These photos show the way the previous owners of my house used a jack post to brace sagging floor joists. I'm not sure of their method, so I wanted to check what the correct method is.

Angle 1 Angle 1 Angle 2 enter image description here

  • It could be that the metal post could be removed, if it's in the way, but a structural engineer would have to sign off on the repair. The metal tension straps on the bottom and wooden compression reinforcement on the top may be enough to make this structurally sound. May 4, 2018 at 9:36
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    @JimStewart - My hunch is that the joist would need to sag a fair bit to transfer tension into those metal straps, i.e. it might be structurally sound but it wouldn't leave the floorboards above anywhere near flat.
    – AndyT
    May 4, 2018 at 13:55
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    There's a lot of wood and metal there doing absolutely nothing. You can simplify greatly.
    – isherwood
    May 4, 2018 at 14:21
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    The root problem is the epsilon-minus who put the pipe in low through the joists, cutting the joists in their tension area. May 4, 2018 at 15:21
  • Would the best technique for installing tension straps be jacking the floor slightly up above flat before installing the straps so that when the jack was removed there would be an increase in tension of the straps which would not allow the joists to sag below flat? Or would it be better to jack the joist perfectly flat and use an angled nailing technique to pre-tension the strap? May 4, 2018 at 21:32

3 Answers 3


Typically, there is a beam put under the joists, then the post are put under each end of the beam. This detail is better suited for longer beams, covering more joists. Somebody has found new and unusual ways do do something there. I can see the logic of what they were trying to do, but makes for a lot of guessing still.

The problem really happened when the insulated pipe was drilled through the bottom edge of the joists.


I would strip everything out of there, completely, after adding temporary wooden posts under the two joists. This includes the steel post, all wood blocking and extra joists, metal strapping, etc., right down to the bare original joists.

I'd then place a doubled joist crossways between the two joists in play, screwing it in place through the original joists. I'd then add double heavy-duty joist hangers, upside-down, on the end of the cross-joist, tying it to the original joists and transferring their loads.

I'd then mount the post to the bottom of the doubled cross-joist using 5/16" x 2 lag screws.

      |x|                     |x|
      |x|\    cross-joist    /|x|  <--- screws/lags through original joists
      |x| |    (doubled)    | |x|
      |x| |                 | |x|
hanger --↑         ||        ↑ -- hanger
                 |    |
                 |    | < -- post
                 |    |
  • That's better than what's currently there, in terms of the cross-joist; but I still have concerns that the original joist is understrength. The metal strapping is better than nothing and you're proposing removing it but not replacing it with anything.
    – AndyT
    May 4, 2018 at 14:41
  • Why would that be in any way necessary if there's a post right there? It's not better than nothing because it serves no purpose, yet adds complexity and effort.
    – isherwood
    May 4, 2018 at 14:42
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    Plus, strapping like that only takes load when the joist flexes substantially. It does nothing for bounce or initial sag unless it's somehow pre-tensioned. The post and cross-joist will eliminate both concerns with gusto.
    – isherwood
    May 4, 2018 at 14:43
  • To clarify further, there's no concern with the pipe bore in this scenario--there's enough remaining of the joists to carry the load near the cross-joist. It's only a problem now because it's out in the middle of the original span.
    – isherwood
    May 4, 2018 at 14:45
  • If we leave the post in then agreed that the shear force is decreased compared with original design, and the reduce section due to the pipe bore is probably ok for that. The designed large sagging moment will be replaced with a smaller hogging moment (US English: negative moment), for which the reduced section is again probably ok. The strapping is not much use in a hogging situation as it will be in compression and at risk of buckling. So, thinking on it further, I agree that it doesn't serve much of a purpose.
    – AndyT
    May 4, 2018 at 14:56

Tl;DR The "correct method" is to remove that pipe and sister in replacement joists. The job that the previous owner did is a hack job / bodge job, and I wouldn't trust it.

A jack post is used to reduce the span of a beam. The problem here isn't that your span is too short, it's that the capacity of your beam has been destroyed by drilling a large hole near the bottom of it. The solution isn't to use a jack post but to repair the beam.

The "best" way to do this would be to restore the beam to its original size by sistering in a replacement joist, but this of course can only be done if you remove the pipe that the hole was made for in the first place.

If you want to keep that pipe, you'll have to investigate other options. Potentially connecting a replacement beam underneath the joist would work, if clearance isn't an issue. You could even put a jack post underneath that additional beam if you want.

The above all applies to each joist that needs strengthening. They way the owner has tried to use a single jack post for two joists looks unsafe to me.


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