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I have a hot tub that measures 7x7 and weight is around 4400 pounds with 5 hefty people in it. I am in the process of building a new 16x40 deck and my wife wants the hot tub on it, the finished deck will be approximately 8 to 9 feet high. My thinking is to utilize this space instead of setting a post every 30 inches in a square which is what i found places on the internet have suggested. Would it be a better idea to build a 7x7 room. Dig down 4 feet which puts the footer 1 foot below frost line pour a footer lay enough 8 inch block to get back to ground level, then build a room with a 2x12 header over the door. Plywood it inside and out with 1/2 inch, and top it with 2x12s on 8 inch center and 3/4 marine plywood. Then build the rest of the deck. Will even put a rubber membrane over the top and even thought of running the 5/4 deck boards over the top of it tieing all the deck together.

enter image description here Just I quick drawing the shades square is the hot tub placement area

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    I personally wouldn't build a "room" as water will be regularly splashed into it and any enclosed-ish space will become mold city.
    – whatsisname
    Nov 3, 2021 at 1:08
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    Hot tubs on decks are heavy point loads -- your AHJ will likely require a set of stamped drawings from a professional structural engineer for your deck
    – ThreePhaseEel
    Nov 3, 2021 at 1:49
  • Does the 2x12’s at 8” on center span the width of the deck? Is this what the hot tub sits on? Where do the posts go at 30” oc? Is the deck 8-9’ off the ground?
    – Lee Sam
    Nov 3, 2021 at 2:19
  • Please add a drawing of the proposed design. Photo of a hand sketch is fine. With more clarity you'll be able to attract some good answers.
    – P2000
    Nov 3, 2021 at 4:48
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    Yes, you can build a building and put a hot tub on the roof. There's a lot to that, though, which makes your question too broad. Also, your "post every 30 inches" thing sounds like a strawman plan. That's not how it's done.
    – isherwood
    Nov 3, 2021 at 18:39

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Disclosure: I'm not an engineer.

Good drawing; we can see what you're thinking of.

The structure and foundation supporting the tub must be sturdy enough to support the weight of the tub, water, and users, and to resist any tendency that it tip over (it's way up in the air), or collapse downward, or twist, or shear.

The mass and density are vastly larger than either the mass or density of the house and deck. Thus, the structure of the tub and its support must be independently able to resist these forces. If the tub and its supporting structure were tied into the house, and the tub or its support structure failed in any way, the falling tub and structure would either pull away from the house and deck, damaging both, or cause the house and deck to be pulled sideways and collapse.

The local building authority will be interested in this as well: the tub's height above ground puts users at risk. This work would surely require a building permit.

Because of the weight of the filled tub and users, the structure to support it safely should be designed and approved by a structural engineer familiar with building codes in your location. While a nonprofessional owner might well be able to accomplish the construction, its design demands a professional.

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