After tearing out the old subfloor in my half bathroom (due to water damage) I am replacing it with 3/4 in (2 cm) plywood. One side overhangs from the last joist about 9 in (22 cm) to the wall, and 11.5 in (29 cm) to the next joist. I plan to tile the floor with ceramics, so I need to brace that side somehow.


My plan was to attach three or four 11.5-inch 2x4s perpendicular to the joists and screw the plywood into those via 3-inch deck screws. My father also had a couple ideas:

  • Cut a couple 4x4s and set them on the concrete below pillar-wise (unfastened), and screw the plywood into them
  • Set a 2x10 on the concrete parallel to the joists, then screw the plywood into it -- this would require more of that black stuff underneath to raise it up to the other joists' level

If it's not obvious, neither of us are carpenters. I already have the 2x4s cut, but is my method unsatisfactory?

EDIT: Here are the three ideas in picture form (my idea, then my father's ideas):





Because of the short span I'd do the job with 2x4. 3" gold construction screws will do nicely.

  1. Run a horizontal ledger along the far joist 3-1/2" down from the top. Fasten it with pairs of screws every 12" or so.
  2. Run blocks across from the near joist on 16" centers (with one near the right end), end-screwing them twice at the near end and resting them on the ledger. Toenail them just once above the ledger to keep them in place. Your subfloor fasteners will do the lion's share of the work after that.

Consider using construction adhesive on everything to eliminate the chance of squeaks.

                 far joist
      | o|                           | o|    <-- new ledger, down 3-1/2"
_____o|  |__________________________o|  |__
      |  |                           |  |
      |  |                           |  |
      |  |                           |  |
      |  |                           |  | <-- cross blocks
      |  |                           |  |
      |  |                           |  |
      |  |                           |  |
      |x |                           |x |
       x         near joist           x

  o = toenailed screws
  x = end-fastened screws
  • Indeed, I was planning on using Liquid Nails Subfloor Adhesive. Do you think my 3" deck screws are adequate instead of the construction screws? – calcium3000 Nov 30 '18 at 15:27
  • Yep. Almost anything except black drywall screws, which are brittle. – isherwood Nov 30 '18 at 15:28
  • Great. And what about reordering #1 and #2 in your answer? Seems like it'd make more sense to put the cross blocks in first, then put in the ledger to add support. (My only concern is that I'd put in the ledger, then the cross blocks wouldn't fit due to operator error in measuring/fastening.) – calcium3000 Nov 30 '18 at 15:37
  • Sure, if you think you can finagle the ledger in later. I wasn't sure how tight things would get. Or lay it in the joist space first and install it later. – isherwood Nov 30 '18 at 15:43

None of the ideas that you outlined are really ideal. Probably the best approach would be to cut some pieces of joist sized material (2 x 10s if I read your question properly) to be used as blocking between the open joist and the one down under that wall. These would be cut to match the spacing between those two joists. You can use metal joist hangers to secure them in place. I would space these equivalent to your normal joist spacing.

This picture shows the idea where you would place these.

enter image description here

Another alternative is to use some joist sized material cut to length and just lay it up against the outer joist that is under that wall and simply screw it through the face into that covered joist. It may take several iterations of this to being out the top of these sistered joists so that the top extends beyond the wall surface. This alternative may be easier so as to avoid difficulty with the electrical wire running through that area unless the wire is already below the bottom of the existing joists.

  • Ah okay, so your second idea is to basically 'fatten' the joist under the wall? That does seem easier, and it should only require two fastened boards since that overhang is about 2.5 in. – calcium3000 Nov 30 '18 at 12:47
  • I would agree with the 2nd option - sistering the joist under the wall. Since both pieces of 2x lumber will be sitting on that concrete ledge, you can be pretty sure that if it ever sags, that little corner will be the least of your worries. – FreeMan Nov 30 '18 at 14:58

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