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I need to repair a cut joist in my kitchen floor. I was going to use a header and sister the joists on either side of it. Question: how do I know what thickness of a header I should use? What about the thickness of the sistered joists? The joists are 16" on centre.

Here is a drawing of the problem area (for context, this is a top-down view of the floor under the kitchen) : enter image description here

Here is my proposed fix enter image description here

  • I'm not sure where the headers come into play here. Are you going to remove studs from the wall when you repair/remove joists from the floor? – JPhi1618 Jan 3 at 19:05
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    How about a few pictures? Our imaginations are weak on Friday. – JACK Jan 3 at 19:06
  • FYI, you're referring to "height" or "depth", not thickness. – isherwood Jan 3 at 19:25
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    I think when OP says "header" he means the doubled-up 2xXthat connects between double up joists. Sounds like he's removing part of a joist, and needs to carry the load from the remaining parts of the joist around the missing part. Like framing in the opening for a stairway. – SteveSh Jan 3 at 19:46
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    I don’t think 2x4 FLOOR joists at 16” on center can span 20’. – Lee Sam Jan 24 at 17:10
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If we're talking about any sort of engineered joist (truss, TJI), stop reading now and consult a local engineer. This answer assumes solid 2x10 lumber.


I'm guessing that you don't mean a header so much as a joist fit between the sistered joists alongside. Since the span is so short (presumably 32" or less), you can use a single joist of the same height as the one to be repaired (so you have full backing for joist hangers).

The sistered joists should also be the same height as the originals. They would not need to have bearing at the ends if they're very well fastened to the original joists (construction adhesive along with 16d nails or structural screws in sets of three at 12" intervals, say).

|  |  |            |  |             |  |  |
|  |  |            |  |             |  |  |
|  |  |_           |  |           _ |  |  |
|  |  |___________||__||____________|  |  |
|  |  |_____________________________|  |  |
|  |  |_          ||  ||          _ |  |  |
|  |  |            |  |             |  |  |
|  |  |            |  |^- hangers-^ |  |  |
|  |  |            |  |             |  |  | <-- original joist
|  |  |            |  |
|  |  |            |  |  <-- cut joist
|  |  |   
|  |  | <-- sistered joist    

It's best to fit all joist components first (with a few toenails in the case of the cross-joist), then install the hangers. This ensures that they're pressed up tight, and that you can get things in place at all. Be sure to use construction adhesive at all points of contact (wood-to-wood and metal-to-wood) to prevent squeaks.

  • Would you change anything if it’s 2x4’s at 16” on center floor joists instead of 2x10’s? – Lee Sam Jan 24 at 17:12
  • Yes, but where have you ever seen that? The floor would be a trampoline. The OP may be mistaken here. – isherwood Jan 24 at 18:46

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