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Community and Q&A

Does I-Joist Spacing Need to be Exact

thevinegaroon | Posted in General Questions on

I am planning on using I-joists spaced 16″ oc to support my second floor. There are two points in the plan where the joists begin on an interior wall to accommodate the L-shaped stairwell, which will be open to the second floor ceiling.  When I lay out my joists 16″ oc, the final joist in each segment isn’t spaced quite far enough to be positioned under the wall above (one falls three inches short and one falls one inch short). I’ve attached a picture. The reference lines in the picture are footers, not first story walls, but just imagine the joists resting on walls that bear on those footers, with the open space being a staircase.
In those two cases, would it be okay to set that final joist 19″ oc and 17″oc respectively? The joists are sized so that 19.4″ spacing is acceptable to the manufacturer. I’m assuming that the other option (which I’ve displayed in the drawing)  is to put an extra joist in each location, which isn’t a huge deal, but these aren’t cheap so I’d rather buy only as many as I need.
So, to sum it up, can I eliminate that extra joist (the one on the right) in those two locations? If not, is what I’ve displayed in the drawing the correct way to handle the situation?
Thanks for advice!

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  1. Expert Member
    Michael Maines | | #1

    Zoe, based on your description, yet you can eliminate that joist. The downside is that the subfloor may have to be custom-cut, requiring more labor and material. Your builder should be able to tell you which one will be less expensive. Production framers usually prefer to have framing "on layout"--in this case, 16" o.c. no matter the situation. But frame-to-finish carpenters and remodelers will more likely be able to work with off-layout framing.

    1. thevinegaroon | | #2

      That's great news, thank you! Using 4x8 sheets of OSB, none of the joints will fall on those joists, so eliminating the extra joist will be pure savings. I will have experienced framers installing, but using the building permit I have I AM the builder...and the designer, estimator, purchaser, HR lady, IT department, the list goes a couple hundred bucks in savings is money in my wallet! Thanks for the help!

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