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12″ foundation walls needed for double stud wall (kolbert style)

dirkgently | Posted in General Questions on

Hi all,
I just received a copy of my house plans from architect. 
 I was HORRIFIED at the 12″ foundation detail. I was able to get a quick response from him on it which is the following paraphrase.
Because I wanted to use a 2×4 exterior wall (along 4″ air space with 2×4 interior wall) then the foundation needed to be 12″ to carry both walls….HUH?
I have worked on a small subdivion 11 years ago of net zero double stud walls which were 2×4.
Yet according to this architects engineer I would need to use a 2×6 exterior wall in order to reduce my foundation wall to 8″ depth.
Before I push back on this I would like to know if anyone else has encountered such a situation….I am not one of those carpenters who “thinks they know everything”…..I find i am wrong every day about something, so do not want to jump the gun.
Thanks for any thoughts on this.

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  1. Expert Member
    DCcontrarian | | #1

    How much house is the wall carrying? For two stories you need 2x6 walls. If there's too much house above for a single 2x4 then the inner 2x4 needs to be structural, if it's structural then it needs to be supported from below. There might be some way of supporting the inner 2x4 without widening the foundation but I can't think of it right away.

    My intuition would be to go with a wall that's 2x6 on the outside, 2x4 on the inside and make only the outer part structural. Note that on the second floor you can go to 2x4 for both.

    1. nickdefabrizio | | #3

      You are saying that a two story house now needs 2x6 walls to be code compliant?

      1. Expert Member
        Michael Maines | | #11

        DC, I don't know where you're getting that info but even the 2021 IRC allows 2x4s 16" o.c. to support one story plus a roof--in other words, a 2-story house.

        1. Expert Member
          DCcontrarian | | #13

          I'm just guessing, framing isn't really my thing. I'm working backwards from why the engineer would spec a 12" foundation, rightly or wrongly he feels that the inner 2x4 needs to be supported by the foundation. Either he's looking at the assembly and saying the inner 2x4 needs to be contributing in order for the outer 2x4 to be sufficient -- or he's just mis-understanding the situation and thinks the inner 2x4 is contributing even though it doesn't need to be. Or he thinks the inner 2x4 needs to be supported even though it's not contributing.

  2. user-6184358 | | #2

    You can call out a 1/2" gap from the top plate to the truss on the inside wall. That makes the interior wall non-load bearing. It allows you to frame the exterior walls & frame the roof - then frame the interior walls. Use a truss clip or tie it to the outside wall. See DC's comment above on the two story issue. It should also be checked for 24" oc vs 16"oc.

  3. dirkgently | | #4

    To clarify my house to be.
    modified cape (shed dormers) with 2 story
    16" OC framing
    I joist for 1st floor
    in NH using 2018 code

    Absolutely will switch to 2x6 exterior wall if needed to save on the concrete.

    1. Expert Member
      Akos | | #5

      Slab on grade or a full basement/crawlspace?

  4. dirkgently | | #6

    Well I feel really dumb right now.
    went to 2018 IRC and seems 2x4 walls are NOT allowed for 2 stories and roof.
    I learned something new today.

    1. Expert Member
      DCcontrarian | | #8

      This is where experts can be annoying -- they gave you want you asked for, not what you want. Instead of drafting up dual load-bearing 2x4 walls with a 12" foundation wall they should have explained the problem with what you asked them for.

      I think you can do 2x4 on the second floor if that helps.

    2. Expert Member
      Michael Maines | | #10

      Per table R602.3(5), 2x4s 16" o.c. are allowed for a 2-story house. Where are you seeing otherwise?

      2x6 framing is required for walk-out basements that have two stories and a roof above. High-wind zones may require different framing.

      If you are trying to do 24" o.c. spacing or your walls are over 10' high, you would need to go with 2x6 for one of the walls. It's not uncommon to have one part of the house with 2x6 framing for the structural part of the wall and 2x4s everywhere else.

      I design a lot of double stud walls, including sometimes on Kolbert projects, and I have never had to use 12" foundations unless there is some other factor that requires them.

      I recommend identifying either your inner wall or outer wall as the structural layer.

  5. dirkgently | | #7

    Hi Akos
    full basement

    1. Expert Member
      Akos | | #9

      With a basement, first floor is usually platform framed, so the wall is not sitting on the foundation. The size of the wall has nothing to do with your foundation width.

      About the only thing you might need with a wider wall is some web stiffeners for I-joists to take the extra load in case the inner wall is load bearing.

      Come to think of it, even if you are not platform framing (ie I-joist hang off sill plate with top flange mount hangers or using a ledger), there is still no need for the thicker foundation.

      1. Expert Member
        Michael Maines | | #12

        The IRC does require that the foundation wall be at least as wide as the wall it's supporting, so if you're thinking of a 12" double stud wall as 12" of structure, the foundation would need to be 12". But double stud walls usually have one layer that's structural and the other is basically furring, so the foundation width should not be constrained by the framed wall dimensions.

        1. Expert Member
          Akos | | #15

          I'm guessing that is left over from the days of all brick houses. As far as I can tell, our code doesn't have similar limits for wood framed walls which makes sense as a wider wall doesn't put any additional load on a foundation. Designating one wall as the load bearing section does simplify a lot of things.

  6. buildzilla | | #14

    i found the drawings available here from 475 useful to visualize options for both exterior and interior load-bearing options (the entire pdf file is like 15mb or i would attach it directly)

    the drawings contain a bit of implied marketing for products that 475 sells (eg pro-clima) and also particular variations of wall assemblies (eg sheathing-less), but the basic structural aspects are generally applicable.

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