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Probably overthinking this. I'm putting up a 2x4 stud wall in my basement. It will sit on the concrete basement slab floor and is attached to joists above. The floor slab is in decent shape but I know it's only ~2 inches thick in places from drilling it for fasteners. The direction of the wall is across the joists. The joists are 2x8 and floor they support is noticeably bouncy. They are in decent shape: some checking but nothing serious that I can see. The joists span 14 feet from beam to foundation and the wall will be 1-2 feet from the end of the span.

The wall will support hundreds of pound of plumbing/heating equipment so it's going to be pretty solid. The room above is the kitchen so there are a dining table, cabinets, stove, etc already weighing down the floor. There is slight motion in the joists from people walking on them. If some of the weight from stuff upstairs is removed, the joists will lift some.

How do I attach the top of this wall to the joists above so that the wall is stable and there are no problems such as damage to the slab floor that it will sit on caused by the bouncy joists?

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  • At a 14 foot open span, no wonder 2x8 joists are bouncy. One or two feet probably will help a bit, but not as much as closer to seven feet(half way). I think just having them tight with the static weight(no people walking) be the best.
    – crip659
    Mar 9, 2023 at 21:18
  • @crip659 The wall needs to be at the 2 feet. My first thought was it could be a bonus to have it steady the floor but now I'm more concerned about avoiding trouble from having that floor above it bouncing around and damaging something below. Mar 9, 2023 at 22:03
  • This is far too many questions in one! Please focus on just one aspect at a time.
    – FreeMan
    Mar 10, 2023 at 12:58
  • have you considered fixing the joists so the floor doesn't bounce? 2x8's at 12 inch spacing should be fine.
    – Tiger Guy
    Mar 10, 2023 at 20:39
  • @Tiger Guy I have considered sistering the joists but that would be more work than I have time to take on. The joists aren't broken, it's just an old house that was built before they started using bigger joists. Not clear to me what you mean by "2x8's at 12 inch spacing." Mar 10, 2023 at 21:45

2 Answers 2

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If you build the wall so that it supports the floor joists then one problem that could occur is that the floor joist will become a lever. When it sags/bounces in the middle of the long span, the tail at the end of the short span will want to lift. What'll that do?

  • if the tail of the short span is near the foundation, and if the exterior wall above stands on top of the floor joists, the joists could try to lift that wall just a little. That might cause cracks in the exterior finish, might cause nails to work loose, etc.
  • if the tail of the short span is near the beam and the joists rest on the beam, you could get some upward heaving of the floor which could also transfer into an interior partition wall (if there is one in that spot), or could cause damage to rigid floor coverings like tile or hardwood.

Mid-span support also changes the strain forces inside the joist. Usually the top of a joist is in compression and the bottom is in tension, but above a mid-span support, the top would be in tension and the bottom in compression. I'm not a mechanical engineer and I don't fully understand the consequences of this, but as I recall the stress on the joist is increased in this condition.

It also sounds like your concrete slab is likely not up to the task of carrying loads from above anyway.

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  • It's balloon framed so there's no wall standing on the joists. It would lift the floor and mess up the interior finish on the wall. Do you have suggestions on how to attach the wall to the joists so that it does not become a fulcrum? Mar 10, 2023 at 12:16
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Use a string line to measure how much the floor above has sagged at the location where the wall will be.

Make the wall tall enough to correct that.

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  • The floor sag is on the order of an inch. I don't see that happening especially by propping them up 2 feet from the end and on a concrete slab 2-3" thick. Mar 10, 2023 at 4:09
  • I mean prop it by how much they sag at the point where you buiid the wall. (edited answer)
    – Jasen
    Mar 10, 2023 at 7:03
  • 1
    I got what you meant. I meant you can't take 1" of dip off in the center by taking 1/4" off at the end. It's more likely to lift the end off the sill. The joists are 70 years old, they're crunchy and well set in that bow and would probably crack if I tried to straighten them completely. I'm sorry but I'm downvoting this answer. Mar 10, 2023 at 12:12
  • Maybe instead of down voting an answer that's based on common sense and a common situation, you should provide details in your original question that would have headed off solutions that won't work in your particular case...
    – FreeMan
    Mar 10, 2023 at 12:59
  • @FreeMan This answer is saying to entirely correct joist sag by placing a wall on a 2" thick concrete floor 2 feet into a 14 foot span. That sounds like a bad idea. Those details were in the question. Exact condition of the joist doesn't change that. If I tried this, it wouldn't take the sag out and would likely damage the floor. Mar 10, 2023 at 13:23

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