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Community and Q&A

Leveling an Existing Slab Foundation

maine_tyler | Posted in Green Building Techniques on

I will be installing some rigid foam and two layers of plywood over an existing concrete slab to create a wood floor.

I know this subject has been covered at length here, but I am hoping to get one more knock on the head from the GBAers with a particular detail, which is the leveling of the slab (also covered at length here, I know).

It’s a garage slab, good shape, but pitched to drain. (It’s all insulated around the exterior of the slab now, 4′ down into the earth from the sill plate and the surrounding earth has proper drainage pitch).

I’ve got about 3 inches +/- to make up over a 20’x28′ space (550 sq. ft.)

The obvious thing is self leveler. But alas contractors are booked and I fear the stuff. There seems to be quite a few horror stories on botched jobs and you have to work quick. I’m not sure I can get enough hands to make it happen this fall. And it sounds stressful.

Hence I am considering stone dust or similar to level. I have extensive experience leveling the stuff for ADA trails. Obviously where it is laid thin it will not compact well. But can I get away with this if I work carefully placing the foam over it as I go. Does anyone see potential issues with long-term movement of this shim layer of stone dust? Other issues?

Thanks for any thoughts.

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  1. kbentley57 | | #1


    I can't offer any advice on the stone dust, but there's another option that I've heard of that might work well here -

    Commercial roof's typically used sloped foam insulation to point water towards drains. If might be worthwhile to see if that can be obtained. You might get lucky and find XPS insulation that has the right taper to match your garage floor. One layer of that and one layer of normal XPS would have you level, and insulated, without the busy work in between.

    1. maine_tyler | | #6

      I had a thought some time back to do some custom foam leveling. Martin correctly pointed out I may be off my rocker. Pre-tapered would be another story, but the pitch in this case isn't exactly even. I also already have the 2" type IX EPS on hand. Otherwise I'd be about it.

  2. Expert Member
    NICK KEENAN | | #2

    Another option would be cutting pressure-treated 2x4's to level the slope.

  3. Expert Member


    I've never heard of it being done, but can't think of a reason it wouldn't work, especially with two layers of subfloor.

    Now - where does the VB go? Ha ha.

    1. Expert Member
      NICK KEENAN | | #4

      The foam is the VB!

    2. maine_tyler | | #5

      I'm thinking of just throwing some poly above the wood finished floor to protect everything from interior moisture.

      In seriousness though, despite my skepticism and grievance about the lack of agreement and clarity on this issue, I might still lean towards an adhered VB, such as redguard. Then the stone dust-- make sure it's dry--then the eps, taped for extra precaution. Also considering stone dust first, then poly/foam.. who knows

  4. walta100 | | #7

    Wood floor over concrete is the riskiest option.

    Note any wood in contact with stone or concrete must be pressure treated.

    I got to ask have you done a moisture test on this slab yet?

    Cut the Plastic Sheeting
    Cut clear plastic sheeting into several 24-by-24-inch squares. Make sure the plastic has no holes or tears.
    2. Secure the Plastic
    Spread each plastic square out flat over a dry area of concrete. Tape along all four edges of the square with duct tape or other waterproof tape. Firmly press the tape to create a moisture-tight seal with the plastic and concrete. Use as many squares as needed to test all the main areas that may get flooring.
    Make sure that the concrete where you place the plastic is dry. If it is damp or even darkened by moisture, it will skew your test results.
    3. Examine the Plastic
    Wait at least 48 hours, then inspect each plastic square. First, check the top surface of the plastic to see if any moisture has beaded up on it. This is an indication of excess moisture in the air. Next, peel up the plastic and feel its underside. Moisture on the inside of the plastic, in the form of dampness or even fogginess, indicates the slab is expelling moisture in measurable amounts.


    1. maine_tyler | | #9

      No wood will touch concrete. This is a fairly common assembly, though not without it's controversies to be sure.
      Not a basement. Will have VB and insulation.

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  7. maine_tyler | | #11

    Why are we getting spammed suddenly?

  8. brian_wiley | | #12

    No idea what’s up with sudden spam, but I hope it doesn’t kill the thread as I’ve got a similar situation and am very interested to see what you land on for a solution.

  9. walta100 | | #13

    Even if the wood magically levitates above the concrete if you fail to pass the moisture test you risk cupping, finish failure and rot.


    1. maine_tyler | | #14

      Walta, are you generally opposed to the countless assemblies detailed here on GBA that discuss this approach? What about the recent spotlights about the concrete free slabs; same issue for you? I'm not sure I understand your concern in this situation.

      I am still flip flopping on attempting leveler vs stone dust. My only real fear is whether the stone dust could be sort of vibrated into unlevel clumps over many years of foot traffic. Not sure if it's a real threat though.

  10. Expert Member
    Akos | | #15

    One option is dry pack. If you can split up the slope into 4' or 8' wide sections with 2x ripped to the correct height, it should be pretty easy to get a good level. You can even leave the 2x in place. Since it takes a long time to set, it is pretty forgiving. You can read here about proper dry pack mixing:

    Another option would be to install the foam directly over the existing slab followed by tapered sleepers over the foam to get the floor flat. If you have low psi foam, you might need to use wider sleepers such as 2x3 on flat. The nice part about this setup is you can go with a single layer of plywood for your subfloor. If it is something like a workshop, you can even skip the plywood and install T&G directly over the sleepers. T&G installed like this tends to squeak, not recommended for a living space.

    1. maine_tyler | | #16

      That's an intersting option. Would you worry about the wood screed sleepers rotting, or since it's down under the foam etc. do we just call it out of sight out of mind?

    2. maine_tyler | | #17

      Does this ^ look like a deck mud? Seems he isn't even using reference sleepers, but he's obviously got experience. Looks intriguing.

      1. Expert Member
        Akos | | #18

        That is pretty much it but he is doing to something solid that can be used for a finished floor. If you are fine with concrete as a finished surface, you can do something similar if you put the rigid directly over the slab.

        Since you are mostly looking for a level surface and don't care about finish or if it cracks anywhere you can don't have to do such a good job. The sand/cement ratio doesn't matter much nor does the water content. Drypack is made with minimal water content so it can be shaped which you don't need here. The cement in the sand is mostly in there to keep it from flowing, as long as the surface is mostly flat, the foam+subfloor will hide most flaws.

        There are many ways to get a flat reference, he is using the a strip of screed that is leveled in one direction as reference for working in the other direction. Wood works well for this as you don't have to take care not to damage your reference strip or wait for it to set. It is easy to slide across it.

        P.S. About the only way to level over long distance is with a laser level, even the best box level won't be accurate enough.

  11. cody_fischer | | #19


    I am in an identical situation...with about 800 SF to do. What did you do? Stone dust or dry pack/screed?

    How did it turn out?


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