my coop is two stories, the upper is lined with shower stall liner from home depot, it's made of recycled plastic and is very durable and relatively cheap. we use the top floor for meat birds and brooders. the bottom floor is dirt. there are pros and cons to both. our layers prefer having a good base to dig into and a nice sandy dirt to dust and sun in. As long as the dirt is covered and kept dry, it's probably the best for the birds, IMHO.