I'm replacing the floor in a ground floor bathroom. How do I mate the old plywood subfloor with the new one? I plan on leaving a gap between the subfloor and the sole plate around the perimeter of the room to allow for the plywood's expansion and contraction but, what do I do where it meets the doorway. Do I need a tight fit with the plywood in the doorway or do I leave a gap?

I'll install blocking and sister the last joist.

Do I need to cut the old subfloor back so that half a joist width is exposed in the doorway, so that the old and new plywood are fastened to the same joist? I plan to sister the last joist. There will be travertine on cement board over the subflooring.

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  • A small gap shouldn't matter unless your finish flooring material would be vinyl where it would transfer through.
    – topshot
    Commented Oct 3, 2016 at 13:04

1 Answer 1


I see you have 3 layers of flooring that was under the old tile. The top layer I see the cement board, under that the underlayment, and under that on top of the joists, the subfloor.

To get a good tie in to the existing, use a right angle grinder or circular saw with the proper blade, and cut back the cement backer about an inch away from tile floor coming through the doorway from the other room. If it is not tile, like I think it is and there is a joint somewhere close to what I described earlier, remove the material up to that joint then. That looks like it may leave about 3-4 inches of plywood underlayment exposed after the cement board is cut and removed. Next set the circular saw to depth for the thickness of the plywood underlayment and make a cut about 1 1/2-2" away from the cement board, finish the cut to the side of the door rough opening with a multimaster or reciprocating saw if you can control the depth of cut, and remove that small piece. Do nothing with the subfloor. Add screws along the edge of underlayment to secure it to the subfloor. Add the ledgers/sister joist, the subfloor. When cutting the underlayment, make sure it is one piece going into the doorway and out into the bathroom a ways so the joints in the underlayment and subfloor do not line up. Use screws to hold the underlayment to the subfloor, use construction adhesive if you choose to as well. The screws go in at 4-6" on the perimeter, 8-10" in both directions in the middle. Do not concern yourself with those screws going into the joists, it is better if they do not. 1 1/2" or so long screws will do well there.

I typically gap all my plywood that is not T&G about 1/8" on all edges. when it is T&G, I still gap the ends 1/8"

  • Thank you for the detailed instructions. Do I need to tie the subfloor into the new subfloor? Or will running the underlayment and cement board through the doorway be enough to tie everything together? Do I leave a gap between the plywood in the doorway or only on the outside edges of the room?
    – Laxmidi
    Commented Oct 11, 2016 at 16:00
  • Gap everything 1/8" or so a little bigger won't hurt on both layers. Make sure the ledger applied to the existing joist is secured very well. use glue between the two if possible or screw it together. Make sure there won't be no movement between the two. With all the new material going in the same thickness, and the lap of the underlayment bridging the joint and tying the area all together will be a good it will ever need to be. It should be as strong as a all new install.. Just as a important reminder, do not let parallel joints line up on each other the 2" separation I mentioned should do it
    – Jack
    Commented Oct 12, 2016 at 4:39

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