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I recently was given a 1 yr old hot tub. Im building a platform in my walk out basement for it. I wanted to pour a slab but i was vetoed.... i will attach a couple pictures. The platform will be 9 x 9. Full the tub weighs 3700 lbs and is 7x7. Plan on 1000 lbs for people. I have 3 beams. Each has 2- 2x6s on a notched 6x6 posts. So there is a 6x6 every 4.5' north south east & west. we want it as low as possible for privacy reasons. If i run 2x4 joists every 6" will that be adequate for bearing the weight? That would be 20 over the 9 feet with the 2 outsides being doubles. Beams under the ends and the middle at 4.5 feet. Should i run 2 more beams inbetween the 3 i have already?
 

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You need to add 2 more beams, and those puny 2x4’s are just going to sag.
I missed that you plan a 2x4 every 6”. That should work.

You might consider moving the platform far enough from the wall to be able to walk behind at least one side.
 

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Even with the 2x4s at 6", at a span of 4.5',some of them may still be overstressed, depending on how the weight of the water in the hot tub is distributed.

If you want it as low as possible, you can put the 2x4 frame on the concrete slab, add 2x4s between each of the ones you have (so they'll be spaced at about 18"), shim or grout under each of the 2x4s, so they're supported by the slab every 30" or less (at the perimeter and 3 places along each 2x4). That's asuming you're using 2x6 decking. If using composite or 5/4" wood decking, you'll need to bring the spacing down to 15" or less (6 interior 2x4s w/ 7 equal spaces).
 
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