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I am building a deck for a hot tub and I want to use composite decking. Composite decking has many advantages as far as I see it. I won't have to paint it and it's water resistant. I have used it for other decks but I have never placed a load on it. I currently have joists at 16" on center. Would 12 inch on center be preferable in this case?

Can a hot tub be placed on composite decking?

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  • Is this an indoor or outdoor project? You shall consider consulting with a structural engineer for safety.
    – r13
    Commented Apr 13, 2021 at 16:56
  • This is outdoor. I have consulted on the deck design. I'm just trying to figure out the decking now. Commented Apr 13, 2021 at 17:05
  • If you consulted a structural engineer, what did s/he say about joist spacing? If you didn't consult a structural engineer, I agree with @r13 - you should do so. Commented Apr 13, 2021 at 17:07
  • It was designed with 16 inches on center. I don't have a lot of experience with composite decking so I thought it might do better with less spacing between the joists. Commented Apr 13, 2021 at 17:11
  • Beware of sunlight Issues. A composite deck in full sun can quickly become too hot to walk on, and children and pets can suffer burns.
    – r13
    Commented Apr 13, 2021 at 17:11

2 Answers 2

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99%* of all composite decking would not only hold a hot tub just fine, it would be preferred over wood. It is preferred because it will not fail long term due to water damage or termites and cause a collapse or tilt.

As for weight bearing I am not sure there is any calculators out there for composite, however when I have gotten architects involved they basically treat it as "better than PT pine".

The two biggest things to think about when adding a hot tub to a deck is:

  1. Joist spacing which you have already thought about. Yes 12" is preferred for heavy load. Adding a 3 joists may be all you need for a hot tub - if you had joistx3 8" apart on each side and dead middle, well that's pretty good against deflection.

  2. If planned well you would be trying to place the hot tub above a load point (or multiple). This isn't viable for all decks - smaller decks may only have load points at the perimeter but that also means on smaller decks you should be able to install hot tub on perimeter. Obviously installing something super heavy in the middle of a deck is dumb and will lead to sagging. For what its worth I have seen houses sag after installing 1500 pound safes in the wrong spot (closet in middle of joist span).

"*" the asterisk for the 1% is because I have seen and replaced hollow plastic decking - just in case someone confuses that for composite.

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  • The deck is actually going to be installed over a level concrete slab. It's needed to bring the hot tub up 9 inches to meet an existing deck. It should be well supported. Commented Apr 13, 2021 at 17:20
  • @Brian Kalski Note, the deck and joist are usually designed for a combined dead and live load of 50 psf, which would be easily exceeded by the weight of the tub and water. Also depends on the location of the tub on the deck area, it might create an unbalanced condition that needs to be addressed by an engineer. Please be cautious.
    – r13
    Commented Apr 13, 2021 at 17:34
  • @BrianKalski - The concrete slab actually has nothing to do with supporting it. Note that having a concrete slab under worries me more than assures me for a hot tub. it makes me think you are using that as footings, which is not normal in most parts of the US. Just know the magic things you need are proper footings and smaller joist spacing - the concrete slab is really a non-factor... unless you go the "normal" hot tub route and put the hot tub on the slab and build around it - so it is sunken. That to me is the way to go with a hot tub.
    – DMoore
    Commented Apr 13, 2021 at 17:46
  • What I am planning is placing a 2 x 10 deck on top of a 6 inch thick concrete slab. It seems like this would be ideal because all the joists would be equally supported. The hot tub only needs to be raised 9 inches. Wouldn't the slab act as one large footing for the deck? Commented Apr 13, 2021 at 18:04
  • It sounds like you are designing a 9" high support for the tub, and the composite deck will be placed on 2x10 joists that are laid directly on the 6" thick concrete slab without posts. Something doesn't add up here.
    – r13
    Commented Apr 13, 2021 at 19:50
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To me, its more a question of the deck structure supporting it. If the posts, beams, joists, etc, are up to it, then your good. The decking itself doesn't have much to do with it, the hot tub spans several joints. The decking is just transfering load from the hot tub to the joints.

Just for good measure, you may want to put some short posts from the slab to the joists under the hot tub.

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  • Also check with the manufacturer. My composite deck needed to have 12" joist spans and it has no unusual loads. In the almost 20 years of owning my composite deck (made 100 percent out of recycled milk bottles), i have never had to worry about foot burns! Best improvement to my home, IMHO.
    – DAS
    Commented Apr 14, 2021 at 4:13
  • I like my composite decking too. If it wasn't too bid a deck, I'd go with 12" joist spans.
    – BrianK
    Commented Apr 14, 2021 at 18:16

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