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So I'm laying vinyl in a bathroom which is currently just floorboarded. From my research, I understand that I should screw down 5.5mm exterior grade plyboard (exterior grade because of the water) before laying the vinyl which is fine.

My problem lies in actually sourcing this exterior plywood. My understanding is that what makes exterior grade plywood exterior grade is the waterproof glue used between the layers. Yet upon googling, most of the links I am seeing don't mention anything about the glue used, and further state that to be used externally the plywood must be sealed! Seeing this I can't help but think i could buy bog standard plywood and seal it myself to the same effect. So my question is:

Is there anything specific i should be looking for (i.e some standard, or something else)? I've seen the standard EN636-2 thrown around a lot. Ideally the ply should be as cheap as possible, seeing as it won't be visible anyway (will be under the vinyl). Or can I buy general purpose plywood, and seal it to the same effect?

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  • What water? I'm always baffled that folks expect their bathrooms to be bathtubs. Water should rarely touch your floor, and that which does should be promptly wiped up.
    – isherwood
    Jun 18, 2020 at 15:44

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"Exterior grade" is confusing. The fact is that much of the plywood you encounter already is exterior grade, which (as you've stated) just refers to the adhesive used to bond the plies. It has nothing to do with wood preservation, for example. CDX is the rough grade I'd encounter in the U.S. for this purpose. The 'X' implies exterior adhesive.

Sealing plywood to achieve the same effect probably isn't a good approach. For one thing it'll change how your mortar bonds. For another, it isn't reliable as you'll be puncturing it with a hundred screws.

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  • One additional consideration: Pressure Treating. While CDX has exterior grade adhesives, pressure treating protects the wood itself against moisture penetration. Don't know if it's necessary, but it's something to consider in a bathroom floor that can get wet.
    – FreeMan
    Jun 18, 2020 at 15:44
  • If your subfloor gets so wet that it'll rot, so will the joists below and the walls adjacent. It should not be necessary.
    – isherwood
    Jun 18, 2020 at 15:45
  • Fair point. Then why the recommendations for CDX? (yours and what the OP has been seeing elsewhere.)
    – FreeMan
    Jun 18, 2020 at 15:49
  • CDX seems to be a little thick for what i'm looking for. Is there something with similar glue properties but thinner?
    – Mr_Random_Guy
    Jun 18, 2020 at 15:53
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    @FreeMan For subfloor CDX is fine, but for underlayment most vinyl flooring manufacturers recommend ACX because the “A” side is plugged, patched and sanded. That way imperfections in the top ply won’t telegraph through to the finish vinyl flooring. Pressure treating plywood weakens it. Not recommended. I agree with Isherwood, adhesive may react with pressure treatment especially because it’s wetter too.
    – Lee Sam
    Jun 18, 2020 at 17:11

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