Best practice for designing a stiff floor under bathtub – GreenBuildingAdvisor

Replies

1. Expert Member

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   AdamPNW | Apr 01, 2026 06:56pm | #2

   Just under 12’ (11’ 8”).

1. Expert Member

   MALCOLM TAYLOR | Apr 01, 2026 07:09pm | #3

   Adam,

   I wouldn’t do anything, but if you want to stiffen it I’d go to 12″ oc, which will also deduce deflection of the sub-floor.

   Make sure your layout works with the drains and any ducts.

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   AdamPNW | Apr 01, 2026 07:17pm | #4

   Thanks Malcolm. I’ll lay out the plumbing first.

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   freyr_design | Apr 01, 2026 08:08pm | #5

   depends on your flooring type but TCNA recommends L360 for ceramic and porcelain and L720 for natural stone. along with higher weight of tile may be something you should consider with your deflection loads

1. Expert Member

   MALCOLM TAYLOR | Apr 01, 2026 08:53pm | #6

   freyr_design,

   The online calculator I use shows the joists should be over L720 with a 16″ spacing.

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   freyr_design | Apr 01, 2026 09:25pm | #7

   to be fair i didnt actually look up spans or loads, just mentioned because most people underspec tile deflections and dead loads.

1. Expert Member

   MALCOLM TAYLOR | Apr 01, 2026 10:17pm | #8

   freyr-design,

   I was at a job last week where the owner decided to use 2’x4′ tiles on the bathroom and kitchen floors – and it was his first time tiling. Laid it over Ditra, with virtually no lippage. Pretty impressive.