I have installed of a 2500 gallon rainwater storage tank on the shop. I built a pad by digging out a hole and placing compacted gravel in that space. I put in a layer of 3/4 inch pressure treated plywood on top of the gravel. This is what the tank will sit on. I connected the tank to the shop downspout with 4 inch ABS pipe. There is a first water reservoir and an overflow that connects to the old pipe that goes out into the yard. It has been raining and it seems to work.