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I have a hot tub on my deck. I'm ready to fill it but now wondering about the weight on it. The deck is 10' wide with 2x8 joists on 16"centers. The tub is 7' x 7'. When filled with water and people it will weigh about 6000lbs.

I know it's not strong enough. I was thinking about sandwiching each joist with two 2x8s then installing a header under each end. I would like to keep it open underneath.

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    I'd recommend getting a real structural engineer to tell you what would be involved in making this work. That's the weight of two cars in a small space, and it isn't a static load once the kids hit it. Given that a failure could be life-threatening, trying to guess seems a bad bet.
    – keshlam
    Commented Jun 1, 2016 at 23:10
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    Hello, and welcome to Stack Exchange. The answer: no, no, no, and more no. As keshlam said, you need more than internet advice; get a structural engineer.
    – Daniel Griscom
    Commented Jun 1, 2016 at 23:30

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Double 2x8's spanning 10' @ 16" O.C. can barely support 160 PSF (safely) if they are douglas fir and the spa is centered on the joist. 150 PSF over ~50 Sq. Ft. would be about 7,500 lb.

As for your end beams, (3) 2x10's spanning 7' would support the weight of the joist ends with a 1.5 sq. ft. footing on each end of the beams; 4 total.

Just to stiffen things up, I would add (2) 2x8s between the existing joist evenly spaced ~5" O.C or so to help support the deck boards. With the beams and the footings, this would give plenty of support to the SPA.

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