Wood can be an issue too for absorbing urine. Started last year (larger breed) using tiles - 12"x12" (some are smaller) ceramic tiles. Easy to wash off, non absorbant, and in warm weather areas can be put in the freezer (be careful not to tap them on anything) and cool porches for them. I have 2/rabbit so I can just exchange them. $1 each, sometimes less if you have a discount hardware store around.