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I am going to tile a bathroom. I have removed the linoleum and all adhesive. There is a 1/4" plywood layer over the original subfloor. The floor is dry and clean. Do I need to remove the plywood subfloor before installing the backer board and tile?

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If your plywood floor is in good condition, I would drive screws to securely attach it to your floor beams. When I did my floor, I saw they were attached with nails which was not ideal even though I had no squeeky issue.

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  • Screws don't really hold in 1/4" – redlude97 Apr 2 at 18:12
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No, if the plywood is sound, clean, and flat, it's a perfect surface (unless the built-up layers cause your finished tile floor to sit too high).

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  • I don't think 1/4" plywood is acceptable as a substrate in any circumstances according to TCNA – redlude97 Apr 2 at 18:11
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The 1/4" layer is useless. I'd remove it. You'll need a proper decoupling layer (Ditra or a generic alternative) between the subfloor and the tiles. The thin plywood will add flexibility where no flexibility is needed and can cause the grout to crack.

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  • Actually it isn't useless. Any additional subfloor increases stiffness, which is good. It just needs to be well secured. Also, since you don't know the thickness of the underlying subfloor, this extra thickness may be necessary to meet tile and mortar requirements. – isherwood Apr 2 at 18:19
  • The "well secured" part is correct but disingenuous. You'd have to glue it down with a construction adhesive or epoxy. Recall that stiffness of a composite plate is maintained by the shear loads transmitted between the composite layers. And in practice that means you're building up more plywood layers manually. If that glued connection fails anywhere, the thin layer will be less stiff than the subfloor beneath. So nope, in practice it never helps. And no, screws or nails will not work on material so thin - there'd need to be too many of them (1" spacing on a regular grid). – Kuba hasn't forgotten Monica Apr 3 at 18:43
  • That's not the only way stiffness is achieved. Tile floors all over the world have two unattached layers of subfloor under them. Let's not get bogged down with pedantics. – isherwood Apr 4 at 15:44

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