My contractor is installing a 3/16" plywood as a shower pan subfloor on 16" floor joists. It is for a curb-less shower. I think they did that because they built the floor too low to allow water drainage and now has to

  1. Cut into the joists for the slope
  2. Use 3/16" plywood as subfloor (44" x 76")

I am very concern about the integrity this flooring. My questions are:

  1. Is this safe?
  2. What are the potential problems?
  3. How long can it really last? It's newly built and seems OK now but over time moisture could erode underneath things can fall off. It just looks very fragile.
  4. What would the fix be?

3/16" plywood subfloor cut joists

  • 5
    Not even close. Decking on outdoor decks is typically 1.25 lumber, and on 16" spans you can feel it flex if you bounce on it. 3/16 it'll sag and crack instantly. Commented Nov 11, 2021 at 7:57
  • 2
    I feel for you... this is a spectacularly terrible job in so many ways. I hate to say it, but I think you might need to start thinking about having a non-curbless shower. Is this bath on the first or second floor? What's underneath? (I'm assuming it's finished space underneath, but if it's a basement and you can steal a bit of ceiling height, you might have some options that keep the curbless.) Commented Nov 11, 2021 at 14:28
  • 3
    As a further aside, start looking for a competent contractor. You might try to get this one to fix things, but I wouldn't hold my breath. Don't pay another dime. (And stop recent checks if you can.) Commented Nov 11, 2021 at 14:29
  • 2
    Those unlevel joists are enough to make the tile crack the first time you step on it. If the contractor knew from the beginning that you wanted a curbless shower and built that floor then don't you dare pay them another dime. If your curbless shower is a "change order" then it's still their fault for not quoting you enough to re-do the floor framing properly. If they "cut you a deal", well you're face-to-face with that ill-fated choice now.
    – MonkeyZeus
    Commented Nov 11, 2021 at 20:51
  • 4
    The contractor built the floor and they knew I wanted curbless shower. This is all on them.
    – manni
    Commented Nov 11, 2021 at 21:47

2 Answers 2


Is this for a threshold free tiled shower floor?

3/16" - that's a joke.

TCNA requires 1 1/4" subfloor for a tile installation.

Assuming the contractor tiles on top of 3/16 and grouts it. The grout will crack almost immediately. What waterproofing system are they using? Hotmop, kerdi, red guard?

The fix is getting a competent contractor.

Also it looks like someone knotched all the strength out of your joists. The dimensional floor members were probably 2x10 but it looks like they've reduced them to be the height of a 2x4 at the notch. For a back of hand strength comparison of those members you take the square of the inverse so a 2x10 is 100 in comparison to your 2x4 which is 16. So your floor member is now 80-90% weaker - it doesn't even look like they left you 4" of depth on that 2x10.

  • 35
    Quick! Call your building inspector in to have him take a look at this! Even if it costs you $100 or more to have him come for an inspection that you know you'll fail, it will force the contractor to fix it all on his dime for creating this mess.
    – FreeMan
    Commented Nov 11, 2021 at 13:21
  • 2
    @JPhi1618 You're talking about shaving off the top, but that big ass notch taken out from below for the Y, and the angled pipe further up already reduced the joist to a thin piece.
    – Logarr
    Commented Nov 11, 2021 at 17:13
  • 2
    @Logarr, You're right. I was focused on one dumb thing and became blind to all the others. Now I have to wonder if they ran the new ABS drain pipe or if it was like that and they "just" shaved the top.
    – JPhi1618
    Commented Nov 11, 2021 at 17:15
  • 2
    @JPhi1618 This is a raised floor about 2' directly above concrete slab.
    – manni
    Commented Nov 12, 2021 at 19:10
  • 2
    So this floor assembly is over a 2' crawl space. This makes fixing it way easier, it certainly doesn't mean the joists are fine. Also they have a 2' crawl space and they decide to cut the crap out of the floor joists to run the plumbing? I am highly resistant to firing people and would almost always get people to fix up a job. In this case I'd fire them. This is one of the most horrible jobs I've ever seen - sorry you are in this situation! Commented Nov 13, 2021 at 3:37

Per IRC-2018 table R503.1, a subfloor on 16" centers requires 5/8" minimum.


The same minimum required by every manufacturer of backer board that I know of.

Is it OK to use 1/4" concrete backer board over 1/2" plywood as a bathroom subfloor? No, 5/8" minimum always for any floor anywhere.

3/16" plywood is for use upright, or as a shim over a subfloor that already meets spec.

"minimum" - Use 3/4" T&G and backer board. I've never even seen 1-1/4" plywood.

If you want curbless you need to raise the height of the entire rest of the floor and have a nasty transition at the doorway. Up to you. Curbless is silly IMO; how you gonna put a door? (no door? That's even sillier; sounds cold).

At this point I wouldn't want anyone to see it or they'll make you redo all of it.

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