I actually raised this topic a little while ago, as I currently use hard wood chips and wondered whether sand would be a better alternative. I considered using the play sand as I thought it would drain better and also dry better when it got wet (rather than builders sand, although cheaper wasn't such good quality). I actually have a covered run, but the rain does blow in through the sides (which are wire mesh) if the wind is blowing in that direction so the edges do get wet sometimes. I thought sand would be better for their feet and as I poo pick several times per day, it would stay fairly clean and it would probably be easier to poo pick from sand rather than wood chip. The only reason I decided against it is because someone raised the point of how to dispose of it when it became time to do a complete clean out and replacement, but if it could be sieved and washed then I suppose it wouldn't need to be disposed of. Anyway, I decided to stay with the wood chip for now and have another think at a later date.