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Community and Q&A

Floor Deflection for Comfort and Sections of Tile

BrunoF | Posted in General Questions on

I am working with the engineer to finalize my new house plans and am trying to sort out how far above code we need to be to get a tight floor and to properly support tile in bathrooms and the mudroom.  Please note that all the floor joists in this house will be 11 7/8″ TJIs and right now the maximum joist span is around 17′.

I understand that L/360 will meet code but may not feel solid and may not be ideal for tile sections.  I also understand that within a set joist depth that I can reduce deflection by reducing the spacing and upgrading the grade of the TJIs.  I also understand that we can design to L/480 or above.

Given this information, what is the best way to quantify a “stiff” floor?  for example, will L/480 result in a stiff feeling floor?  Is there a guideline for staying a certain % below max spans listed on TJI tables that will give a stiff floor?  Would my tiled sections need to be even stiffer?

Thanks!

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Replies

  1. Expert Member
    Akos | | #1

    What we humans terms as good floor is actually the natural frequency of the floor. Natural frequency is a combination of span and deflection so deflection alone does not guarantee a good floor.

    Generally bellow 20' span, anything near or a bit above code min is a good floor, once you get past 20' is where you have to be careful with natural frequency. This is a good read about the topic:

    https://seaoo.org/downloads/Presentations_CONF/seaoo_2012_conference___woeste___design_of_wood_floors_to_mitigate_floor_vibration_problems.pdf

    Tiles only care about deflection. Some large format tiles might need L/480 or higher, I would check the installation requirements of the tile of your choice.

    1. BrunoF | | #3

      Akos, Thanks! I assume that you comment on sub-20' spans applies to engineered lumber as I am in a house with 2x10 southern yellow fine floor joists spanning 15' or so on 16" centers and they are bouncy.

      1. Expert Member
        BILL WICHERS | | #4

        When you get up near the extremes in the span tables for any side of lumber, you start to notice more "bounce" in the floor. Sometimes solid blocking will help a little, but usually you either have to sister the joists, add a 2x4 on the lower edge as a lower "flange", or put a header under the center of the span to essentially cut the span in half.

        Bill

      2. Expert Member
        Michael Maines | | #5

        Sawn lumber floors tend to have less vibration than an engineered wood floor with the same calculated deflection, because the variation in wood density makes it less likely to find a common harmonic frequency than with the more-uniform density of engineered lumber.

        I typically specify that at and above a 16' span, deflection should be L/480, even when L/360 meets code. I also follow the lead of professional engineers I work with and limit maximum deflection to about 0.6", regardless of the span.

      3. Expert Member
        Akos | | #7

        Code tables have a lot of legacy values that don't necessary meet even code min deflection values. I've recently run into this while sizing a beam, the code min beam was way undersized for deflection based on detailed calculation.

        The 2x10 at 15' is in this category, at best it is borderline.

        1. Expert Member
          Michael Maines | | #8

          I was going to argue with you, as I often find it faster to size my own beams/joists/rafters/headers but then check code tables, and I usually find that they are right in line. But I just checked your example, using #2 SPF (MOE=1,400,000 psi), 40 psf LL + 10 psf DL and L/360 deflection, and found that the joist should be 9.8" tall, not the standard 9.25". The MOE would have to be 1,700,000 psi for a 2x10 to work mathematically, which might be possible with southern pine or fir but more than I've heard of for SPF.

          1. Expert Member
            KYLE WINSTON BENTLEY | | #9

            Don't forget that SYP underwent a design value revision in 2013. It's no only about as good as SPF or white pine. No. 2 visually graded is still 1.4E6 psi.

            I'm surrounded by it here in the south east. The growth rings on newer stuff could be up to a half inch apart in some lumber I've come across. I use to prefer it, but now it's just heavy, dense, and harder to work with IMO than SPF.

          2. Expert Member
            Michael Maines | | #10

            Response to Kyle, #9: NELMA revised the values for SPF as well; smaller dimension members went from 1.4 to 1.1 E6 psi. I believe the idea is that larger-dimension members come from higher-grade trees, which matches what I've seen--sometimes I'll buy a 2x10 or 2x12 and rip the sides off for beautiful, straight-grained, quartersawn spruce. I don't believe the code council adopted the NELMA changes but I could be wrong.

          3. Expert Member
            KYLE WINSTON BENTLEY | | #11

            I'm guilty of buying oversized and ripping out the middle for a good quarter sawn board as well!

  2. Expert Member
    MALCOLM TAYLOR | | #2

    BrunoF,

    Depending on how far you want to get into it, John Bridge deals in depth with these questions.
    https://www.johnbridge.com/

  3. Expert Member
    Michael Maines | | #6

    I specify L/480 deflection in kitchens and bathrooms, and L/720 when the tile is large-format, with one dimension 24" or more, after reading on the John Bridge forum and checking with manufacturers.

    1. dan_saa | | #12

      The Tile Council of North America (TCNA), also specifies minimum L/720 for any stone flooring of wood framing.

    2. Expert Member
      MALCOLM TAYLOR | | #13

      Michael,

      I suspect the trend for oversized tiles is going to come back and haunt us.

      1. Expert Member
        Michael Maines | | #14

        Malcolm, I suspect the same, though I also love the look of larger tiles.

        Dan, you're right--that had escaped my memory!

      2. Expert Member
        Akos | | #15

        Tiles are only going to get bigger. I'm seeing showers with the full size tiles, it all had to be water jetted to fit.

        1. Expert Member
          MALCOLM TAYLOR | | #16

          Akos,

          I like the lack of grout lines and all the attendant problems they can cause, but on floors it's asking a lot of the tiles, which are bound to have stresses in some parts under load. For now I'm favouring a pre-fabricated base, and big tiles on the walls.

      3. BrunoF | | #17

        I don't think we will have any tiles greater than 12" in any one direction in this house.

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