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I'd like to know if this deck plan is strong enough for a 7'x7' hot tub. The max weight with water and seven people is 4,800lb. I'd like to keep the four post design because there is an 18" Oak stump in the middle, and I want to avoid the root system as much as possible. I'm going to dig and cut the roots as much and as deep as necessary to get a big flat holes.

This picture should have enough information to understand the plan.

[Edit] This would be built with treated Southern Yellow Pine.

hot tub deck plan

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  • Do you mean (4) 2x12’s? (Two on each side?)
    – Lee Sam
    Apr 14 at 23:39
  • No, triple 2x12 beams. Beam on post. Apr 14 at 23:40
  • What's going to go around/next to this? A 2ft border around the tub will be rather awkward. Apr 14 at 23:54
  • Oh, good question! I'm going to put railings on three sides, and it's going to connect to an existing deck that is 16' wide. But it's going to be freestanding. because the existing deck is much older, so I will probably have to replace it before this one. Apr 15 at 0:44
  • You indicated there is (3) 2x12 beams, but there are two beams shown that running N-S on top of the posts. Where is the third beam? Also, is the beam a built up by sistering 2, or 3 2x12? Please clarify.
    – r13
    Apr 15 at 1:27
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With the given tub load and 50 psf deck load, I've checked the reactions of the 2x10 joist, and 2x12 beam, assuming there is one beam on each side of the tub. You shall add at least 10% to the design forces indicated in the graph below to check the adequacy of the members.

For foundation, you shall divide the post design load by the footing area to get the bearing pressure, then compare it with the allowable soil pressure. Note you shall leave a safe margin for the weight of the concrete footing, which wasn't included in the calculation.

enter image description here

Notes:

  1. Under tub joist load = (Tub Weight/Tub Area) x joist spacing = (4800#/7'x7') x 8" = 65.3 plf, then add joist and misc. weight rounding up to 80 plf. (*Please double-check the full weight of the tub, it seems low).

  2. Suggest adding bracing between the posts in each direction to resist the lateral loads (wind and from water splashing).

  3. The most critical bending moment of the joist and beam is when the tub is loaded full, the surrounding deck is free of live load (empty).

This is an engineering matter that shouldn't be expected to get a simple answer "Yes" or "No" from a stranger you don't know. Please consult a structural engineer before proceeding. Good luck.

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    I’m not sure you answered anything. The op wants “to know if this deck plan is strong enough”? You say “add at least 10% to the design forces to check the adequacy of the members.” Where’s your answer?
    – Lee Sam
    Apr 15 at 4:22
  • @Lee Sam Please re-visit this post. Someone obviously does not know his limit of capability. diy.stackexchange.com/questions/221971/…
    – r13
    Apr 15 at 4:50
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    This is more of a comment than an answer. You’ve converted the total load in gross pounds to the load in pounds per linear foot.
    – Lee Sam
    Apr 15 at 5:05
  • This is really helpful. The pounds per linear foot metric is what I was missing. So I should be able to find charts for dimensional lumber that give plf values? Apr 15 at 12:30
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    @Josh Pearce I see. No, it does not matter in load distribution, it matters only in deflection calculation. 5/4 x (1 1/4" I guess) would deflect more than 2" x, but in your case, deflection is not really a big concern, so all the reactions provided are still valid. IMO, you have a sound plan (quite conservative for the given load), but please do not take my opinion for granted. As suggested previously, you shall consult with an engineer or experienced wood deck designer before moving forward. Good luck.
    – r13
    Apr 15 at 16:38

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