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Replacing damaged parts of a plank floor that is 7/8" thick. Need a level surface to lay another 1/2" plywood subfloor layer. Can I 'glue and screw' a two thinner pieces of plywood (1/2" and 3/8"), or do I need to source 7/8" plywood? I would use pl premium and screws for compression between the layers. My other idea was to add 1/8" hardboard strips on top of the joists in those sections and just using 3/4" instead. Any help would be appreciated.

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  • Depends on how far the plywood is spanning. How far apart are the floor joists.?
    – Lee Sam
    Commented Oct 15, 2018 at 14:32
  • Right. Forgot to mention the joists are 16" OC
    – Eric HS
    Commented Oct 15, 2018 at 16:52
  • I'd use 3/4" for strength on top of 1/8" shims on the joists. (In theory, your 1/2+3/8=7/8, but I'm a skeptic.)
    – Aloysius Defenestrate
    Commented Oct 16, 2018 at 0:43

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I wouldn’t try to laminate two pieces of plywood together.

Group 1 type plywood is rated for various spans. Using 1/2” plywood to span 16” will span the distance without failing, but will have too much defection. That is to say, it will be springy. It’s designed for 2 span installations. If it’s a one span situation, it will be noticeable.

Adding additional plywood layers does very little to decrease the deflection problem and if the adhesive is not installed correctly (and cured correctly) it could create other problems, like 1) bulging, 2) warping, 3) delaminating, etc.

I’d use 3/4” plywood with the wood strips you suggest. 3/4” plywood will be stiff enough that you could use in a single span situation and still be acceptable for ceramic tile floor, etc.

Also, remember to install with the face grain perpendicular to the joists. (Screw and glue...adhesive... to insure no squeaks.)

Also, this is for “subfloor” only. You still need an underlayment. Check with the manufacturer of your finish floor material to see what they require for an underlayment. (It’s different for ceramic tile, vinyl, etc.)

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  • Thanks very much for the detailed response. Before seeing your response, I actually tried the 'glue & screw two layers' approach (making a complete disaster of a couple sheets of plywood and a v-notched trowel trying it), and then ended up ripping them up when it didn't work out... then I smartened up and bought some 3/4" sheets and 1/8" boards and cut strips to fasten to the tops of the joists as you suggested. Fast and easy. Thanks
    – Eric HS
    Commented Oct 18, 2018 at 22:59
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Your plan should work just fine, assuming that the layers are bonded as you describe. I don't know that I'd bother screwing between joists, though, or maybe do so sparingly just to ensure a snug bond.

You'll want to apply the adhesive with a controlled bead (maybe 3/6" in a consistent pattern) or v-notch trowel to prevent waviness. The 3/8" layer is vulnerable to bulging due to excess adhesive.

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  • Thanks for confirming and for the tip about bulging. I'm pretty sure I dont have v-notch trowel... would a drywall or putty knife work to smooth out the adhesive?
    – Eric HS
    Commented Oct 15, 2018 at 17:51
  • Nope. You need to create ridges and valleys so that the adhesive can spread and make good contact while remaining flat. You'll have a difficult time creating a flat surface with a straight trowel. They're inexpensive.
    – isherwood
    Commented Oct 15, 2018 at 18:25
  • @isherwood Plywood is made from plies installed 90 degrees to one another. Adding another sheet of plywood will have two sheets running in the same direction, which could cause warping. See this. en.m.wikipedia.org/wiki/Plywood The plies are glued together using heat and pressure. “Home-made” plywood seems dangerous and could cause problems.
    – Lee Sam
    Commented Oct 16, 2018 at 1:14
  • Won't cause warping. Will be plenty stiff. There's nothing homemade about using several layers of plywood in conjunction. It's done every time birch underlayment is added to a subfloor, with obvious benefits.
    – isherwood
    Commented Oct 16, 2018 at 1:43
  • @isherwood But that’s not for structural. That’s underlayment (for floor finish) over subfloor (structural).
    – Lee Sam
    Commented Oct 16, 2018 at 3:12

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