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Unread 01-21-2019, 02:46 PM   #6
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Join Date: Jul 2001
Location: Boerne, Texas
Posts: 89,974
Welcome, Jesse.

You do understand that the joist deflection is calculated over the entire unsupported span of the joists, not just the portion under the room to be tiles, yes?

I would also like to know more about that "concrete" layer you have. It's much more common to see poured gypsum-based underlayment in applications such as yours.

Originally Posted by Ali
CX (resident pro builder) recommended 1/2" exterior grade plywood with a face grade no less than C glued and screwed to the existing subfloor (not to the joists) to eliminate lateral deflection.
I bet if you remember further, Ali, CX told you that might be an option if your existing layer of plywood was in near pristine condition and the second layer was laminated with a full spread of wood glue and lots of mechanical fasteners. With the glue-lamination it is not important that the mechanical fasteners do or do not penetrate the joist structure.

And please note that the purpose was to decrease the between-joist deflection, a vertical component, and has nothing to do with "lateral deflection." We do wanna be careful not to confuse new visitors, eh?

Tell us more about that two-inch layer of material, Jesse, and we'll work from there.

You are/are not planning to tile the bathroom floor as well as the shower?

My opinion; worth price charged.

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