I used a 4 x 8 sheet of glass board from 'nards on my floor. The technical term for the material is FRP. The interior walls of my coop are 1/2" OSB primed and painted with a Pittsburgh oil based gloss enamel and I plan to caulk the floor/wall intersection. The Amish coops that you can buy from Horizon Wood structures online use the stuff. Heck, if it's good for the Amish, it's good for me.