I learned the hard way about building a heavy weight-bearing deck. I had an existing deck that I wanted to expand to put a hot tub on. A large hot tub full of water is rather heavy. I contacted a contractor to build my deck expansion, and told him what I wanted it for. Well, he did not do it properly. He built it like most decks are built, with a rim joist around the edge sitting on the supporting posts, and then the cross stringers were merely nailed in from the sides of the rim joist. In other words, the only thing holding the weigh of the deck was the nails coming in from the side of the rim joist. Well after I had installed the hot tub and filled it with water, I noticed that the stringers were sagging, because the nails were bending. I had to disassemble the deck supports, and install a perimeter beam underneath the stringers, consisting of three 2 X 6's screwed together and installed on top of the vertical supports. The stringers were then supported securely on top of the perimeter beam. Make sure your contractor knows that you want a heavy duty deck that can take weight and is built like this.