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room without subfloor Current area, 10' x 12'

closeup of subfloor 3/4" subfloor meeting the existing subfloor, 4mm too tall

I've had to replace the subfloor in my living room, but my existing subfloor is 5/8" plywood and I can't find that size locally with tongue and groove.

I can find 5/8" without tongue and groove and I can find 3/4", but it is 4mm too tall.

One option a guy at the lumber yard recommended is to plane down the underside of the 3/4" where it touches the nearest joists so it is level with the existing floor and ramps to it's full height after 5 or so joists.

Is this an option?

What is the recommended method of making up 4 mm of difference between the subscore heights?

I want to put down vinyl planks, and they required a variance of not more than 1/8 inch over 6 ft according to their manual.

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    You could do that, or install a threshold between the rooms. Any particular reason not to use a threshold?
    – Cheery
    Sep 16, 2023 at 0:57
  • You can also use blocking between sheets in place of tongues and grooves... Sep 16, 2023 at 1:04
  • The main reason I didn't want to use some kind of transition is it's smack dab in the middle of my livingroom, so I'd basically be dividing the livingroom floor in half.
    – Jane Panda
    Sep 16, 2023 at 1:11
  • About 1/4 of the joists in that room have HVAC or water running through them; the others could have blocking though.
    – Jane Panda
    Sep 16, 2023 at 1:11
  • What are you putting over the top of it? Couldn't you just use some feather finish and float it out?
    – matt.
    Sep 16, 2023 at 2:54

3 Answers 3

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Your new subfloor does not have to be T&G. It may be preferred if it is a large area, but if it is a few pieces, the gaps between the regular plywood can be backed up with scraps of 5"wide plywood, glued and screwed, or 2X4 blocks instead. The main issue stopping the deflection between the joists where the plywood spans between the joists. Block those spans, which may be needed anyways at the end of the run where the new plywood meets the plywood from the other direction, if applicable.

One last thought, the tongue is typically centered in the edge, if you could plane it down, you would still have a difference on top. Besides T&G is a real bear to get together sometimes.

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  • The area is about 12' x 10', I can block about 75% of the spans, but several of them have pipes and HVAC running through them
    – Jane Panda
    Sep 16, 2023 at 1:32
  • Surprising everything is that close to the floor so that piece of plywood can't fit in somehow. It does not need to be full width if there is a space of 4" between the pipes, that can work. For insurance, add it to either side as well. Where the ductwork is, even a piece of 1/4" will work, since it will be glued and screwed. Consider how thick the tongue is that goes in the groove. 1/4" plywood should be a bit thicker than the tongue....
    – Jack
    Sep 16, 2023 at 2:11
  • I see you have the plywood already....
    – Jack
    Sep 16, 2023 at 2:12
  • If the edge that meets the original floor is only the 4' side with the square edge, planing that down will be the simplest way to resolve the issue. If possible I would do it over a few joists so it will fall within the guidelines of the flooring install.
    – Jack
    Sep 16, 2023 at 16:46
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What I would do is glue down some 1/8" hardboard over top of the existing subfloor. That would bring the floors flush. $15 per 4'x8' panel at HD, so not too pricey.

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  • This is a very good solution if you’ve already bought all the 3/4 t&g. (You’d still need to rip the tongue off at the transition and block underneath…) Sep 16, 2023 at 12:39
  • Or simply butt the groove end against the old subfloor...glue a strip of "something" (1/4" square dowel?) in the groove to fill it if you're worried about it cracking.
    – Huesmann
    Sep 16, 2023 at 12:42
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So the real issue is that the new subfloor is 4mm higher where it meets the old subfloor. I had this problem in the past. Here is my solution.

You are installing LVP...so sand the edge of the 3/4 down with an 80 grit on an orbital sander. Then float out the edge with floor leveling compound. Not the concrete power that is mixed the leveling paste. You only need to spread it out about 8 to 10 inches.

This will soften the transition enough that it will not be felt under a good quality LCP that is at least5 mm thick. ( anything thinner will give poor performance and does not have a thick enough core to be very durable.)

To help soften sound and give more padding to the transition, add an underlayment. There are many types. I have used the Quiet walk with great results.

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