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Deck is 12’x 8’with mid span posts(6’). Double 2x10 beams around. 16” center 2x10 joists. All posts 6x6 treated S-Pine as well as all lumber. Tub is placed over the 6x8’span. (1/2 of the deck. Will this construction support tub?

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  • You should pay an engineer for this. But how high up is it?
    – popham
    Nov 1, 2023 at 3:53
  • And show a diagram Nov 1, 2023 at 10:21
  • Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking.
    – Community Bot
    Nov 1, 2023 at 10:22

1 Answer 1

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By the International Residential Code, a deck gets designed for 40 psf live load. Water at 8 inches depth is about 40 psf. Assuming the 40 psf for people plus your 3500#, that's 120 psf. So your deck is probably not good enough unless it was ridiculously over built. To spit out a strength, an engineer should examine the connections and foundation of the deck, not just the beam and post sizes. The post lengths are also relevant. And you probably need lateral bracing assuming that the deck is at the non-trivial elevation implied by your 6x6 posts.

If you were sizing the joists and beams for the normal 40 psf live load, then the IRC's R507.5 and R507.6 would be where to find the sizes (the tabulated beam sizes are for perimeter beams, not interior ones which have twice the load). That section of the IRC tells you all about 40 psf decks. The beam sizes suggest to my intuition that you've got a 40 psf deck. The joists seem to have some reserve strength, but I'm making assumption about your exact framing layout, so I don't want to speak too confidently. Even if the joists were already big enough (they're probably not), the connections may not be developing their full strength. An engineer should look at your structure and prescribe adequate modifications.

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    I agree 100%. I hope you don't mind that I reinforced the critical point.
    – FreeMan
    Nov 1, 2023 at 13:12
  • A second vote for your estimated load: I happen to own a hot tub with a footprint of 6'8" x 7'9" and height of 33"; the manufacturer in their specifications indicates dead weight of the tub with occupants averaging 175 pounds is 105 psf. The 120 psf target reflects a healthy, conservative approach.
    – Greg Hill
    Nov 1, 2023 at 16:16
  • @GregHill, do you remember if the manufacturer actually specified design load? Something that potentially put them on the hook for structural problems? Because I was a little bothered by the weight distribution. The hot tub has its load somewhat concentrated toward its center, where uniform load distribution implies half the midspan bending moment versus idealizing all of the load concentrated at center. I rounded up to the 120 psf, but I contemplated adding some more because of the load distribution. In real life it would be great to start with a design prescription from the manufacturer.
    – popham
    Nov 1, 2023 at 17:50
  • @FreeMan, I changed it back. I get to the point quickly in the 4th sentence. This is a good answer for somebody asking the question. I don't believe that people asking the question are eager to skim. I'll add the word "engineer" once or twice, though, to be more explicit.
    – popham
    Nov 1, 2023 at 18:00
  • This manufacturer doesn't specify a design load; they just say "The weight per square foot must not exceed the structure's rated capacity." I have to imagine at hot tub is semi-flexible and thus can bend along with the structure on which it rests, increasing the loading at the center of the span. It's not all the load concentrated at center, though. I wish I had the engineering background to understand the implications of this!
    – Greg Hill
    Nov 1, 2023 at 19:17

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