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I just pulled up the hardwood floor in the house I just bought, and found that there is 1/2in plywood subfloor. House was built in 1982. Joists are 2x10 16"OC.

The room is 16x13.

The person who built the house used 1/2 subfloor throughout the house.

In my kitchen, is it OK to put thinset on top of the 1/2in subfloor, then 1/2in hardiebacker on top of that, then porcelain tile? I spent most of the day reinforcing the floor from the basement with small pieces of 2x4's between the joists and along the floor seams. Then went over the entire floor to secure it tightly to the joists with 3in nails from the nail gun.

The only issue is if we put down 5/8 on top of the 1/2 subfloor, then thinset, then 1/4 hardiebacker, then tile, the floor is almost an inch higher than the dining room floor coming off the kitchen. Not sure what else to do since the entire house is using 1/2in subfloor.

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Sounds like you got it under control, but what you might consider instead of 5/8 inch plywood is 1/2 inch plywood (so now the floor is 1/2 inch higher). This is plenty thick for a finish tile floor. It would be wise however to install the new plywood sub-floor perpendicular to the plywood floor it will cover. Also consider fastening it with screws and or ring shank or spiral nails. Bridging between floor joists was the way to go for added support under the floor. Give a final check by walking the floor listening for squeaks and looking for any deflection (bounce). To accommodate the 1/2 inch difference in floor heights, purchase or cut a transition strip of hard wood.

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According to the three most prevalent manufacturers of backer board in the US, no, 1/2" ply is not acceptable. All of them require at minimum 5/8ths plywood.

Is it OK to use 1/4“ concrete backer board over 1/2” plywood as a bathroom subfloor?

That being said, I'd lay down some 3/8" or 11/32" over the 1/2" and then backerboard over that, like ojait suggests.

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