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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

manni
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2 Answers2

23

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.

Fresh Codemonger
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4

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

https://codes.iccsafe.org/content/IRC2018/chapter-5-floors

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.

Mazura
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