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My upstairs sub flooring is 5/8" plywood, which is screwed to the joists. I want to add an additional 3/8" layer of plywood, followed by floating 1/2" bamboo flooring. I plan on using a 2mm underlayment between the second layer of plywood and the bamboo flooring.

Would it be OK to install a 2mm underlayment between the first (5/8") and second (3/8") layer of the subfloor?

Other than some additional expense, are there any downsides to this?

Also would I be correct in orienting the second layer of plywood so that the gaps between boards do NOT align with the first layer boards?

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  • The 3/8" plywood itself would be underlayment. If the 2mm layer is below the 3/8" plywood, then technically the 2mm is not underlayment. You should be more specific about this 2mm. Is this a layer of sound dampening?
    – popham
    Commented Feb 11 at 20:34
  • you can do it. Your floor ends up slightly more bouncing but that is the general effect of underlay. The floor manufacturers typically spec the compressive strength of the underlay so that you don't end up with so much deflection that you break the click lock of the floating floor. is your floating floor tng with tng glue or is it a click lock? Commented Feb 11 at 20:40
  • @popham - it was just an idea that came to mind. Wondering if it might be worth it for additional sound deadening and eliminating potential squeaks. I plan to screw the 3/8" plywood down to the 5/8" layer of plywood. Commented Feb 11 at 20:48
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    Not to beat this ailing horse, but asking whether it's "ok" and for any "downsides" implies that you're beyond questioning whether it's even going to accomplish the goal.
    – isherwood
    Commented Feb 13 at 19:10
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    Is it "OK"? I'm sure it is. I think that the question we're trying to get at is what benefit do you expect to gain by doing so? If you'll address that, you'll probably get a "Yes, it'll achieve your goal", or "No, you're headed down the wrong path". Other than adding a few mills of thickness to the floor, I'm not sure what benefit you'd gain, but, once you've nailed down the 2nd layer of sub floor over it, I don't really see any downsides other than the added expense.
    – FreeMan
    Commented Feb 13 at 19:45

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I always go to tiling sources for subfloor details. If you look at the specs for span rated plywood, you'll discover that it has the typical L/360 deflection limit, but they use 100 psf instead of the floor's 40 psf rating. I assume that this is to avoid squishiness under foot presses. I imagine that way back they measured the squishiness of an old floor built from 3/4" boards and conjured a spec to replicate the feel, but I'm just speculating.

The TCNA Handbook (a tiling source) has a 5/8" subfloor plus 1/2" underlayment spec for floor joists on 16" centers. I recall that Schluter (a tiling system manufacturer) has a spec for 3/8" underlayment, but that's below one of their Ditra products, so it's not applicable here. Operating within this context, I wouldn't spec 3/8" underlayment for new construction.

By gluing the subfloor and underlayment into a composite, you can still get the stiffness of a 5/8" subfloor plus 1/2" underlayment. I don't like it because it's extra work. Further, anybody who tries to remove the underlayment in the future is in for a big surprise. I would use the "premixed thinset" that tilers use ("mastic" or "organic adhesive" are other common names). A 3/16" v-notch trowel would be the tool to get a nice even distribution without too much thickness. Both the subfloor and the underlayment must be Exposure 1 rated (the "X" in CDX means Exposure 1, and OSB is generally Exposure 1 rated).

Your 2mm layer sounds like more cost and work. Additionally it would interfere with the composite floor if you use the 3/8" underlayment plus organic adhesive.

The typical detail of underlayment calls for 1/8" gaps between sheets and staggered to miss the subfloor joints. I think that the sheets tend to have a bit of extra thickness at the cuts, so I imagine that part of the justification is to avoid adding this extra thickness twice at the same spot. Additionally, the interior spans of plywood away from the ends have more stiffness than the end spans. The other justification for the staggered joints, I imagine, is to avoid stacking the underlayment's soft spans on top of the subfloor's soft spans.

The TCNA Handbook's F150 detail calls for 1/8" gaps between underlayment panels. It doesn't specify fasteners and it doesn't specify a fastener spacing. Every 8" sounds familiar.

And don't orient the underlayment incorrectly. Its face grain should run perpendicular to the joists. Just align the long edges of your underlayment parallel to the long edges of your subfloor, and you should be fine.

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