I recently had a contractor repair damage to my subfloor and joist. In addition to installing the new Joist's he also cut pieces of wood (same wood as the joist) into 2ft pieces and screwed them into the existing joist. He spaced the 2ft pieces about 2ft apart in an attempt to level out the floor. The subfloor turned out horrible. I am unable to put any flooring down due to the high and low spots all over the floor. I now have concerns about everything he has done. My questions are -

  1. Can you use screws to connect these 2ft pieces of wood to the joist as he did? Can the screws handle the load? Do I need to take the screws out and use nails or remove the boards and replace the entire joist with a new one(s)?
  2. Should I revisit the joist/subfloor or use the self-leveling compound as suggested by some of the contractors that have since visited to evaluate.

I will need to do the work myself. The self-leveling seems easier but more of a band-aid. Any advice would be welcome.

  • What is the difference between the high and low spots? Were the pieces of wood screwed into the side of the existing joists or the top? Oct 19, 2021 at 17:36
  • 1
    Pictures of the floor and the joists underneath would help. I would like to have it fix than patch myself, but it depends what is causing the problem. It sounds maybe new measurement joists were used beside old measurement joists.
    – crip659
    Oct 19, 2021 at 17:50

2 Answers 2


Can you use screws to connect these 2ft pieces of wood to the joist as he did?

Sure thing. It's become very common over the last 5 years or so to use screws instead of nails for construction carpentry.

Can the screws handle the load?

Screw strength varies, but GRK R4™ screws are fairly commonly used and according to their ICC code approval report have a shear strength of 428 to 655 lbs (depending on screw diameter) per screw. Add up the number of screws holding up a board and that's the minimum amount that piece of joist can handle (there's friction between the boards, and that adds some strength - I don't know the physics to know how much additional).

The GRK RSS™ "structural" screw's ICC approval report shows that they have a shear strength of 754 to 1231 pounds per screw.

This is going to be more than enough to support these scabbed on floor joist sections.

The odd thing to me is that your contractor put on short 2' sections of joist. Granted, lumber is a bit pricey these days, but the normal method of leveling a floor is to sister a full new joist in instead of little pieces. Usually an entire joist is out of kilter and it doesn't have little dips that need to be shored up.

Because he did this, it's not surprising that the floor isn't particularly level. Since there's already a sub floor down, self-leveling compound may fix your problem, depending on how wavy the surface is.

If the sub floor is screwed down, you may consider unscrewing it (much easier than pulling nails!), then unscrewing (again, much easier than pulling nails, thank goodness your contractor used screws, not nails) these joist scabs and look at reattaching them to provide an actually level surface. You may find that just moving one or two is sufficient to get your floor level enough for your purposes. You may need to redo the entire thing.

NOTE: This isn't an ad for GRK screws. They happen to be plentiful at my local retailer and I've become fond of them, but I don't work for the company or any of their marketing firms. Just a happy customer.

  • "there's friction between the boards, and that adds some strength - I don't know the physics to know how much additional" - FYI friction provides basically all of the strength with most wood to wood construction, in normal circumstances the shear load on screws is 0, and their pull out strength is substantially higher than their shear capacity. Oct 20, 2021 at 2:31

I don't understand the addition of the 2' pieces of joist.

If the 2' pieces of joist are at the same level as the existing, I have added some splices on some jobs where the joists were not installed at exactly 16" or 24" and the plywood subfloor/roof deck did not meet at the center of a joists. The splices were installed so the plywood edges would have a better surface area to rest on. Nails or screws both work fine for this. You might look at whether the subfloor (lumber or plywood) joints needed the additional splices for support.

If the 2' splices are above the existing joist (which your description sounds like) they were installed in an attempt to level the floor due to sagging joists. I would not consider this a good installation.

You should check local code framing tables to determine if your joists are adequate. You can check floor (and roof) framing requirements by checking your city code or calling a your building inspector. You'll need to know the size, spacing and span of the joists to see if they are to code. Interior walls resting on the span will affect joist sizing and spacing. If they are not the correct size, you may need to install new joists beside the existing or possibly a beam below the joists to support the span.

If you plan on being there long term I would not spend any money on floor leveling compound until I determined the cause of the sagging and unevenness as it will probably continue to worsen with age.

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