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Old 06-09-2011, 10:05 PM   #1
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Hot Tub Foundation

Hello everyone!
I am getting a hot tub and am building a foundation for it. I need opinions to see if this will work.

I bought (4) 8' treated 2X4 and will use a total of 12 galvanized nails to make a box out of them to sit on top of the ground. Then, I am going to pour pea gravel inside the box (and make it level.) I am going to put treated plywood on top of the pea gravel and the hot tub will sit on top of that. Any reason for this not to work?

Thanks all!


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Old 06-10-2011, 06:24 AM   #2
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doesn't the tub manufacturer have support specs for your unit ? if it were me, i wouldn't bother w/the plywood & substitute compactible granular base ( any quarry ),,, pea stone shifts & can't be satisfactorily compacted,,, you've already considered existing soils condition/type & susceptibility to freeze/that, correct ? that's MY opinion


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Old 06-10-2011, 07:39 AM   #3
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...yea...cant imagine pt plywood is going to last very long. Since you're making a frame for it any way, why not fill it with concrete?
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Old 06-10-2011, 07:53 AM   #4
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The actual per square foot load of a hot tub is not that great, so it will generally work on decent soils with limited improvement. For example, a four foot deep hot tub exerts about 250 pounds per square foot. Compacted angular gravel or road base can support easily 2000 psf.

The key is to remove unacceptable soil first, before you place your support layer. Unacceptable soil would include organic soil, uncompacted fill, wood, debris, or other waste material, soft clay, and soft silt. Once you get down to decent mineral soil, meaning sand, gravel, hard clay, glacial till, or dense silt, you can backfill to grade with structural fill, compacting in six inch lifts as you go.

Of course, you can read the manufacturer's recommendations on base preparation, as previously suggested. You can also discuss foundation requirements with the building inspector, if a permit is required. I second the opinion to skip the plywood, it is unnecessary, and likely to rot over time. Similarly, if you feel you need a barrier for the fill, make sure the PT you use is rated for ground contact, most PT sold at the big box stores is NOT rated for ground contact.
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