats early last summer. There were hundreds of them. Our hen house is a pole barn with a liner, so the rats were running through the walls. I finally opened up some of the walls to figure out what was going on. The concrete for the floor didn't flow completely around the poles and left some gaps where the rats were burrowing under the pad and coming in and out. I sealed up the holes with hydraulic cement, patched a few other holes in the foundation, and completely framed up some incomplete doorways where the rats were able to enter and exit the walls. In between flocks I laid out pounds of D-Con. They scarfed it down and I was picking up dead rats for days. It's been over a year since I've seen a rat out there. The mice have moved in now. They are impossible to exclude, but I use a lot of Tin Cat traps to keep the population down.