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Unread 01-11-2015, 02:35 PM   #26
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Join Date: Jul 2001
Location: Boerne, Texas
Posts: 95,342
Scott, you'll find some contradictions between that MIA manual and the stone installation methods in the TCNA Handbook, which began to publish a few stone methods in, I believe, 2012. And you'll find some clerical errors in each, some of which have caused a good deal of confusion in past. Such as their requirement for substrates to be "level" to a specific tolerance when I'm sure they mean flat. And for the joist deflection in the TCNA saying L/360 is required when the MIA still requires L/720. You've gotta overlook some of that with an eye towards the intent and that's not always an easy thing.

You can find in the TCNA Handbook a published method for stone installation over wood framed subfloors using CBU, for example, that requires a first layer of nominal 5/8th" and a second layer of nominal 1/2" plywood. That method requires a maximum of 16" joist spacing, though. I think they would accept 1/2" over 3/4" over 19.2" joist spacing, but I can't prove that and Kevin's recommendation of two layers of 3/4" might be safer.

The method of gluing of the plywood layers with beads of construction adhesive I would take issue with and the ceramic tile industry actually has testing that indicates some small measure of uncoupling the tile installation exists when the second layer of plywood is mechanically fastened only to the first layer and not to the joists and definitely not glued in any manner.

Whole lotta things to consider and we'll always try to lead you down the most correct path. But if you find evidence you like better, you're certainly free to use that instead.

If you decide to use two layers of 3/4" plyoood, I'd very strongly recommend you pre-drill the screw holes in the top layer to prevent screw jacking.

My opinion; worth price charged.

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