My run is 25'x50' and slightly downhill so I don't have a drainage problem. The thing that I would be concerned about with an all sand run is the girls love digging for bugs in the dirt. Maybe a partial sand run would do, but I don't think it would be the same for them if they couldn't dig around in the real dirt. Right now I do have a huge sandbox where they take their sandbaths. So I guess my question is, don't your chickens miss digging in the real dirt for bugs??