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The floor joists in my unfinished basement of a 1950s house have nails that the previous owner probably used to hang items on. Many of the nails have caused the floor joists to split.

crack in floor joist

I've been removing the nails because I'm not hanging stuff from the ceiling.

Should I bother to repair the joists or fill in the nail holes? If so, what should be used?

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Nail holes in a joist wouldn't concern me unless it affected a significant proportion of the joist. You can safely drill a hole 1/3 of the width of the joist in the middle for running wiring.

Cracking or sagging that is allowing the home to go out of level/plumb is a concern. It's best to get a structural engineer to evaluate this problem. The standard solution is to sister a second joist next to the failing one, and possibly add some support columns to a footing in the floor. You would first remove any blocking that's interfering with the new joist, jack up any low points on the old joist until things are level, install the new joist so each end is supported, glue and bolt the new joist to the original joist, install new blocking, and remove the jacks.

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the photo you have in your post is a joist failure. plain and simple. the nails are coincidental or related, but not relevant. cracks like that in the joist are not abnormal, but they do need to be repaired. they often happen at knots, checks or notches/cuts in a joist. however, when the crack is along the lower edge of the joist and running longitudinally down the joist, the joist is failing. it usually starts with one, and then the adjacent ones fail trying to take the load from the initial one. you need to sister it with a new joist and/or support both sides of the failed joist with a post.

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This joist can be repaired using a joist repair plate. These are made from structural steel and they have holes in them so you can lag bolt them into the framing member to reinforce it.

I haven't been able to find a good image of one, but it will be similar to this.

Joist Repair Plate

The one pictured is not properly sized for your needs, and the construction doesn't appear to be heavy enough either, but you will get the idea. You most likely would have to have one custom fabricated.

To install it, you would need to jack the joist back in place and place temporary supports on both sides of the area to be repaired. Then slide the plate into place and attach it with bolts.

This crack doesn't appear to be very serious, so you could probably fix it on your own. If you see more signs of failing joists, you should consult an engineer to assess the issue.

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