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Community and Q&A

Hardwood Installed on Slab

craigcarter | Posted in General Questions on

I have a master bedroom addition we are building for a client.

Foundation will be a slab with masonry perimeter similar in design in the attached document.

Existing finished flooring is over a framed floor system. A few factors provided the decision to use a raised slab instead of a crawlspace for the addition.

My initial plan was to install 2×4 sleepers flat to the slab, Advantech subfloor and then 2.25″ hardwood flooring. The finished flooring will be at the same height as the existing finished flooring of the house.

I felt that the 2×4’s attached to the slab and then the subfloor with the hardwood may provide a similar feel to a framed floor system.

In doing some research I came across this Joseph Lstiburek article “Walking the Plank” also attached.

The recommendation in the article was to forgo sleepers as a past method and install the plywood directly to the slab after the installation of a proper vapor barrier.

I feel like the 2×4 sleepers plus the subfloor and then the hardwood floor makes sense…

Would I be better simply gluing down the plywood and then fastening the hardwood flooring to the subfloor and forgo the sleepers?

Would I be creating other issues with having a closed cavity created by the sleepers that could create other potential issues?

Then there is the last couple of sentences discuss a floating floor? Would I not glue the subfloor to the concrete slab? I planned to weave into the existing floor or even if I head off the floor at the new opening?

Am I overthinking this assembly?

Thanks in advance.

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  1. Expert Member


    Are you sure you can set load-bearing exterior walls on one width of brick? It's not something I've seen before.

    1. craigcarter | | #2

      Good question....does it change when the cells are filled and slab poured up against the brick?

      1. Expert Member
        MALCOLM TAYLOR | | #5

        Maybe, but the brick would need some capillary break and insulation or the slab will always be damp and cold.

        If you want the look, your best bet is probably those thin fake bricks.

        1. craigcarter | | #11

          Thank you. While I see things as they have been done locally it doesn't always mean it is the right way and I should ask more questions.

      2. Expert Member
        NICK KEENAN | | #10

        It seems the reason they laid it out the way they did is so there is a layer of insulation between the slab and the wall. But that means that the masonry part of the wall is narrower than the wood frame wall above.

        There aren't dimensions on the drawing but if it's to scale the wall looks like a 2x8. A 6" CMU is 5-5/8" thick or 1-5/8" less than a 2x8. I think that's OK. There's be no problem putting a 2x6 wall on that and I assume the 2x8 isn't needed for strength but because you want the wall thickness for insulation.

        1. Expert Member
          MALCOLM TAYLOR | | #13

          Everything looks proportionate to me. The wall is 2"x6", the slab 4", and there are four courses of brick. They show a 4" wide half-height CMU to the interior of the brick.
          You might be able to get away with a single course of 4" CMUs instead if they were reinforced.

  2. Expert Member
    NICK KEENAN | | #3

    Are slabs usually insulated in your climate? Is the slab insulated already?

    1. craigcarter | | #4

      Slabs typically insulated at the perimeter and 24" inward.

      1. Expert Member
        NICK KEENAN | | #6

        OK, I looked at the diagram, shows 24" of insulation. There are no dimensions but it looks like a 2x8 wall with 2" of exterior foam. That seems like an enormous amount of wall insulation for an uninsulated slab. The 24" around the perimeter protects the footings from frost heaving but doesn't keep the interior warm.

  3. Expert Member
    NICK KEENAN | | #7

    Read the Lstiburek article. I'm dubious of his initial recommendation to keep the slab dry by waterproofing the bottom and sides, I think his later recommendation of a fluid-applied coating on the interior is safer.

    He doesn't say exactly how his floating floor goes down. I've done click-lock over concrete and as long as the concrete is reasonably flat it works well. But I'm not sure how a nail-down wood floor would go. Generally glue doesn't work unless both surfaces are very close to flat, and concrete is hard to get that flat. You'd also need really short nails to nail tongue and groove through the tongue and into 3/4" plywood over concrete, maybe 1" nails. That just sounds really short for hardwood.

    An advantage of sleepers is you can shim them to account for irregularities in the floor. And you can use regular length flooring nails. The sleepers don't have to be that well attached to the concrete, once the flooring is nailed on it creates a very heavy, very rigid structure.

    1. craigcarter | | #9

      I am in agreement in feeling subfloor directly applied to the concrete slab may have difficulty...I've thought about the nails and leveling as well...if using sleepers and then subfloor with hardwood installed on top I think that is the way to go. Coming to a final direction sleepers or no sleepers is needed as that will determined the finished foundation/slab height.

  4. craigcarter | | #8

    In hindsight I should have left the foundation detail off...what we are doing is similar to this not exactly and maybe unrelated to my main question of the hardwood flooring. However, the detail I have is gravel, perimeter foam, poly and then concrete slab followed by either 2x4 sleeper, subfloor and then I really need the 2x4 sleeper is my primary question.

    1. Expert Member
      NICK KEENAN | | #12

      For a nail-down floor the only other option I can think of is a double layer of subfloor. The subfloor has to be a single rigid piece.

      Not sure that the sleeper has to be 2x4, 1x4 might work just as well.

      I'd definitely put a fluid-applied vapor barrier between the slab and the sleepers.

    2. Expert Member
      MALCOLM TAYLOR | | #14

      I agree with you and DC. The sleepers will be a more forgiving, resilient, and easier to build detail.

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