I was going to us sand also in the new run we are building this fall. We live in Texas, and it is so hot here in the summer. Regular days can hit 105 and the extreme day can see 115'. I had not thought about sand fleas??? Really. Maybe the girls would eat them. I just know WHEN we get rain, it is a mud hole now. I suggested to my husband that we do concrete, and he said they would not be able to stand on the floor when it is that hot. So I think I will go with the sand, and put down Diatomaceous earth to help with the fleas if they become a problem. I read that if you put a little in the roosting boxes it helps keep ants out. We had a problem this summer with fire ants in the coop. YUCK. Thought my girls were breaking their eggs, and then the ants would come. Oh no, not that simple, hubby went out to close them up and a six foot Bull snake was the egg eater. So many issues this year. When we bought our place a couple years back it had a cute little hen house for two chickens, I have enlarged the coop, but have not really prepared a run. So now, we are going to work on coop and run. Let me know about fleas please.