it is fairly large I'm suprised it is on 2x4 and not 4x4. Ck with the town most have stipulations about a building that large. I have a 10x20 shead. It is built on 4x4 and I layed down 6" of 3/4" stone first. taht is the least I would do for 2 reasons. It will drain the water away from the bottom. You don't want wood laying in water or mud even though it is pressure treated. It allowed me to level it and it won't sink in an area. As for the brick it is better or even concreat but a truck load of stone was easy.