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I'm putting in a 3' x 6' shower, enter at the short end opposite the control wall with a 4' half wall (it will be 8' tall up to an existing overhang for the first 2.5') along the long edge (exterior wall on the other long edge). The half wall runs parallel to 24" on center I joists (TJI⁄ Pro 120TS 11 7/8" Joists) and will be between two joists.

I'm wanting to make the half wall rock solid. I prefer to only cut into one joist bay, but can cut into the adjacent ones if needed.

My plan was to put 2"x8" blocking on the flat glued and screwed to the top flange and then use structural screws to screw the wall to them. I was also going to put two 2"x8" boards as an endcap (going to have a ledge on the inside of the wall) that extends down to the bottom of the joist bay and is screwed to a 2"x8" block on the flat attached to the top and bottom flange.

Looking at the documentation for the joists it says "DO NOT use sawn lumber for rim board or blocking", because "Sawn lumber may shrink after installation". It seems like this is referring to blocking used for structural integrity of the I beams. Does this restriction also apply for the 2"x8" on the flat blocking? I found this DIYSE question, which says to use joist hangers and dimensional lumber for a similar I joist model (there's the same use case (H2) in my I joist's documentation, except without the need for a backer block on the backside). Can I use dimensional lumber blocking (on the flat or with a joist hanger) or do I have to use engineered lumber such as plywood or an I joist (on the flat or with a joist hanger)?

enter image description here Here's my quick attempt at mocking up my plan, hopefully it helps. Sorry for everything extending too far. The 2x8s will stop at the bottom of the joists and not go through the joists. The 2x4s will stop at the floor. There'll two ~5/8s plywood/osb sheets between the joists and the 2x4 wall.

SkroobLamarr
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1 Answers1

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This isn't really a matter of joist requirements. You're not blocking for a rim or a beam. Regular kiln-dried lumber (which has been stored properly) is fine, especially since the blocks are so short. Any lengthwise shrinkage will be negligible.

Some wisdom from experience: The screws you mention will not make your wall stable. This is not the fault of the screws, but the inherent instability of a stack of narrow softwood boards. Instead, you want a continuous post (ideally doubled, as a pair of studs at or near the end of the wall) extending from the bottom of the joists to the top of the wall (or just under the top plate). It should be fitted well, anchored robustly at the bottom and top of the joist cavity, and glued copiously with construction adhesive to stabilize and fill gaps. This is really the only practical way to make such a wall stable in a durable way. (On re-reading I see that this may be what you were planning.)

Here I have the post resting on a full-span block with blocks alongside the post, screwed to the block below. The upper blocking will just be split blocks, though you could place full-span blocks alongside the post, too. (Really, the subfloor itself would provide adequate anchorage if fit well and glued.)

Be sure the post is plumb before buttoning everything up. You can shave one side later if needed, but ideally it's not needed.

ELEVATION (SIDE) VIEW

                      |  |  |<-- post
                      |  |  |



==========================|  |  |====================== <-- subfloor
         |____|...........|  |  |..........|____|
           ||             |  |  |            ||
           ||             |  |  |            ||
           ||             |  |  |            ||
           ||             |  |  |            ||
           ||.............|  |  |............||
          ____............|||...........____
         |____|............................|____|
======================================================= <-- ceiling


              ^-----------------^---- blocking (also above)
isherwood
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