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.

  • I would not use 5/8” plywood but most building codes still today will allow it. You most defiantly can tile directly to plywood but it’s not the best idea. Custom products mortar as mentioned above will warranty and it does plain as day state you can use their product in residential and light commercial area’s. Every one on her is going to have their own opinion on a lot of things like using 5/8” plywood but some things the opinion has been taken out when a manufacturer of a product states you can use it. So take every answer with a grain of salt and remember that all the new top of the line p Commented May 27, 2022 at 16:58

4 Answers 4


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:

enter image description here

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.)

enter image description here

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)

enter image description here enter image description here


For most homes you want a minimum of 3/4" plywood + 1/4" backer or membrane (Kerdi). This is bare minimum.

But there are a lot of factors to your install. What is your joist spacing, how big are your joists, is there currently any deflection in your joists, where in the joist does the bathroom sit (on top of a support beam or in the middle of a run?), what type of tile... and so on. Any answer that gives you specifics without knowing these things could waste your time and money.

If you want to be really safe you can go 1" subfloor plus 1/2" backer and this would be OK for 98% of the homes built in the late 70s (assuming they met building codes then). But this doesn't help your transition issue. You could lose maybe 1/4" by putting a membrane in. Still have a transition step with thinset plus tile.

So you have a couple options:

1 - do it wrong, hope flex and movement in floor doesn't cause cracking in tile or grout lines.

2 - do it right, live with transition. You can put this right under the door and no one will notice.

3 - go with something else like rubber planking - some look just like tile. These would float right over plywood. You can redgard floor if you want to add some water protection if needed.

  • Thanks for the feedback. I updated the OP with answers to your questions as best I can.
    – kinar
    Commented Oct 26, 2014 at 16:32

I've seen pro tilers install tiles right onto a plywood (not particle board) subfloor by putting down Ditra mat. I'm not a tiler so I can't say 100% that this is advisable but it's up to commercial code anyway. It would solve you're height issues and its easier to install than backer.


Install your tile directly to the 5/8" ext. grade plywood. Use a high-grade polymer fortified thin-set mortar specifically designed for direct application to plywood flooring. I have had very good results, proper application is key. Check it out: Custom Building Products. Your finished height should end up just slightly higher than your old floor, a nice transition bar will prevent any tripping hazard between your carpeted floor and your beautiful new tile.

  • I don't think you're eligible for Custom's Lifetime Warranty if you install directly on plywood.
    – Mazura
    Commented Oct 26, 2014 at 7:26
  • @mazura, did you even follow the link to the Custom spec sheet for the referenced mortar? What part of "Suitable Substrates... exterior grade plywood" and "exceptional adhesion to plywood subfloors" did you not understand? You "don't think"? How about: do not comment unless you know, and know by experience... not just a lame guess. Commented Oct 26, 2014 at 19:46
  • 1
    Found here, custombuildingproducts.com/homeowners/… it's part of step 1 for installing over a wood floor.
    – Mazura
    Commented Oct 26, 2014 at 20:07
  • Personal (long gone) installations of mine have failed due to the plywood yielding and delaminating due to lack of a moisture membrane (which backer itself somewhat provides); not from mastic failure.
    – Mazura
    Commented Oct 27, 2014 at 0:18
  • 2
    Traditionally, for wet applications, the arguably proper method is a "dry pack" mortar bed sloped to a tile drain followed by a hot-mop asphalt application, followed by floated cement followed by tile set in thin-set mortar. It is a method that is quite labor intensive involving specific steps. This is still a common method for commercial and residential applications, but becoming less so with the advent of modern alternative materials and techniques. Commented Oct 29, 2014 at 1:49

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