I am putting in a hot tub outside and the guy who built my house gave me some advice. He advised me of putting down about 3-4 inches of pea gravel and then 4 inchs of blue board that over laps to cover the 8x8 square. He said that would be perfect as a foundation because it spreads the weight. I like the sound of it because I am doing it myself and on a budget. But then again it sounds strange. Any advice?