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).
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.