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I'm building an 8x10 freestanding office shed. I used 2.5 sheets of non-T&G 3/4" plywood for the subfloor (I know this was not a good call but there's a plywood shortage in my area). I glued it down with PL400 and screwed it to the joists (most are spaced 16" and the rest are 12"). However all the seams fall halfway between joists, since I unthinkingly placed the half-sheet on the wrong side (I've been kicking myself over it ever since!).

I've already put foam insulation between the joists and it would be extremely challenging to add more joists from underneath.

I'm worried about movement and the edges "sawing" together. The floor will eventually be covered with T&G pine.

How can I keep the plywood from flexing? I'm thinking of cutting the edges of the plywood back to the joists and replacing them with 16" wide pieces of plywood. I would have 2 seams for every 1 seam but at least they'd fall on joists.

Buying T&G plywood is unfortunately not an option.

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  • Add a layer of plywood making sure to have seams fall on joist centers. Or tear up and redo. Get under the plywood and add support between joists to catch the seam. At least use a pocket screw jig to screw sheets together at 5 inch or less intervals. The pine t&g flooring should run perpendicular to the joists. I would be concerned that you will have an unsatisfactory result when end joints of t&g land on unsupported sub floor seam.
    – Kris
    Oct 25, 2020 at 22:27

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While t+g might have been a bit better, as long as your sheets landed well (1/2 on, 1/2 off) the joists, it'll be fine.

If you're still concerned AND you can get underneath, you could install solid blocking (2x4 on the flat) perpendicular to the joists under the seams.

Edit to reflect new information (seams not on joists):

Soooo... this isn't great, but still not the end of the world. If you run 3/4" t+g pine flooring perpendicular to your floor joists, you'll get a little flex, but your plywood underneath won't saw itself to death.

If you really wanted to fix this, you'd cut out the ply in the joist bays where the seams are. Cut next to the joist, so that the plywood that stays behind is on the entire joist. Attach something (could be as small as a 2x4, and doesn't, strictly speaking, have to be continuous) to the sides of the existing joists, and set a complete section of plywood on that. Glue/screw/etc. Use a 2x4 on the flat if you have any seams perpendicular to the joists.

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    I upvoted this because you specified that the joints in the sheets need to be about 1/2 on the joists. Also it should be noted that the 3/4” plywood is rated to span up to 24” spacing s for floors. (1/2” can span 12” and 5/8” can span 16”.) Your floor will be plenty stiff , without the T&G edges, so there will be no flexing and rubbing.
    – Lee Sam
    Oct 25, 2020 at 18:07
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    Thank you for your reply. I have edited my question with some additional detail (unfortunately seams don't land well on joists). Sorry for the lack of clarity.
    – fisherpeak
    Oct 25, 2020 at 20:46

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