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Hot tub foundation – Stone dust?

JML | Posted in Construction Techniques on October 22, 2005 12:47pm

I am going to install a hot tub and I am considering options for how to prepare the site for placement.  The filled weight of the tub is around 6000 lbs.  The hot tub supplier suggested a 4″ bed of packed stone dust under a deck made of 2×6 pressure treated with the joists on 8″ centers.  I think that concrete is the best option but the homeowner is reluctant to let me put in a 4″ slab given that they may want to re-landscape in a couple of years and want to avoid the possible demolition of a 4″ slab.

Does anyone have thoughts on this?  Will the stonedust / deck option be adequate?

Thanks

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  1. DavidThomas | Oct 22, 2005 03:51am | #1

    Will the stonedust / deck option be adequate?

    That's what I'm using, kind of.  Somewhat less has worked just fine.

    I've got a 2x6 frame, filled with pea gravel with 2x6s laid flat (and cut into the frame) where the manufacturer called out their pressure points (four of them in the middle for mine). 

    It's been good for 4 years.  4 freeze-thaw cycles.  Temps to -25F.  No pitch changes, no plants going out from underneath (I put down typar).

    Concrete pad or gravel/stone filled frame, either will be fine.

    David Thomas   Overlooking Cook Inlet in Kenai, Alaska
    1. brownbagg | Oct 22, 2005 05:03am | #2

      I would put in a six inch slab, no rebar , no wire. cut the slab on 1 foot square and cut deep, like three inches. If she has to remove later you can pop the cuts with a twenty pound sleds and it will pop at the one foot cuts. easy to haul off. the extra two inches of concrete will counter act the rebar in the four inch slab. use a weak concrete like a 2500 mix pea gravel.

      Edited 10/21/2005 10:03 pm by brownbagg

  2. User avater
    Matt | Oct 23, 2005 01:57am | #3

    Personally, I think a slab under a deck is overkill, although I'd use what we call ABC gravel or crush-n-run.  It's a mixture of fines, and larger sized aggregate up to 1.5" and compacts very well.  If you do use the stone rather than a slab, you could pick up a couple of bags of Portland cement and put the stone in a mixer, maybe 1 shovel full of cement per mixer.  Then compact it in dry.  That would make the stone  set up a bit so you would not have to worry about erosion destroying the "building pad", however it would still be fairly easily removable later.

    Or, how about this: HT sits directly on slab and decking is built up around the HT to make it easier to step into/out of.

  3. dustinf | Oct 23, 2005 02:01am | #4

    I'm starting a project in a couple weeks with similar situation.  I'm going to used a 4" thick compacted stone base with concrete pavers at the required pressure points.



    Edited 10/22/2005 7:03 pm ET by dustinf

  4. storme | Oct 23, 2005 04:46am | #5

    Assuming crushed granite is the same as stone dust, I've been impressed with how stable the stuff is. I would guess it would work fine but I'd worry about it getting saturated. A suggestion, why not mock up a section pack it well then flood it and see how stable it is?

    The other thing is, if she's thinking of moving the hot tub it may not matter if it shifts 1/2" make the plumbing joints so they'll handle some movement, put a disclaimer in your contract and consider it an experiment.

  5. User avater
    Mongo | Oct 23, 2005 07:21am | #6

    Stone dust will be fine as long is it is contained on the sides.

    If you dampen it and compact it with a plate compactor, it'll be pretty darn stable.

    Stonedust is what I use as the bed for all dry-laid bluestione patios. I'll often use it as a base under slabs as well.

    1. gvriskbusiness | Oct 23, 2005 05:28pm | #8

      Sorry I'm late on this thread, but since Mongo mentioned the bluestone...do you think it would be okay to put the HT on top of a dry laid BS patio on 4 inches of compacted stonedust on 12 - 16 inches of compacted of 3/4 crushed stone (the area is part of a 2500 sqft BS patio on the side of a 'mountain' that runs right up to my foundation, so there's a lot of perf pipe drainage in the CS)?  BS has been there for 3 years, very stable through lots of hard freezes and very wet thaws.  I jsut don't want to ruin the BS patio while thinking I'm putting in an 'improvement' - yeah, something SWMBO wants and I get to maintain....Looks good from my house....

  6. Bruce | Oct 23, 2005 05:17pm | #7

    I used 1" washed stone about 8" deep under a foundation of 4x6 pressure treated timbers under our tub.  The stone has no way of holding moisture as could happen with finer material compacted in place.  This prevents any heaving if you're in a freeze/thaw area.  Haven't seen any movement in 2 years.

    Bruce

    Between the mountains and the desert ...

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