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I ripped out my old hardwood and left the old 3/4" diagonal subfloor down. The floor is mostly level. As for flatness, 3 spots need to be taken care of - each spot is about 3/16 to 1/4" deep. Where I can I am going to shim/shore up under the subfloor and screw down the floor with 1 3/4" subfloor screws. After that I am going to use ram board, tar paper, drywall shims to get it flat. After this is where I am getting confused.

  • Do I put a underlayment like rosin paper first, shim, then put down the OSB or skip that?
  • Do I lay down the OSB like I would normally with the ends landing on the joists then secure it to joist like normal?
  • What about screwing the OSB to the planks as well?
    • Since they are are 7" wide put a screw every 7 in-between the joist to hit a plank or not needed.

I am using laminate so there will be a underlayment on that.

Also I am using 3/4 to match the height of old flooring and it was cheaper by a lot.

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  • What is your intent behind fastening the OSB to the boards between joists? To avoid squeaks? And why the 1-3/4" fasteners when the OSB fasteners will clamp everything to the joists? And drywall shims? As in paper shims?
    – popham
    Nov 27, 2023 at 8:30
  • You shouldn't be shimming between the old and new subfloors. Is that what you meant? Level the old subfloor, then install new.
    – isherwood
    Nov 27, 2023 at 19:54

1 Answer 1

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For me, it's fairly simple.

  1. Level the original subfloor. Shim it from below or lift the boards and raise the joists.

  2. Screw or nail down the old subfloor so it's silent. Preparation is key, as they say, and here's your chance. If you can get ahold of a framing nailer, 8d ring-shank subfloor nails would be great (and fast). Walk it thoroughly and deal with any lingering squeaks. Shims from below and heavy-duty construction adhesive can help with troublesome areas.

  3. Lay new subfloor on a zigzag bead of heavy-duty construction adhesive. This locks the layers together to further reduce the chance of noise.

  4. Nail or screw the new subfloor to the joists, staggering butt joints. Some carpenters like to keep the layers in isolation. I haven't found that necessary. You'll want full-length framing nails or 3" screws for this.

  5. Walk it all again to thoroughly test for noise. Deal with that now, before you install your finished floor.

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