Raised my coop on concrete blocks, elevated by 14 inches. It’s slick enough rats slip instead of climb, haven’t seen one burrow underneath since.
Installed solar-powered ultrasonic stake near the coop, but placed it far enough so it doesn’t confuse the chickens. Rats seem to avoid that corner like the plague.
No single solution works forever. I’m using hardware cloth, bucket traps, motion lights, elevated feed, and cayenne, and it’s finally stayed rat-free.