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Updates: I was working on the house last year and asked about this project here previously. I never got around to tackling the living room as other rooms had priority. But now I am returning to this.

Reading comments again, I see that people had asked for more info, which I'm providing. Let me know if any additional info would help.

House built in 1975. Room is about 16' square built on 2x10 joists spaced at 16". House and room are structurally very solid but the existing subfloor is "wavy" - specifically due to some joists being up to 3/4" lower than others. None of the joists are at the same height. By my determination, it was just built that way. Floor was originally carpeted (on top of 5/8" particleboard that I removed) and you couldn't tell it wasn't flat. The plywood subfloor is 5/8" and is nailed (was existing) and screwed (I added) down to joists. I can access the floor from below (prev post was incorrect - basement is finished but not the part below living room). The joists have wood cross-bracing.

The hallway had been tiled in marble by a previous owner. This raised the floor by nearly two inches. Obviously it was already a big step as there is now a 9.5" step down from hallway level to living room subfloor level. I am putting down LVP atop a new 5/8" AC plywood underlayment - both of which I already have as I'm doing same in 3 bedrooms. In bedrooms, I only had to raise the floor by 1.5", which I did with strips of 3/4" plywood below new 5/8" underlayment.

There are four approaches that I've considered or had suggested to me. Looking for feedback - including pros and cons.

  1. 2x4s on edges, orthogonal to joists, every 8", scribed and cut make floor flat

  2. 2x4s on edges, parallel to joists (one on top and one halfway), scribed and cut make floor flat

  3. 2x4s on sides, orthogonal to joists, and shim to flatten

  4. cut with circular saw 1.6" wide channels, orthogonal to joists, for 2x4s on edges to fit into, trimming joists at intersections as needed to make level

Regardless how how I shim up the new underlayment, I plan to have supports under the new underlayment at no more than 8" spacing.

I already have LOTS of shims cut of various thicknesses of plywood from 1/4" to 3/4" for use in approach #3. I already have the AC plywood and LVP.

Someone in previous post had suggested "sister 2x8s beside the existing joists". I don't think that would make sense. Too much work - including removing cross-braces from joists.

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  • Why not have a single step (4 3/4") or a small ramp? You'd keep the high ceiling and have a smaller area to adjust
    – Jeffrey
    Commented Apr 28, 2023 at 18:45
  • They lie when they say 2x4. It is only 1.5x3.5. 2x3 plus 1/2 inch plywood will give 3 inches, but it is a bit thin(bouncy) if you want tile or laminated flooring. The 3/4 inch change is something to check first, it is quite extreme that close together.
    – crip659
    Commented Apr 28, 2023 at 18:48
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    I am liking the "countersink" approach the more I think about it. Hear me out. I would put down rails, flat and level, and then use my router with a 1.5" straight bit. Commented Apr 28, 2023 at 19:22
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    Rout, rout, rout..Bang! That's the sound of the router bit exploding when you find a fastener you missed removing while routing. There may also be screams of pain from chunks of router bit impacting flesh, and/or an underwear change required.
    – Ecnerwal
    Commented Apr 28, 2023 at 19:40
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    Building a an additional step in place seems to me much less effort and expense than any of the whole-floor approaches you're considering. I'm not following how routing out individual channels in the subfloor would improve the level (flatness) of the floor. Using the existing subfloor to guide a router would make all of the grooves just as uneven as the current subfloor, no?
    – spuck
    Commented Apr 28, 2023 at 21:09

2 Answers 2

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Use 2x4 on edge, and a reliable level reference, and scribe them to fit the floor and your desired height. 2x3 could work if you are OK with it being up just about 3 inches at whatever the lowest spot now is, 2x4 will give you a bit more to fit the whole floor to level and 3" up without being as constrained by whatever that point is.

Probably best to run them crossways to the joists.

Mark lines on the floor where the 2x4 goes and number them. Mark as often as suits your desired degree of precision along the lines. Check the height to your level reference from each point, and mark that on the 2x4 you have numbered to be fit in that spot, also keeping track of which end is which. Connect the marks & cut to the line. If you find a high spot between lines, measure an additional spot and cut away. When you install, using framing or subfloor adhesive that cures hard under each 2x4, which will deal with any low spots that are not excessive.

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  • Hadn't considered that. What exactly does it mean to "scribe", and what tools would be needed? Glue and screw them down I assume? You think that would be easier than cutting 1.5" strips out of subfloor, which would also be a means to level. Commented Apr 28, 2023 at 20:34
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    Scribe is the fitting process, described in this case in the last paragraph. If you want to bother to level each 2x4 to the same plane above where they go you can scribe the mark in with dividers as is commonly done for scribing counters to fit walls, but the many measurements to a level reference method is actually simpler and less overall work for shimming a floor or ceiling to level. As for routing the floor, the time, effort, and opportunity for error in leveling your rails to cut out for each strip is huge, and then there's the exploding router bits.
    – Ecnerwal
    Commented Apr 28, 2023 at 20:39
  • Or there's 2" of rigid foam and a bunch of self leveling compound, but that stuff is expensive. I'd say it's idiot proof, but the idiots would prove me wrong... but it is pretty much "how to make weird lumpy stuff level without really measuring a lot." The foam is just a less-expensive and lightweight filler in this application.
    – Ecnerwal
    Commented Apr 28, 2023 at 20:46
  • Here's a quick video showing how to scribe the 2x4s.
    – pdd
    Commented Apr 28, 2023 at 21:45
  • @Ecnerwal You had said "Probably best to run them crossways to the joists". But in the video that pdd shared, the 2x4s are parallel to and on top of the joists. Which is best and why? Thanks. Commented Dec 7, 2023 at 3:30
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This is expensiver and work, but you could strip off the subfloor (don’t have to remove screws or glue from the top of existing joists — bear with me!) and sister 2x8s beside the existing joists. You’d control level and flat by tweaking the sister up or down a bit. (A laser would be helpful for the leveling, as would deep throat clamps for initial locates before fastening.)

I’d bang lots of normal nails to set the joists, but then add a few structural screws to each for peace of mind.

With perfect joists, you can lay down your subfloor with screws and glue.

Oh, forgot to say: you can work your way across the room and have the old subfloor to work from if you cut one bay worth of subfloor for each new joist sister. (Assuming 16” oc, your strips of subfloor would be 16”.) You need to figure out your height in advance (laser!) and eventually, you’ll paint yourself into a corner.

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  • I had considered stripping off the subfloor - especially if I went with the plan of an intermediate step. But so much work! Also the 16' side walls are mid-joist. Commented Apr 29, 2023 at 2:10
  • Walls falling in the middle of a joist bay do make things a little more complicated. But a bit of blocking and a custom rip of a 2x4 sitting on the blocking and nailed to the bottom plate would fix that. Again though, I agree this is a ton of work for a benefit that’s variable according to you. Commented Apr 29, 2023 at 23:26

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