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


I just bought a house built in 1978 and I'm planning on putting tile in the bathrooms (currently linoleum).

The fixtures are removed and the flooring under the linoleum is particle board. I think its 3/4". It has a couple weak spots (one over a vent and the other around the toilet that was leaking).

I was planning on pulling this up and replacing with plywood and concrete backer board. However, my question is, can I put 1/4" backer board over 1/2" plywood, so that the level of the floor is pretty close to where it was before and not have a large transition between the rooms?

-edit to add-
1 bathroom is along an outside wall of the house.
1 bathroom is in the middle of the house with a support beam running along one wall (beam runs the whole length of the house).
Bathrooms share a wall in between them (total span of 10ft from outside wall to middle support beam of the house).
Both bathrooms are on the 2nd floor.
both bathrooms are above another heated/insulated room.
16in joist spacing.
I don't know how big the joists are because I haven't removed the existing subfloor yet.
We will most likely be installing ceramic tile.

My main goal here is to create the smallest transition as possible between the bathroom flooring and the carpet (installed on the same particle board) outside the bathroom.

If there are other options outside plywood + backer board that would be available, I am 100% willing to listen to them.

Best Answer

1/2" plywood is for temporary applications, exterior sheathing and used as gigantic shims. It does not belong as a floor or a roof; minimum 5/8" exterior grade.


What subfloor should HardieBacker board be installed over? The minimum subfloor specified is 5/8" exterior grade plywood or 23/32'' exterior grade OSB subflooring. HardieBacker board must be installed in accordance with local building codes and the floor must be engineered not to exceed the L/360 deflection criteria, including live and dead design loads, for the specific joist spacing used. HardieBacker board is not designed for use over concrete.


Subfloor should be minimum 5/8" exterior-grade OSB or plywood.


For the subfloor, securely glue and fasten minimum 5/8" (16 mm) exterior grade plywood or OSB panels (PRP-108) to the floor joists. Floor joists should be spaced a maximum of 16" (40 cm) o.c.; a 3/4" (19 mm) Exterior Grade Plywood or OSB subfloor with I-joists spaced a maximum of 19.2" (48 cm) o.c. is also acceptable.

For the threshold you don't want something that overlaps, like this:

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For a Hardwood transition, you want the type that abuts the tile, like this one-sided beveled threshold: (I've had luck filling against them with grout; complementary colored caulk is also available for tile-to-wood transitions. Standard thresholds can be ripped down, length-wise, to remove one bevel and then the bottom sanded down for a custom fit. Endeavor for absolutely perfect cuts along the doorway.)

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Since you are transitioning to Carpet, I'd use something like this or this: (an abrupt leading edge is more of a concern then a reasonably high threshold)

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