Predator safe floor ideas needed

[FMF1986]

My chickens are in a corner of the old bank barn. The rats come through the walls, fairly impossible to keep them out. Best thing I did was hang an old skid with some extra perches added, suspended from the ceiling, so the chickens all perch up there at night. They used to be easily accessible to the rats, but are not now. I'm waiting for the snow to stop to start construction of my summer run. I tried free ranging last summer, which was a colossal failure, so next plan is to have them in the barn in winter and the outdoor run in summer. Once the chickens go outside for the summer, I am going to re-parge (concrete around the stone foundation) of the barn, and going to put poison into their holes then parge over them. Best thing I can think of.