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I purchased a house from 1900 about a year ago. One of the biggest problems I have unsurfaced is cut and cracked joists. My questions are as such with pictures below:

  1. Can I just jack them up, insert wood glue into the cracks, then sister with 3/4 plywood?

or 2) Should I replace with LVL or I-beam?

  1. If I sister the joists with 3/4 plywood, should it be one long piece of plywood (16' appx) or 2 - 8' pieces?

Or, should I go about this a completely different way? If so, what are the recommendations?

If I missed anything that would be helpful in troubleshooting/planning, please let me know and I'll do my best to get it.

Joist 1cracks along length of joist

Joist 2 enter image description here

enter image description here

Joist 3 (notched and sagging floor above at corner of stairs) enter image description here

Joist 4 (vertically cut all the way through and wedge inserted and long horizontal crack) Vertical cut enter image description here

"Wedge" enter image description here

enter image description here

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    Is the floor above these joists pretty solid or does it feel bouncy? Those don't look horrific to me. Well, except for the "wedge" one. Can you add a pic of that from the bottom?
    – FreeMan
    Commented Nov 17, 2023 at 1:20
  • Swapped out a duplicate for the wedge from underneath. As for the the floor, there is a definite downhill slope on the first floor towards where the stairs are. On the second floor, things have sagged enough that the doors are out of alignment for two of the bedrooms. Tomorrow or this weekend I could get my line level or just a line and figure out the total sag we are experiencing.
    – Eric D
    Commented Nov 17, 2023 at 1:56
  • Settling leading to a slope in the floor is one thing, bouncing because the joists are failing is another. If you want to fix the slope, you'd fix the foundation wall. If the floor is bouncy, you worry about one or more joists failing. Also, the link target [9]: URL for your last image is missing and you've got [9] twice, so you may want to try that again.
    – FreeMan
    Commented Nov 17, 2023 at 12:26
  • Do the "cracks" shown in the joists show up on the other side as well, where they cut through the full joist thickness? Or are they just located on the shown side? Wood "creeps" over time, where the sagging could just be from weight plus time. Is this precious space down below? Can you install a beam line and a few columns below the midspan of the joists to halve the spans and push the sag out?
    – popham
    Commented Nov 17, 2023 at 18:54
  • Is there any evidence below that 4x4 "wedge" that there was previously a post at this location? Is your theory that those dowels were driven in to dilate the wood and generate pressure in the joint?
    – popham
    Commented Nov 17, 2023 at 19:05

1 Answer 1

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Those aren't cracks. They're "checks", and generally aren't considered a problem. It's what wood does and their presence doesn't significantly affect the members' strength. See also any log rafter system.

Plywood isn't an appropriate beam material. Half its plies are oriented the wrong direction. Beams are never built from it or repaired with it, and installing it with a joint in the middle negates any value anyway.

Any engineered beam will use OSB, which is made with wafers all oriented in roughly the same direction (TJI joists), or plies of solid wood laid in the same direction (glue-lam beams).

If you want to repair or replace a joist or beam, add something that's up to the task by itself. This can be attached to the original member (assuming that it has proper bearing) near the ends with suitable structural hardware, or it can rest on the original beam supports. The act of "sistering" isn't inherently needed, and its primary benefit is noise reduction and stabilization of anything attached to the original member.

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  • Use of OSB in I-joists is about its shear strength and its cost. The shear strength is independent of the fiber orientations. Scroll tables M9.2-1 (page 52) through M9.2-3 from awc.org/wp-content/uploads/2022/01/AWC-2018-Manual-1810.pdf. Watch the OSB column and the parallel 5 layer plywood column. You'll see that they align pretty well with plywood always tied or winning. Now check out Table M9.2-4. You'll finally see OSB winning. But note how the strength axis versus weak axis values in this table match. Fiber orientation doesn't matter in the one instance where OSB wins.
    – popham
    Commented Nov 17, 2023 at 19:48
  • To make sure I am understanding...(1) the cracks aren't cracks and I don't have to worry about them. (2) Use OSB or, preferably, glue-lam/LVL beam directly next to the beam I am wanting to support and/or replace. (3) if I want to reduce and stabilize the whole joist system, I should install blocking and/or bridging. Is that correct?
    – Eric D
    Commented Nov 17, 2023 at 20:01
  • If using OSB (as opposed to an engineered joist containing OSB, you'd need to sister it to the existing with appropriate fasteners. Plain OSB is not a joist as it has no lateral stability. I'd expect you to use solid lumber here or TJI joists.
    – isherwood
    Commented Nov 17, 2023 at 20:37

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