I have no grass in my backyard since it is a pretty steep slope. The only flat piece of land I have is a concrete pad that the previous owners were using as a patio attached directly to the back of the house. I was wondering what considerations I need to make for the run. Would it be better to fence in the concrete slab and put down some gravel or sand for their run, or should I try and fence in part of the slope (about 45 degrees) and let them potentially create a landslide with their digging. I know some people have sand runs, but does anyone have sand directly on concrete?