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Diagonal Wood Bracing Instead of Gypsum Wall Board for Shear Resistance

alderwiser | Posted in Building Code Questions on

I am trying to follow Prescriptive Code to get my house through permitting in Island County,WA without an engineer (the tech rat race has escaped the locked down city and made its way into the surrounding countryside, so now us regular folk have to compete with spending happy millionaires inflating prices and enticing their personal GC’s to forget about affordable, necessary housing and instead to demand that the shallow pool of structural engineers time be focused on underground swimming pools, 10′ tall quadruple sliding doors and other ego boosting moment frames that of course will only be used for 2 months of the year).

Anyway, my design has changed to easily show continuous sheathed wall panels.  IRC R602.10.4.3 states that I need GWB or equivalent as the interior finish, as I don’t meet the exceptions. However, we have a mill onsite and are hoping to mill 1/2″ hemlock for interior wood paneling finish instead of GWB. The paneling will be installed vertically over horizontal, milled 2×3 chaseway (over Intello AB).

My thinking is to treat the 2×3 as Diagonal Wood Bracing DWB (45 degree angle)in the hopes that this has an in-plane shear resistance equivalent to GWB. Does anyone have any idea if this will work?

Much appreciated,

Kevin B

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  1. Expert Member


    Practically I'd imagine the strapping would be at least equivalent to drywall, but I'd consult your local building inspector to see what they will accept. They are the only option that matters. Have you thought through how you will run services in the diagonally oriented chases?

  2. Expert Member
    NICK KEENAN | | #2

    The whole idea of prescriptive code is that you do things a particular way so that a non-engineer can assess whether you meet the code.

    What if you just put a layer of drywall under the paneling?

  3. gusfhb | | #3

    Drywall is 'wicked cheap' and the board hangers will do your house in a day.
    Probably less than the complications of working around diagonal braces.

    Travelling in Arkansas 25 years ago or so I drove by a high end house being constructed in a country club setting. I was astounded that they were hanging the sheetrock on interior walls before the roof went on. Exterior walls were foam then siding. So I guess sheet rock is structural, who knew?

    1. Expert Member
      NICK KEENAN | | #4

      If you've ever worked with metal studs you'll know drywall is structural. It goes on and suddenly a rickety flimsy partition is a solid wall.

  4. Expert Member
    Peter Engle | | #5

    If I'm correct, it looks like Kevin is trying to use horizontal 2x3 strapping and vertical wood plank as the finish. So the answer is, no. This does not have equivalent shear strength to GWB. Malcolm and the others assumed that your 2x3 strapping is diagonal. Diagonal strapping provides shear resistance. Vertical and horizontal provide much less. The suggestions to just put a layer of GWB behind the lumber finish are on point. Or, if you need a nailbase for the vertical plank, use OSB or plywood sheathing. These are even stronger than GWB, though they are getting pricey recently.

    That said, the prescriptive rules for wall bracing are complicated, and even more so in high wind and seismic areas. You might eventually need an engineer anyhow. Many local code reviewers/inspectors will insist on it.

    1. Expert Member
      MALCOLM TAYLOR | | #7


      The assumption about the strapping being diagonal comes from Kevin's last paragraph. We may have misunderstood his intent.

      1. Expert Member
        NICK KEENAN | | #8

        In the preceding paragraph he describes it as " horizontal, milled 2×3 chaseway" and then he says "My thinking is to treat the 2×3 as Diagonal Wood Bracing."

        Horizontal doesn't provide the same shear resistance as diagonal. This is why when using the prescriptive code, you have to do things as prescribed.

        I still think a layer of drywall is the way to go.

        1. Expert Member
          MALCOLM TAYLOR | | #9


          Thanks, that makes sense. I assumed he meant to re-orient the strapping diagonally.

  5. Expert Member
    Akos | | #6

    Without a sheet good on the interior, it is almost impossible to create a warm side air barrier. Without a warm side air barrier, your walls are going to leak, this is bad for energy efficiency and you can end up with a lot of moisture in your walls creating mold risk.

    Wood panel walls look great, but they need to be installed over a good air barrier. As others have said, drywall is cheap, satisfies your official and a good air barrier when detailed right. Since it is not your finished surface you can do a very rough taping job with no sanding to seal it up.

