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My second floor has an existing subfloor constructed with 5/8" plywood. I am installing a second layer of 3/8" plywood to raise the level of the subfloor prior to installing bamboo flooring. I plan to both glue and screw the second layer down. The total area of the project is over 1000 sq ft.

Would it be best to use construction adhesive applied from a tube or from a bucket and troweled on? How much coverage of adhesive should I use?

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  • See diy.stackexchange.com/questions/293268/…. 100% coverage. Technically there's an ASTM spec for subfloor to joist glue, ASTM D3498, where that stuff would probably do a better job. It probably costs 10x as much too, though, and the premixed thinset is what people use (apart from actual thinset). Here's a page that aggregates a bunch of the D3498 glues: grainger.com/category/adhesives-sealants-and-tape/….
    – popham
    Commented Feb 15 at 20:38
  • And your underlayment screws are supposed to miss the joists.
    – popham
    Commented Feb 15 at 20:40

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Because 3/8" plywood is pretty flexible I'd want a uniform, consistent application of adhesive. You'd have a hard time doing that from a tube. Even 1/2" plywood is rigid enough to do it that way, but not 3/8.

Then, a 1/8" v-notch trowel would probably be fine, but I'd look into what the manufacturer of your product recommends.

However you go, be sure to roll or walk it down well to get good initial contact. Even if the layers later spread a bit it'll still be supported by the adhesive.

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  • Is there a specific construction adhesive product you would recommend? I find that when I search for construction adhesive and select subfloor as the usage on the Home Depot and Lowes websites that the only products it displays are in a tube. Commented Feb 15 at 20:06
  • Doesn't really matter that it comes in a tube. It matters how you apply it. Squirting it out if a tube is just fine if you then trowel it to distribute. "Heavy duty construction" or "subfloor" adhesive is what I'd use.
    – isherwood
    Commented Feb 15 at 20:08
  • Got it, thank you very much! Commented Feb 15 at 20:32
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    You're welcome. Please take the tour so you know how to respond to answers. "Thanks" comments are discouraged.
    – isherwood
    Commented Feb 15 at 20:38
  • @Troy, your Home Depot search was surfacing ASTM D3498 adhesives. Those are intended for joist to subfloor applications. They're "gap filling" adhesives that are calibrated to develop the 1-1/2" wide contact area between joists and subfloor sheets into tee shaped composite beams. These adhesives are unnecessarily strong for full sheet-on-sheet bearing surfaces.
    – popham
    Commented Feb 15 at 20:54

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