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My 1952 ranch has a saggy spot in the floor. It's centered on a vent that was cut into the top of the 2x10 joist.

The cut is about 12" wide, 5" deep. There is about 4.5" of joist height remaining below the vent, and in the middle of that section is a big knot! Adding to this dysfunction - a non-load bearing wall sits right on top of and parallel to this joist.

I should be able to get the HVAC duct pieces out of the way enough to get a jack and sister in place. A sister 2x10 would have to be spaced out away from the original joist to accommodate the vent that protrudes to the left of the notch.

Or should I use a piece of steel angle? I see some joist repair plate products for sale online, but nothing with the width required to bridge this gap.

Any tips from those with experience?

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  • A non load bearing wall with bottom plate on top of and parallel to this floor joist would seem to stiffen the floor and counteract sagging. It would be a benefit. You could just place a steel flat bar maybe 6 ft long on the bottom of the joist. Jack in the middle under the flat steel bar to remove any sag and then fasten with nails along its length. Removing the jack would tension the bar and take the sag out. Alternatively put a pier under the joist. How much is it sagging? Might just ignore it. Nov 27, 2023 at 6:02
  • The amount of steel necessary to restore the stiffness and strength for something like this is probably much larger than you imagine. See diy.stackexchange.com/questions/287084/… for comparison against a 2x8 notch.
    – popham
    Nov 27, 2023 at 6:30
  • What's below the joist in question (besides more ducting? Is there a slab?
    – Huesmann
    Nov 27, 2023 at 14:15
  • I have used the steel joist repair product and read the documentation very thoroughly. You cannot use it there. Any repair product would be so large, and expensive, and makeshift you'll just end up wishing you sistered.
    – jay613
    Nov 27, 2023 at 14:56

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For a floor joist like this with redundancy from other joists, I would sister a 6 ft chunk of 2x10 centered over the notch. Install a 6ft long spacer first, though, to avoid the duct interference. Use an adhesive to join everything, with screws drawing things tight for a thin glue line.

Ideally you would use a low-creep adhesive like resorcinol. Because of the low stress levels, low humidity, and redundancy from other joists, I might settle for a PVA glue like Titebond. I expect that the "waterproof" Titebond III has the best creep characteristics of any PVA, so that would be the stuff I would use if not resorcinol or epoxy.

Before gluing everything, you want to use a jack or wedge to remove the sag from the joist. Avoid hydraulic jacks to avoid possible pressure bleeding overnight. Remember that the joist will deflect after you remove shoring, so push it up a little too far to compensate.

Note: This is what I would do. If you hired a contractor to fix this, then he would need to obtain a permission slip from an engineer to follow this proposal. I'm skeptical that an engineer would provide this permission slip. Rather than get permission, this hypothetical contractor would sister the full joist length to stay in conformance with IRC prescriptions. That solution avoids the cost and lost time of involving an engineer. (The engineer would probably follow @isherwood and provide a mechanical fastener schedule.)

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    Glue applied in the field should not be considered structural. Proper mechanical fastening is required.
    – isherwood
    Nov 27, 2023 at 14:28
  • @isherwood, even with the redundancy, the IRC admittedly doesn't admit structural gluing in the field. Without the redundancy I agree with you for the PVA glue. Without the redundancy and for a slow setting epoxy (which is very forgiving in spite of poor fit up), I disagree. Without the redundancy and for resorcinol, I would want third party quality control and a written procedure that would involve sanding and flatness constraints to disagree. Gluing in the field is not blessed because carpenters are untrustworthy, not because it's impossible to get right.
    – popham
    Nov 27, 2023 at 17:56
  • @isherwood, edited to acknowledge that this answer is not by-the-book. I probably would have stuck to the book if not for that resorcinol I have that's about to expire.
    – popham
    Nov 27, 2023 at 19:04
  • Can't argue with that. We're a DIY site, not an engineering site. The carpenters are not trustworthy. I'm not even sure I'd trust myself to get it right, so I fall back to conventional strategies.
    – isherwood
    Nov 27, 2023 at 19:04
  • Thanks for your thoughts. The affected joist is also cupped, concave on the side where the sister/partial-sister would need to go (left side in my photos). Getting a tight glue bond would be tricky. I'm going to see whether I could get a full sister beam-to-beam in place - it's in a tight spot in the basement, there is wiring and plumbing and HVAC all over the place, but this spot might have some room. I would add a spacer and screw/bolt the sister to the weakened joist after jacking it level (or slightly above).
    – NATHAN
    Nov 28, 2023 at 15:25

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