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My title pretty much asks the question, but for slightly more detail: I am prepping to put in some 3/4" hardwood flooring on the upstairs (2nd floor) of my 1970s house. My subfloor has had only carpet on it for the life of the house (I'm pretty sure) and is in pretty good shape. However, there are a few plywood butt joints that are not flush with each other - they are off by as much as 1/8" (see photo. yes, that's a Lego to show scale - it was sitting nearby).

enter image description here here's a more zoomed out photo enter image description here

I'm wondering how best to fix the problem - I'm no expert, but even I can tell that hardwood flooring will not sit nicely across that 1/8" height difference.

The plywood is nailed down, not screwed, and one idea is to put some screws in and see if that tightens it up. I thought I'd ask the collective wisdom on here before doing that.

Another interesting and possibly salient detail is that the plywood subfloor is nailed only every 4' which I am pretty sure is because there are not typically-spaced floor joists but instead are larger beams spaced every 4'. Evidently, these beams used to be exposed to downstairs but were drywalled over before we moved in to create a ceiling. So I'm also pretty sure that there is more than just a single layer of plywood in the subfloor - I don't know what's underneath it, but it's perfectly solid in the middle between the beams.

  • Is the offset uniform over the length of the edge? – Hart CO Aug 10 '17 at 16:38
  • In one case yes, but in two cases no - the offsets are greatest in the middle of the edge and nearly flush at the corner. – susie derkins Aug 10 '17 at 16:41
  • Try using screws like you mentioned. If they don't pull the floor down, you can unscrew them. If the plywood won't go down, you could try planing the edge and see if that makes the floor flat. The nails would have to be pulled before planing to prevent damage to the planer. The original installer may have put down a thick bead of construction adhesive and allowed it to dry before laying down the high sheets. – Jim Stewart Aug 10 '17 at 20:04
  • The other possibility is that the high sheets may be bowed up by being compressed from the sides by other sheets. This would occur if the installer did not leave sufficient gap between sheets. You should be able to see that if it is the case. – Jim Stewart Aug 10 '17 at 20:08
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Use a large hand plane like this:

enter image description here

I installed vinyl plank throughout my first floor; 1200 square feet. The rooms I did before thinking about this show every little bump or seam. Most people don't see the defects, but I sure do. They drive me nuts. I wish I thought of this sooner.

Make sure the blade is set to take tiny ribbons of wood and is real sharp. It will go through the wood like butter. Makes quick work and you can get that area completely smooth.

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I would use latex modified concrete leveler or tile thinset.

Get a long straight board or piece of angle steel to use for a screed.

You may also want to apply a bonding agent to the sub-flooring to keep the leveler or thinset from separating from the sub-floor.

Good luck!

  • This would mean you couldn't nail the hardwood down, right? You'd have to use an adhesive? – mwwalk Aug 12 '17 at 23:35
  • Your third sentence is unclear. – isherwood Oct 17 '17 at 13:10

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