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Old 01-11-2018, 04:26 PM   #1
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hot tub base - pour concrete into a retaining structure?


Hey all - I plan to put my hot tub on the ground, but there is a 12 inch drop across that area. See the attached image for my solution. Basically, I'd build a "box" with 6x6 planks, two levels high. I'd dig down 12 inches on one side - the other side would sit on the ground.

After a bunch of research, many have been successful setting their tubs right onto crushed stone/crusher run. That was my plan - I'd build my retaining box then fill with crusher-run to the top, compact (gas powered) and level it, then sit the tub on the top.

However, it voids the warranty if it's not on concrete.

OK..so then, I'd fill in half with crusher run, the top it off with poured concrete?

I plan to drive rebar into the ground on all sides. Large steel L brackets on all corners (through bolted). It may be overkill, but I also planned to take a steel rod and span from low-to-high-side and bolt it on the outside.

Full, this tub will be like 5000 pounds.

I'm suddenly concerned my retaining structure (especially on the low side) may not support the weight. I'm not sure how much weight is pushing down, and/or if that weight is also pushing sideways (wanting to explode out my 6x6 beams). I suppose the weight of concrete and crusher run is similar...but concrete is solid, so presumably would exert less outward pressure.

Thoughts here?
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Old 01-11-2018, 04:47 PM   #2
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Re: hot tub base - pour concrete into a retaining structure?


If you are pouring concrete there is no need for the 6x6 unless you like getting slivers in the feet.
Would you be looking for a step on the 12" end?
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Old 01-11-2018, 04:52 PM   #3
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Re: hot tub base - pour concrete into a retaining structure?


No way would I do it that way!
No location in your profile so hard to say what the correct way to do it is.
If I had to do it no matter where you are I'd be poring proper footings for that location and using, concrete block not pressure treated lumber.
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Old 01-11-2018, 04:56 PM   #4
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Re: hot tub base - pour concrete into a retaining structure?


Should have mentioned - I'm in Atlanta. The earth is red clay.

In my image - the face that your looking at will actually be a deck that meets the hot tub about halfway up.

So the hot tub isn't down inside the 6x6 structure, I need to fill this box to the top. Based on the grade, the "top" would be level with the earth on the high side.

Finally - I wanted to add an addendum - on the far back/lowest corner, I'm considering digging 3 feet down, putting a 12-inch concrete form in it with rebar sticking out about 12 inches. The 6x6 corner would sit on top, and I'd drill holes in the 6x6s and push them down over the rebar. But maybe this is overkill...
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Old 01-11-2018, 05:00 PM   #5
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Re: hot tub base - pour concrete into a retaining structure?


Quote:
Originally Posted by denemante View Post
Should have mentioned - I'm in Atlanta. The earth is red clay.

In my image - the face that your looking at will actually be a deck that meets the hot tub about halfway up.

So the hot tub isn't down inside the 6x6 structure, I need to fill this box to the top. Based on the grade, the "top" would be level with the earth on the high side.

Finally - I wanted to add an addendum - on the far back/lowest corner, I'm considering digging 3 feet down, putting a 12-inch concrete form in it with rebar sticking out about 12 inches. The 6x6 corner would sit on top, and I'd drill holes in the 6x6s and push them down over the rebar. But maybe this is overkill...
So you have already jumped into the deep end. No advice needed.
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Old 01-11-2018, 10:18 PM   #6
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Re: hot tub base - pour concrete into a retaining structure?


Whatever you do don't leave any of the rebar exposed, it all should be covered with concrete. If any of it is not covered by concrete it will rust and the rust will work its way into the concrete in the expansion caused by the rust will split the concrete.
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Old 01-12-2018, 05:37 AM   #7
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Re: hot tub base - pour concrete into a retaining structure?


Quote:
OK..so then, I'd fill in half with crusher run, the top it off with poured concrete?

I plan to drive rebar into the ground on all sides. Large steel L brackets on all corners (through bolted). It may be overkill, but I also planned to take a steel rod and span from low-to-high-side and bolt it on the outside.

Full, this tub will be like 5000 pounds.

I'm suddenly concerned my retaining structure (especially on the low side) may not support the weight. I'm not sure how much weight is pushing down, and/or if that weight is also pushing sideways (wanting to explode out my 6x6 beams).
Ayuh,.... Once ya get the stone compacted, there's virtually no outward forces, just downward,....
A 4" cap of concrete is more than enough,...
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Old 01-12-2018, 10:26 AM   #8
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Re: hot tub base - pour concrete into a retaining structure?


If using a pad - I think you need to bond the tub to the rebar.
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Old 01-12-2018, 12:41 PM   #9
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Re: hot tub base - pour concrete into a retaining structure?


Overthinking, maybe.

If your hot tub is 6' x 6' = 36' SF.

5000 lbs. divided by 36' SF = 139 lbs per SF.
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Old 01-12-2018, 10:03 PM   #10
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Re: hot tub base - pour concrete into a retaining structure?


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Overthinking, maybe.

If your hot tub is 6' x 6' = 36' SF.

5000 lbs. divided by 36' SF = 139 lbs per SF.
I think your 5000 pound estimate is way too high and the size you estimated is way too small. A few years ago we had one that was about 8'x8" and it held about 280 gallons. So even if you go on the high side and say 300 gallons that's only about 2500 pounds. Using those numbers you come out with about 51 psf. Which is less pressure than you plan something when you are standing on it.
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Old 01-13-2018, 06:39 AM   #11
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Re: hot tub base - pour concrete into a retaining structure?


Agreed Msradell,

We have had 3 - 8' x 8' spas at our different homes and I just built CCA decks under them. Kept the decks wood properly sealed and loved them all.
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Last edited by Gregsoldtruck79; 01-13-2018 at 04:11 PM.
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