Definitely put a slope in it so it drains when you hose it off or it rains. I have a 8' x 16' pad that slopes about 8" over the 16 feet, and have a trough with a drain into a pit I dug that has 1/2 of a 55 gallon plastic drum dug into the ground. The trough has a grate on top, and the waste/water drains through pea-rock into the tub. The tub has holes in the bottom with more rock on the bottom of the tub-about 4 inches deep. I have an access door that I use to add lime occasionally, but the waste usually leeches into the ground no problem.