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We just bought a new (to us, built in '52) house with an unfinished basement. They've added - relatively recently (visible in listing pictures but not older pictures) - a pair of joist supports with a 4x4 bridging a few joists, one in particular looks weak and I'm guessing needed work before they could sell.

Those supports are anchored to the concrete and could probably stay, structurally, but we want to finish the basement and they're in a terrible place. We want to strengthen the joists they support so we can rip out the jacks.

What I've seen online suggests getting an identically-sized piece of wood, jack up the failing joist, glue and screw the new wood to it, let it set, remove the jacks. They seem to have made a very half-hearted effort to do so, but didn't sister it along the whole length - perhaps due to the difficulty imposed by existing water/HVAC lines.

Are there any concerns or things I should be aware of before I attempt to do it properly? I plan on matching the length exactly, adding a couple temporary jacks to hold everything in place, removing the not-really-a-sister joist they attached, disconnect any lines necessary to put the new wood resting on both the concrete wall and the middle metal support beam, then construction adhesive + screw the joists all over the place - finally, put everything back, let it set for a day or so, and remove all the jacks.

Reasonable?

Joist support jacks Supporting a joist that has a big (falling apart) knot

The "sister joist" they added stops at the HVAC, needs to extend a couple more feet to the metal support beam running down the middle of the house (where all the other joists terminate)

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  • What's above this part of the ceiling? Is there tile or stone floors? Seems like a regional thing between having to rest the ends of a sister but around here what you have would not pass inspection both for that and too few fasteners.
    – aquaticapetheory
    Apr 25, 2023 at 18:15
  • It's a bathroom above here mostly with tile floors, some of the area above is hardwood. I don't like the current solution at all.
    – user112697
    Apr 25, 2023 at 18:19
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    Unless there is something that can't be moved over the duct, not including blocking between joists, there should not be any problem to place a new longer sister joist there. Depending on the weight on the floor above, a sister joist usually only needs to extend beyond the damaged joist by enough, does not need go to supports usually.
    – crip659
    Apr 25, 2023 at 18:21
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    Aha. Are those 2x8 joists? Probably put all the junk there because the floors were bouncy and it would crack the tile. If you go monkeying with it now like jacking up joists, it will almost certainly ruin that floor. Sounds like you're doing a lot of work already and the tub is magical in all the wrong ways, might as well redo the bathroom.
    – aquaticapetheory
    Apr 25, 2023 at 18:24
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    @crip659 Inspectors here let you put on a short sister if you're repairing a broken joist but want it to rest at the ends if you're trying to reduce bounce. Looks like bounce needs to be reduced here. Asking the local inspector is probably in order here (unless the town's not involved).
    – aquaticapetheory
    Apr 25, 2023 at 18:28

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Rip out the bathroom: tub, walls, floor, and all. Take down all the stuff you've got supporting those joists in the basement. Reinforce all the joists underneath the bathroom and tiled areas with equal sized (length and height) sisters resting the ends on the beam and foundation. Jack up the joists, glue and screw the joists on. New lumber goes on crown up. Use Strong-Drive SDWS FRAMING Screws (predrill), 3 per every foot (or better yet, reach out to simpson and ask) and loctite pl premium.

Go to John Bridge tiling forums for help with floors. They have a calculator for what size joists are good for what span of floors. They're also very helpful and knowledgeable in general but don't take everything as gospel, eg the full length sister thing. Reach out to your local building inspector to see what's acceptable. I know that doubled up 2x8 joists over a 12 foot span will be stable enough for tile.

Be prepared for adjacent wall finishes to crack when you jack up the joists like the other sides of the bathroom walls. I won't go into detail on how to do that, I'm not an expert, but

  • Jack up 1/8 - 1/4 in at a time.
  • Put 2x8 or similar on basement floor. That concrete has finite strength.

The new joist sisters don't have to hold up the floor themselves so they don't have to be tight against the floor boards above for the entire span. They hold up the old joists. So if you want to make it easy on yourself, you could get some LVL boards and notch out the bottoms of the ends so that sits on the sill and beam and has some space over the top. (Glue won't stick to LVL coating.)

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