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Understanding Rim Joists

josephny | Posted in General Questions on

I hope these are appropriate questions for this forum.

1)  Is it okay to use pressure treated lumber for rim joists?  I’ve seen mixed recommendations and comments about shrinkage.  But, all else the same, I’d sure like the extra protection.

2)  My floor joists extend completely on top of the foundation walls such that the rim joist (nailed or structurally screwed to the outside edges of the floor joists) would not be supported underneath (and not be supporting anything on top of them).  My understanding is that rim joists are solely for the purpose of providing lateral support for the floor joists (so they don’t tip over or move closer/farther from each other).  If that is correct, then unsupported rim joists are fine, right?

Thank you!

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Replies

  1. Expert Member
    Michael Maines | | #1

    Joseph, I am not aware of any reason you couldn't use pressure treated lumber for your rim joists, other than the shrinkage you mention, which can be extreme--I've seen as much as 1/2" shrinkage on larger-dimension PT.

    Unless you have a cantilever or other unusual situation, you don't need a continuous rim board but you do need blocking: https://codes.iccsafe.org/content/IRC2018/chapter-5-floors#IRC2018_Pt03_Ch05_SecR502.7. (The iCodes links aren't reliably going to the correct section--check out R502.7.)

  2. Expert Member
    Peter Engle | | #2

    If you are using I-joists, or in certain loading conditions with sawn lumber, the rim board is also required for compressive strength. In this case, it must sit on the sill. The construction you describe in #2 would not be allowed. Your detail also makes air sealing between the framing and foundation a bit more difficult. Is there a reason you need to build it this way?

  3. josephny | | #3

    Michael: Thank you for the advice. I was going to use blocking anyway.

    Peter: Using 3x10 joists. It's already built that way. I'm planning to spray foam along the inside of the rim joists.

    Here's a pic:

  4. Expert Member
    Michael Maines | | #4

    Joseph, it looks like you have balloon-framed walls, with the studs extending down to the sill. That's a different situation; the studs brace the joists from rolling. If you don't have fireblocking at the subfloor level you should add some. I'm not sure what your code official would say but I don't think you need to add a rim joist unless you want it as a nailer.

    1. josephny | | #5

      Yes, that's exactly the kind of framing.

      The wall studs are nailed to the floor joists and are supported by the sill plate.

      So the fireblocking would be just to the interior of where I would put the rim joist, like in the attached?

      1. Expert Member
        Michael Maines | | #6

        Yes, or it can be horizontally installed between studs at the same plane as the subfloor. 2x lumber is probably the simplest. Anywhere a vertical cavity meets a horizontal cavity should have fire blocking.

        1. josephny | | #7

          Got it -- makes sense.

          Thank you again so much.

          While I know far far more about construction than most people I know, I understand that I know so very little compared to experienced builders and building scientists. I think the vast majority of people don't have a clue how much knowledge and skill is required to do this.

  5. DrJimesTooperMD | | #8

    I'm hoping to piggyback onto this question. My house is not balloon-framed, but has a similar issue to what Joseph posted, where my floor joists extend completely on top of the foundation walls and the rim joist is attached to the ends of the joists, so the rim joist is not supporting any load above and is unsupported below. What are the implications if one wanted to attach a deck ledger to this rim joist?

  6. Expert Member
    PETER G ENGLE PE | | #9

    You can't attach a deck ledger to a cantilevered rim joist. Most of the load on a deck ledger is vertical, and with the rim joist unsupported, you're counting on only the bending strength of the nails fastening the rim joist to the floor joists. That's a very weak connection.

    In your case, it would be best to support the deck independently from the entire house. Run PT posts down the exterior foundation wall to the footings, and set a beam across the posts to support the house side of the floor joists. You should leave a small gap between the deck beam and the house for drainage and debris removal. Depending on height, you will need some sort of diagonal bracing to stop the deck from swaying. You can still fasten the deck to the house, using Simpson deck ties to hold it in if you want to. There are several different options.

    1. DrJimesTooperMD | | #10

      Hi there Peter. Would using structural screws (let's say Simpson SDWS timber screws) to attach the rim joist to the joists be acceptable? My understanding is that some of these washer-head structural screws have impressive amounts of holding power.

      1. Expert Member
        NICK KEENAN | | #11

        The problem is the rim joist has no supportive power. And it's probably attached to the joists just with nails.

        I second the idea of having the deck be free-standing on posts.

        1. DrJimesTooperMD | | #12

          I totally agree it would be a bad idea if the rim joist was attached with just nails. I'm wondering in the hypothetical situation where the rim joist was attached to the joists with structural screws.

          1. Expert Member
            MALCOLM TAYLOR | | #13

            The problem is that the structural screws will be going into the end grain of the joists, which has very limited holding power. Typically ledgers are attached to a rim-joist which relies on sitting on the foundation for its support.

            The only way I can think of this working is by using some mechanical attachment between the existing rim and floor joists. It might be as simple as some Simpson hardware, but it would have to be specified by an engineer.

