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Related to this thread: Step down into livingroom. Too tall? split in two?

I think raising the floor with another plywood subfloor layer is the best option. Solves three problems: 1) the big step, 2) some small damaged areas of particleboard, 3) plywood base to new floor gives me more options for that floor. Can float or glue or staple whereas with particleboard I can only float.

But now I am adding another 5/8" of plywood. The current underlayment is particleboard. Below that is plywood. Both are 5/8". So with my new 5/8" plywood, I am screwing through almost 2" of material. My plan is to use 3.5" screws. Then on top of that will be 5/8 engineered hardwood. So in the end there will be 2.5" of flooring involved. Should I be concerned? About the weight for instance? And should I put tar paper or something beneath the new plywood for acoustic dampening?

Another option would be to replace a 4'x4' piece of particleboard where damage is present, and just float the new floor. But then I'll have an 8" step instead of 7.25"

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  • What's below the living room? If you have height to spare, could be an opportunity for thermal insulation or soundproofing.
    – bobflux
    Commented Oct 6, 2022 at 11:35
  • There is a crawlspace below. Is about 5ft tall. Commented Oct 6, 2022 at 13:05

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There is no problem with adding another layer of plywood- if this gets you what you want with the 7.25" step.

It is fine to use the long screws to get into the wood joists but also smaller screws just into the other subfloor ply would be fine. What you are adding is basically another underlayment layer to your floor and not a structural component.

I can't say that a layer of tar paper will aide in acoustic dampening but I guess it cannot hurt either.

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  • Acoustic dampening seems like a non-function with crawlspace below. I don't think anything will have a lot of effect on the noise level above once it's under the 5/8" of plywood. That would be much more affected by what's on top as a finish floor.
    – Ecnerwal
    Commented Oct 6, 2022 at 16:31
  • Tar paper would off-gas nasty VOCs, definitely not appropriate indoors. Perhaps the asker meant red rosin paper, which is used as a slip layer between layers?
    – twm
    Commented Oct 10, 2022 at 6:39

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