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I am in the middle of replacing my subfloor. The old subfloor was two layers, each about 1/2 inch thick, totaling 1 inch.

1-1/8 inch OSB is pretty much out of stock everywhere at the moment so I'm thinking of putting two layers back in. 19/32 + 7/16 = .578 actual + .4375 actual = 1.0155 actual.

I will be installing LVP or laminate as the floor. Is it okay to secure both layers of subfloor to the joists if there wont be tile going on top?

If that is not okay, how would you go about securing the second layer of subfloor to the first?

Thanks!

2 Answers 2

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NO!


No you should not attach them to the joist and no you should not have them match the pattern of the initial subfloor.

Disclaimer: I take subfloors way more serious than the average human or contractor. I will answer this question but must confess my deep distain for OSB - unless the house will be carpeted. However I am OK (not happy) with OSB in other installs including floating vinyl types.

Your first layer of wood, OSB in this case, is your subfloor. Subfloor should be nailed directly to joists (duhhhh where would you nail it!).

Any other layers that you add is an underlayment. You should not attach underlayment to joists (yes there are cases you would be these are one-offs and not here).

Any scientific study done by (building science, the APA, whoever) will show you that if you attach both to the joists you are building in fact a "swaying floor" in that the pieces will sway with the joists during seasonal changes. By offsetting the underlayment you are deflecting the sway from the joists and allowing the underlayment to move with the subfloor, which should be less noticeable than the subfloor moving with the joists.

So if I am installing OSB subfloor:

  1. I would really think about starting out with 1/2" plywood (or my go to 3/4"). Plywood is better and holds nails and screws better for underlayment. But if dead set on OSB make sure you have the tongue and groove on yours so you can lock them together.

  2. First layer (subfloor) should be staggered as you would when drywalling horizontally. You can start your edge in the corner but your next row should start at halfway point of the first piece in the first row.

  3. I am explicitly not talking about fasteners as it depends on materials AND what the joists are made out of.

  4. Felt paper (15lb) or similar material should be used as a buffer between subfloor and underlayment.

  5. The underlayment (in your case the second tier of OSB) should be offset from the subfloor. I know there is a recommendation of more than 6" but I would rather just go halfway again.

  6. The 2nd layer of OSB should not be attached to joists. Like I said this causes the most seasonal deflection - let's just call it sway. What does sway do? Over time it loosens up your fasteners and creates random gaps between subfloor and underlayment (your layers of OSB). These gaps to you means a squeaky or squishy floor.

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    This answer is fundamentally flawed, and dangerous. Whenever you use two layers of material to build up to the required minimum thickness of subfloor they both need to be nailed to the joists. If you installed the second layer as you described in your answer you would not have a subfloor that passed code (at least in the US). You are exactly correct about the installation of underlayment, except that neither of the the two layers in question are underlayment.
    – David
    Commented Mar 12, 2021 at 1:24
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    That's the key. Underlayment is one thing. Subfloor is another. Also, "any scientific study" needs citation and correlation.
    – isherwood
    Commented Mar 12, 2021 at 13:47
  • Interesting stuff. I see you here again isherwood! This is exactly the thing I'd like to get more understanding of. Can I just ensure my subfloor(first layer) is screwed to the joists and then the next layers above can be floating? Why can't they float, as this @DMoore person advises?
    – Mark S
    Commented Jul 4, 2023 at 22:12
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Here’s some expert advice. Coincides with @DMoore ‘s post https://www.huberwood.com/uploads/documents/technical/documents/2-Layer-Subfloor-with-AdvanTech-subflooring-Technical-Tip-Subflooring-AdvanTech.pdf

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    – Community Bot
    Commented Feb 12, 2023 at 1:35
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    Welcome to Home Improvement. Link-only answers are discouraged. It is best if you summaries the main features from that link in your answer. Commented Feb 12, 2023 at 1:36

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