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Why are I-joists 11-7/8″

DCContrarian | Posted in General Questions on

Just something I was wondering about today and I figured the brain trust here would have an answer. So why are I-joists 11-7/8″ and not either a round number like 12″, or the same as dimensional lumber like 11-1/4″? 


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  1. Expert Member
    Akos | | #1

    You can get into real trouble mixing dimensional lumber and I-joists. I think the thinking was that by going to this size, builders would not be able to use dimensional lumber for rim joists which can create a big issues. Plus 11 7/8 works out to 300mm. I know this sounds like sacrilege on the other side of the border, but mm are great.

    Same with 9.5 instead of 2x10.

    P.S. Just though of one more reason. If you are starting from 4' wide OSB for your webs, 11 7/8 is about height you end with after ripping the sheet into 4 and tapering the ends to fit into the cut on the flanges. Same with 9.5 (4' cut into 5 strips). The 14" doesn't work though, so it might just be coincidence....

  2. Dan Kolbert | | #2

    LVL’s already came in 11-7/8. Unless it was the other way around.

    1. Expert Member
      DCContrarian | | #3

      The "four from a 4x8 sheet" theory would make more sense to me with LVL's.

    2. Expert Member
      Zephyr7 | | #4

      LVLs are also 1.75" thick, so you don't need any plywood between two of them when sistering them to make a header.

      BTW, Akos might be onto something with the metric measurements. Plywood has been going that way for years. Other things are already there -- fiber optic stuff is all defined in metric units, for example, as are semiconductors. Why not lumber? It's lots easier to remember the conversion from meters to kilometers than it is for feet to miles.

      The only screwy thing I remember is all the width options for sliced bread were in mm except the thickest, which was in inches. Apparently metric units are only good for thin-sliced bread, and you have to break out the old school ruler when you measure the thick stuff. This was at a Longo's market in Hamilton if I remember correctly. I probably posted the picture of the sign on GBA once for fun in a similar mm/inch thread some time back.


      1. Expert Member
        DCContrarian | | #5

        I've been noticing lately that the sheets of plywood I get are metric, they're 243.84 cm by 121.92 cm.

        1. Expert Member
          Zephyr7 | | #7

          Hehe, the thicknesses are often (but not always) metric. they might be shown as a decimal inch value, but a lot of those work out to be a nice number in millimeters. Why this is, I don't know, and why it isn't the same for ALL plywood I don't know, either. Lumber dimensions seem similar to those for pipe: none of the stated dimensions actually measure anything on the actual product :-)


  3. Expert Member
    Deleted | | #6


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