I planned mine out for drainage because I knew that the ducks would be messy with their water and pool. So I put a layer of gravel in, lined it with landscape fabric and then a layer of course sand. I chose course sand for the grit and I used landscaping fabric so I wouldn't lose my sand in the gravel as time went on. It does get wet at times but is quickly absorbed into the sand. I also keep the pen covered with a large tarp. This helps give more protection form the elements like rain and sun and somewhat the wind. Plus with sand it is easy to just rake out the poo and keep the pen clean.