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I've seen many variations of my situation discussed here, but I'm new to all of this and I want to make sure my plan is sound.

I'm finishing a bonus room over the garage that currently has only a 1/2" plywood subfloor. That's too thin, so I need to install a 3/4" inch plywood underlayment on top of it. (There will then be LVP flooring on top of that.)

One unusual twist: because of the arrangement of the stairs, it is impossible to get a full-size 4x8 plywood sheet to the room. So the 3/4" plywood is all in 4x4 half-sheets.

From what I've read, it sounds like my approach should be to offset the underlayment so none of the edges line up with the edges of the subfloor sheets. (There's no question of whether the sheets will be perpendicular to the joists, because they're square.) I will use adhesive and screws to secure the underlayment to the subfloor (but the screws will not reach into the joists).

One question: since I'm not attaching the underlayment to the joists, I am a little unclear on exactly where the screws should go. Around the edges, obviously; but where should I place screws in the field? Just evenly spaced (i.e. in an eight-inch grid) throughout?

Anything else I've failed to consider?

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  • Why are you using 1-1/4" (or shorter) screws to hold this together? Using 2" screws would at least put 3/4" of screw into the joist below to help hold the whole thing together and add shear strength to the floor.
    – FreeMan
    Mar 18 at 12:15

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If it were me, I would act as if this new layer of 3/4" plywood were the only one going down. i.e.

  • Lay down adhesive around the perimeter and and along each joist line below
  • Screws around the perimeter and along each joist below
  • Screws long enough to reach through both layers and into the joists.
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  • Hmm. Most sources I've seen say that the underlayment should not be screwed to the joists, but attached only to the subfloor. Some even advise a floating underlayment that is not attached to the subfloor at all. That surprised me a little, but it seems to be the prevailing opinion in most places.
    – rrberry
    Mar 18 at 16:14
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    If that's what the installation instructions for your LVP call for, @rrberry, then by all means follow the instructions! In my understanding, though, "underlayment" and "subfloor" are two different things. "Subfloor" is what supports the weight of all the things on the floor (like people and furniture), distributing the weight to the joists, while "underlayment" separates the finish flooring material from the subfloor. I could be wrong.
    – FreeMan
    Mar 18 at 16:17
  • Well, we haven't selected the LVP yet. Perhaps I should look at whatever information I can find for whatever product we're likely to choose. But yes, I am using "subfloor" to refer to the existing 1/2" plywood that is already nailed to the joists, and "underlayment" to refer to the new layer of 3/4" plywood I need to install. The question is just how the underlayment should be attached to the subfloor, and whether it should be attached to the joists at all.
    – rrberry
    Mar 18 at 16:21
  • However, changing my terminology a bit may yield a different answer. Perhaps I should be thinking of this as two layers of subfloor.
    – rrberry
    Mar 18 at 16:24

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