Our run is concrete and open to rain (it's got a chain link top), and our coop is elevated like yours. I tried leaving the concrete bare and hosing it clean but it ended up being a lot of work and smelled poopy. I found that old leaves and pine needles in a thick cover works well, smells less, and the girls like it much better than bare concrete. We have lots of woods around the run, so I rake the fallen leaves into the run, let them scratch in it until it needs cleaning (once every several weeks depending on how much I pile in) and it's easily raked back out for us. I thought about sand, but I did not want to scoop/rake poop regularly (I do that in the coop under the roosting poles). I've read about people bagging their fall leaves for use through the summer, and I've read about people using wood chips as well. Good luck!
