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Is it better to sister a 2x8 floor joist (cracked somewhere in the middle) using same size lumber or is something like 3/4 plywood better? Consider that both ends of the newly sistered joist won't rest either on sill plate or on the mid-span beam. Most likely, I will be using two pieces of lumber or plywood as sister joist to span 11 ft. If it is plywood, then should I sister it from both sides of original joist or one side is fine.

Lastly, since both ends of sister joist won't be supported by the beam/foundation wall, is it just waste of time and wont add any strength?

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  • Is the joist cracked or looks like a crack in it? A picture of the crack in your question will help. Some wood cracks need fixing, some are normal, and do not need fixing. A crack going from top to bottom of joist is bad.
    – crip659
    Feb 12 at 22:38
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    Did you look around this SE site a bit? There have been several Q&A's regarding sistering just over the last year or so.
    – SteveSh
    Feb 13 at 2:31
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    You wouldn't use plywood. Roughly half the plies would have grain in the wrong direction, making it ineffective as a joist patch. Use solid lumber of the same nominal size as the existing joist. I'd like to see more of the situation before offering specific solutions.
    – isherwood
    Feb 14 at 14:41
  • The total length of the joist is 11', how far from the end of the joist has it failed?
    – FreeMan
    Feb 14 at 15:54

2 Answers 2

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Since your joist seems completely severed at the crack, it's not really doing any support now. Any partial length sistering would thus concentrate the load in the shorter overlap section on each side of the break. That might be good enough, but if it were me I wouldn't mess with sistering but would instead install two new joists flanking the broken one, maybe 8" away on either side. Easier to just secure them at the ends. Fwiw.

This answer to another question suggests cutting out the damaged joist end section (as long as it is less than 1/4 of the total span) and splicing in a replacement via sistering with plywood and specific fasteners.

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I'd be inclined to get a U-shaped piece of steel (channel) made up, that would fit the joist snugly, probably sliding along between joist and pipes. Then bolt through both sides with 4-6 bolts. 12" either side of the split should do it.

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