We used to have a wooden ramp for the chickens to come out of the coop on. I was dealing with bumblefoot with 2-3 of my chickens. The vet said it was caused by the bare wood ramp or roosts giving the chickens splinters. We switched our ramp to cinder blocks piled like steps and made their roosts out of natural branches. I have not had bumblefoot since. Maybe a coincidence but it worked for me.