wood subfloor in bathroom

I have a small 5 x 11 bathroom in 1961 coop/condo, wood subfloor (pictured) is not level. We don’t think water damage just building settling. This is a gut job, all fixtures being replaced. Would it make sense to use purlins then 3/4 plywood to level floor, then place new tub on new plywood? Or backer board & tile entire floor and put tub on top of tile?

Can’t put tub on joists or current subfloor as uneven. Can’t use self leveling compound as depth of pitch too deep (1/2 an inch or more).

  • You are putting a roof under your tub?!
    – virtualxtc
    Mar 21, 2018 at 16:41
  • I don’t understand your joke. The subfloor is not level. I have to level floor. Purlins are thin triangular shims used to level floors (among other things). My question is logistics of using these shims under a bathtub. Can’t use self leveling compound. Picture of subfloor to clarify it is composed of planks with gaps.
    – Joe
    Mar 21, 2018 at 16:52
  • No, they aren't: en.wikipedia.org/wiki/Purlin
    – virtualxtc
    Mar 21, 2018 at 17:39
  • 3
    @virtualxtc Joe, take a deep breath. Virtualxtc is correct. Purlins are structural members not something to be used for leveling floors. You’re thinking of “leveling shims” as opposed to “framing shims”. Even the Code defines purlins, See ICC R802.5.1.: “Framing members to reduce joists spans.” You are looking for a non-structural member to lay on your existing floor. BTW, both your examples call for leveling shims, not pulins.
    – Lee Sam
    Mar 21, 2018 at 22:56
  • 2
    There is the presumption that question posters on this site might be novices, or they would not need to ask. Terminology is often incorrect (and also differs according to region). It is not hard to correct it and preserve the OP's dignity at the same time; it is also important for OP to gracefully accept corrections. Take it easy guys. Mar 22, 2018 at 1:29

1 Answer 1


Assuming your plan is to level the entire floor area, I feel that using tapered shims with the 3/4" ply would be the best of the options presented. You will have to deal with the extra floor height. I just don't like extending tile under the tub.

Anyhoo, if you were to level backerboard you would need leveled plywood under it anyway, for support, no?

  • Thanks to Jimmy-Fix-It for his knowledgeable reply; the only difference in my query is whether this approach to leveling is appropriate in a bathroom. The contractor I spoke to wants to do a mud bed which would probably be easier (to shim would have to scribe shape of floor which would be difficult on an old uneven floor).
    – Joe
    Mar 24, 2018 at 20:21
  • Virtualxtc nothing you said was helpful, nor did you refer to other topics, and quoting Wikipedia to complain about a term, while making no helpful comment on the issue at hand, doesn’t make you look smart. If you don’t know, you don’t know - no shame, just get off the topic if you have nothing to add.
    – Joe
    Mar 24, 2018 at 20:29
  • I’m on the 3rd floor of coop building - there’s no way for me (or contractor) to get beneath subfloor. Downstairs neighbor would not let us in to destroy her bathroom ceiling. If a tile bed, am assuming a waterproof layer (?? Roofing felt??) then wire mesh then concrete. (I wouldn’t attempt this, am trying to see if I can level floor myself). That’s why I ask about shims...
    – Joe
    Mar 24, 2018 at 20:39
  • @Joe, if you want to level it without shims and all that, you could "dry pack" it. It's not hard, you could do it yourself. Staple roofing felt (no mesh needed) then use a long level to create level bridges of "dry pack" (moist, not wet, mixture of sand and portland cement), with wet redwood strips on top. Fill the areas between the bridges by packing mixture in with a wood trowel and screeding level from redwood strip to redwood strip. It's a time-tested technique for leveling floors as a prep for tile, and it's hard to describe. youtube.com/watch?v=a6GGwSY8oj4 Mar 24, 2018 at 22:04
  • This will be easier for you than the shim option, I didn't mention it in my answer because your question seemed to ask which of the presented options would be better. There's a lot of info on the web about dry-pack floors. In a wet area like a shower there would be significant additional considerations. Mar 24, 2018 at 22:07

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