  6. alderwiser | | #10

    Thanks for all the great responses, sorry it took a bit to get back. In clarification we wanted to run horizontal 2x3 with vertical wood panelling (easy for running wire), but knew this wouldn't have shear so the question was would the 2x3s diagonal be equivalent to drywall. It seems like probably not, and to DCContrarian's point, I gotta keep in the bounds of code here.

    So, we will definitely throw up drywall, but are still debating on best/efficient way to do the interior Air Barrier: Either mud and finish GWB or leave the seams etc and use a smart membrane (Intello) underneath. If the latter, then best practice is (yes you guessed it) run 2x3 strapping over membrane for a chaseway gap and separation from drywall work.

    Any more thoughts here?



  7. Expert Member
    BILL WICHERS | | #11

    Airtight drywall is pretty easy to put up. I've done airtight drywall (I actually ALWAYS do it that way, since it doesn't hurt and usually helps), and I've detailed vapor retarders to be airtight too -- often in the same wall (belt and suspenders and all that). I would use the vapor retarder if you need it as a vapor retarder, but I wouldn't put it up only as an air barrier -- I'd just use the drywall for that. If the drywall isn't going to be your finished wall surface, you only need to mud and tape the wall -- you don't need to do a full sanding to get it paint ready, you just need to knock stuff down until the surface is flat enough to not interfere with your final wall surface material.

    Regarding drywall as structural, it's really not supposed to be. Yes, drywall does lock metal studs together to solidfy a wall (metal studs are amazingly floppy before the drywall goes up), but you're not supposed to rely on drywall as a structural material for something more critical like a shear wall. In the metal stud example, if you are using heavier gauge structural metal studs to support a second floor, the studs are what holds that floor up -- not the drywall. The studs can hold the floor up even without drywall in that case. The drywall is not a structural part of the wall assembly as a whole.

    If you need a real shear wall, I WOULD NOT trust the drywall for this. A simple test to show you way: try pulling a fastener sideways (shear force) through the last 1/2" of a drywall panel. It's not super difficult to do. Try the same test with the same fastener in a piece of even 3/8" sheathing and you'll not be able to get the fastener to budge. The reason drywall works at all is that it is held up with around a bazillion fastners compared to what would normally be used for wood sheathing. If you need a real shear wall, I'd put 3/8" or 1/2" sheathing under the drywall, then 1/2" drywall over that. With a wall like this, 5/8" drywall doesn't gain you much, since you have that wood sheathing directly under the drywall to beef up the wall. For normal walls, I use on'y 5/8" drywall since it's a pretty cheap upgrade that makes a very noticeable difference in the solidity of the wall (better sound blocking, less waviness, much more solid-feeling).


    1. Expert Member
      Michael Maines | | #12

      Bill, while the main purpose of drywall is obviously not to provide lateral support, it is one of many code-approved options. It won't work in every, or even most situations, but you can read about the options in table R602.10.4:

      The tables above that one show the requirements in different situations. Not every project needs the highest levels of lateral support.

      1. Expert Member
        BILL WICHERS | | #13

        Thanks for that info, I was not aware of some of those limited locations it's permitted. It's worth noting that the code info requires pretty aggressive fastening schedules in these cases, not the minimums the drywall manufacturers specify.


    2. alderwiser | | #14

      Bill thanks for your input.

      I am not looking to use the gypsum for the primary structural sheathing. Prescriptive Code just requires it as the interior finish, although there are exceptions based on how much exterior wall sheathing you have as a percentage of your wall length. From IRC:

      "R602.10.4.3 Braced Wall Panel Interior Finish Material
      Braced wall panels shall have gypsum wall board installed on the side of the wall opposite the bracing material. Gypsum wall board shall be not less than 1/2 inch (12.7 mm) in thickness and be fastened with nails or screws in accordance with Table R602.3(1) for exterior sheathing or Table R702.3.5 for interior gypsum wall board. Spacing of fasteners at panel edges for gypsum wall board opposite Method LIB bracing shall not exceed 8 inches (203 mm). Interior finish material shall not be glued in Seismic Design Categories D0, D1 and D2."

      In regards to the Air Barrier I'm still figuring out if I need it to be a vapor barrier also. After an hour or so on GBA it seems not. Would it be possible to get a secure air barrier by just taping with airseal tape (Vana or Siga tape) and caulking seams (like you would for an exterior AB on plywood) on the drywall, as it will be covered anyway? My thinking is that would be a lot quicker than taping and mudding the regular way.

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