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I'm preparing my floor to install new 3/4" solid hardwood flooring. I started off ripping up the old carpet and 5/8" particleboard. That left the 1/2" plywood subfloor.

My plan is to lay down new 1/2" plywood underlayment above the 1/2" plywood subfloor. I've read so many conflicting descriptions on the best approach—mostly around whether to screw into the joists or not.

  • Should I nail or screw? (My gut says to screw.)
  • What size fastener should I use? If not going into the joists, I'm not sure I can find 1" decking screws.
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    – isherwood
    Jun 24 at 19:59

2 Answers 2

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First of all I would check with the manufacturer's installation instructions and see if they have any recommendations? If so, do that unless you have a compelling reason to do something else.

Lacking any guidance, I've seen nails, staples, screws, and construction adhesive used.

If this were my project, however, I'd probably choose screws because they are simple enough to deal with and hold securely. Builders probably choose staples or nails because they are cheaper and go in faster. But their priorities are not always to do it the best it can be done.

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  • I've never seen a builder use staples for subfloor (as opposed to 1/4" underlayment, where they're appropriate to maintain surface quality). I'd be interested to hear of a specific example with details.
    – isherwood
    Jun 24 at 16:32
  • Come see my house, they attached the underlayment to the OSB subfloor using staples. I'm not sure what these are classified as but they about 1" wide and have a flat top. The problem is that they don't hold very well and the underlayment came loose in spots, which I was forced to fix about 10 years down the road.
    – jwh20
    Jun 24 at 16:49
  • Yeah, underlayment. That's different. They should've used narrow-crown stapes on a 4" grid, though. What you describe sounds like roofing staples.
    – isherwood
    Jun 24 at 18:14
  • Good call—I'll reach out to our supplier to see if they (or the manufacturer) can offer any guidance. Jun 24 at 19:13
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  • You should absolutely use screws into the joists (or ring-shank pneumatic nails, if available). That's a no-brainer if you want a solid, quiet connection. For your application, 2½" partial-thread screws are appropriate. Anything shorter will probably strip before pulling flush much of the time, or will grip the bottom layer and leave a gap.
  • Screws aren't used in the field because they won't pull in. The bottom layer will strip and the screw head will not pull flush.
  • Staples are used for underlayment, not a second layer of subfloor (which is what you're installing). By "underlayment" I mean 1/4" birch plywood installed for sheet vinyl. No one should use staples for subfloor. In many years as a builder I've never done it or seen it done.
  • If you want a bond in the field, use construction adhesive. Either apply a very thin bead in S-curve lines, or use a notched trowel to distribute. Walk it down well to eliminate any bulges. Use fasteners to bond any troublesome areas while the adhesive sets. In my opinion this would be overkill for hardwood. Ceramic tile, sure. You need extreme rigidity there.
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  • Thanks for your input! From the beginning, I was planning to screw into the joists. But then I read several threads saying that you should never screw into the joists example. I'm not sure why this seems so unsettled. Jun 24 at 19:24
  • That post seems to me much ado about nothing. I live in a very volatile climate and have never noticed the effects of "sway" in any of the many floors I've built. I'm not even sure what that's supposed to look like.
    – isherwood
    Jun 24 at 19:59

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