      2. Expert Member
        NICK KEENAN | | #15

        I would read the "Ledger Connections" section of this page:
        https://www.strongtie.com/resources/literature/deck-connection-fastening-guide

        1. Expert Member
          NICK KEENAN | | #16

          I'm thinking the Deck Tension Tie - DTT2 - for your situation.

          1. DrJimesTooperMD | | #17

            I plan to use those for lateral-load support, but it sounds like they could potentially perform double-duty to alleviate concerns about the vertical load on the extended rim joist. Thanks!

      3. AlexPoi | | #25

        The problem is not just the connection but the bending moment it would apply on your joists. Think of it as a big lever. The further away the ledger is from your vertical support, the bigger the lever effect (bending moment) is.

        This moment can generate some pretty big tension forces on the top surface of your joist, enough to split them.

  7. DrJimesTooperMD | | #14

    Hi there Malcolm, in my situation the foundation is a little unique, where sitting on top of the concrete is a 6"x6" beam, then the floor joists sit on top of that beam. If I were to mount a ledger in the current setup, half of the attachment would be face-screwed into the joists, the other half would be screwed into the side of this beam, which sits on the foundation without issue.

    1. Expert Member
      MALCOLM TAYLOR | | #18

      If you can get support from that 6"x6" I don't see an issue.

      1. DrJimesTooperMD | | #19

        The 2x10 ledger will have around 2/3rd of it backed by the 6x6 beam, definitely enough room for at least a couple rows of structural screws. I also could install blocking between the joists and screw the top half of the rim joist into that blocking.

        1. Expert Member
          BILL WICHERS | | #20

          If that 2x10 is mostly secured to the 6x6 along the entire length, and just sticks up above it along the rim joist to make the floor levels even, then you should be OK -- the 6x6 will carrying the actual load. I would try to avoid anything fancy to try to adapt the rim joist -- try to get all the load carried by that 6x6 if at all possible.

          If you find you have to do something with that rim joist, I'd have an engineer look at this and recommend something that will work.

          Bill

          1. DrJimesTooperMD | | #23

            Thank you Bill, I appreciate it! When I position the ledger I will have it so that as much of the 6x6 is available to screw into as possible.

      2. Expert Member
        PETER G ENGLE PE | | #21

        I would still say no, or at least not without an engineered design. Wood members cannot be supported only by friction, or by fasteners in bending. The wood should be solidly supported from below, with the fasteners being used to hold the wood in place, not to transfer vertical loading. Ledger-to-supported-rim joist connections are a special case where the engineering work is already built into the prescriptive requirements of the code and manufacturer's (Simpson) handbooks. If you attach your ledger directly to the 6x6, then you're much closer to the prescriptive design conditions than you are if you attach a ledger to an unsupported rim board that is fastened to the 6x6.

        1. DrJimesTooperMD | | #22

          I see what you're saying, and I appreciate the advice. It sounds like I should just forgo any sort of a rim joist solution and instead look at securing the 2x10 ledger directly to the 6x6 as much as possible.

          If I were to keep the 2x12 rim joist, I would have the ability to fasten the ledger to the 6x6, by going through the rim joist, if that makes sense. I would just need to use some 10" Simpson SDWS timber screws, but it would go through the ledger, then the rim joist, then the 6x6 beam, and still have a small screw tip poking out of the backside of the 6x6 beam for the inspector to see.

          1. Expert Member
            BILL WICHERS | | #24

            Do whatever gets you connected to that 6x6, don't rely on the unsupported rim joist as a structural element.

            You're going to have a sandwich here that might be a problem with the load tables for those fasteners. The usual application is to screw one thing directly to another, you're going to have something in between that adds thickness. This may or may not affect the allowable shear loads for those fasteners. I have built somewhat related assemblies with steel, and it IS possible, the the way the loads get transferred between structural elements gets more complex. You might try asking the Simpson people about this if they don't specifically mention it in their engineering data for those fasteners.

            BTW, BE SURE to use enough of those fasteners for your deck's design load. DIY decks have a reputation for failing at parties because some part wasn't strong enough, and the ledger is a classic failure point. As I have said on here many times, Always put SAFETY FIRST. Structural failures can and have killed people.

            Bill

  8. DrJimesTooperMD | | #26

    Will do Bill, thank you! I plan to go very much overkill with the Simpson timber screws to attach everything, while sticking with their recommended minimum distances, of course. I have reached out to Simpson about some of these questions, but they are understandably reluctant / unwilling to provide structural advice, due to codes / laws / regulations.

    1. Expert Member
      MALCOLM TAYLOR | | #27

      This may be one of those occasions when a free-standing deck makes sense.

      1. DrJimesTooperMD | | #28

        I think you may be right. I did have a lengthy conversation with an engineer at Simpson Strong-Tie today, who stated that the "ML"-series angle brackets may be a doable solution for strengthening the connection between the rim joist and the floor joists. As always, he suggested consulting with an engineer or code official.

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