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I have had to raise the subfloor in multiple rooms of my house to bring everything up and level with my kitchen floor. The worst discrepancy is the 1" between the kitchen and den. To level off I will be raising the den floor with 2 layers of 1/2" fir plywood. How should I best fasten these layers of plywood to the old plywood subfloor?

My current plan is to apply lepage PL400 in lines 12" apart and on the edges, use 2 1/2" flooring screws and screw into the joists (12" apart), fill seams with floor patch on each layer, and criss-cross the plywood so that no seams align.

I have read elsewhere that the fasteners for additional layers of the plywood subfloor should not penetrate into the joists, but I could not find a reason why this shouldn't happen. Is my method here out of line? Wouldn't affixing the plywood layers to the joists reduce squeaks?

Thanks

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What is the flooring going on top of the subfloor?

To eliminate squeaks with the joists add more screws to the bottom layer into the joists.

The reason to keep them separated and not screwed into the joists is to allow independent movement of the layers due to expansion/contraction, especially for install of tile which is most susceptible to deflection and seasonal shifts.

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  • I am installing floating vinyl plank, not tile so I should be okay there. Would the floor patch and adhesive be suitable, maybe I should move to 1" flooring screws to ensure I don't penetrate too far into the joists?
    – GLid
    Jan 5, 2021 at 23:40
  • I would only correct for any large discrepancies between the two layers, a combo of planing/grinding the high points and using like rosin or felt paper to shim. Some people say to use asphalt shingles but I've never done this myself. An abundance of screws should close any gaps. Then address flatness on the top layer. Others can probably address the adhesive but my understanding is to add strength you would need to trowel it for full coverage not just lines
    – redlude97
    Jan 5, 2021 at 23:46
  • Thanks again. In that case I will move to a 1 1/2" screw. I have received two different pieces of advice on glue... smeared white or wood glue covering the whole panel, and beads of PL400...
    – GLid
    Jan 5, 2021 at 23:55

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