For several years my girls would get bumblefoot and I would take them to the vet for treatment at a substantial cost. The vet told me that one of the causes has to do with the circulation when chickens roost on a flat surface such as the sanded 2x4s that I was using. He suggested wrapping the boards with a coarse rope material to give them a surface that was not flat. Instead I bought a textured shower mat, cut it into strips and stapled to the top of the roost boards. It's been going on four years now without a single case of bumblefoot. The mat I use looks like the photo, but any textured mat should work as long as it has pressure points for their feet.