Dirt floors can work but I'd put something on it. At least a good layer of shavings. Over time dirt floors will either wear away if you are sweeping/cleaning them directly or build up with waste if you don't clean it off. We used to have all dirt floored buildings at the old place and after many years they could become a nightmare especially during years we ran short in the winter and didn't use enough bedding for the animals. Reason we did crushed limestone as the base for all the buildings on our new place. Not as hard as concrete, way cheaper, but it hasn't worn away like the dirt or built up that layer of crud that can't really be removed without digging up several inches of the floor. Plus when we have any spilled water buckets or pipe leaks the limestone just compacts down more where the dirt turned to a muck hole for weeks to months and was uneven when it finally did dry.