TCNA Requirements for stone tile
Hello all,
I am installing a marble mosaic tile in a half bathroom and I just have a quick question.
The current subfloor is Advantech 23/32 T&G installed over 16″ o.c. joists. My original plan for installing the tile was to simply use Schluter Ditra, but after looking more closely at their installation requirements and calling technical support I no longer think the product is appropriate for in installation. They require a minimum of 2″x 2″ tile to be installed and the mosaic has smaller pieces that are smaller than the 2″ x 2″. So, I went down a rabbit hole of purchasing the TCNA handbook and looking at the requirements for stone flooring over wood and I must say that I am a bit confused. The TCNA handbook tells me that for a stone installation over wood that I need to install at a minimum 15/32″ plywood over the Advantech plus 1/4″ cement board and then the tile. This seems a bit excessive to me. Am I going about this all wrong or should I follow this guidance from TCNA?
Also, Advantech is technically an OSB product, granted far superior to other OSB’s but there is still only one detail in all of the TCNA handbook that even references tile installation over an OSB subfloor and it calls for 19/32″ plywood underlayment and then says that you can lay your tile directly on top of that. This detail also seems wild to me.
Any help with this issue would be greatly appreciated.
I am looking for a sturdy installation that will last for a long time, but without having to build the floor up excessively. 15/32″ plywood plus 1/4″ cement board plus the height of the tile and thinset is close to a whole inch above the level of where the floor is at currently.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
My TCNA book is a few years out of date but I do see only two options for natural stone tile over plywood or OSB floor sheathing. 19/32" plywood will be nowhere near the required L/720 deflection so I believe the 15/32" underlayment is to add strength. I recommend checking out the John Bridge tile forum, or if you can handle Facebook groups, check out Tile the World there.
Michael,
Thanks for the feedback. I did also post my question in the John Bridge tile forum. So, hopefully someone there can help me out. Yea the newest version (2024) TCNA handbook only has one portion where it mentions OSB and its for ceramic tile and not natural stone.
Do you think that I should just follow the guidance for natural stone and suck it up and do the 15/32" underlayment and the 1/4" backerboard and just deal with the flooring height difference?
Advantech performs structurally very similarly to plywood so I don't think you have to worry about plywood vs. OSB.
For what it's worth, I'm about to tile my own bathroom with real slate on the floor; I'll use joists 12" o.c. and 3/4" Advantech floor sheathing with 12"x24" slate tiles over Ditra Heat. I can understand why they would advise against small tiles over Ditra because of the uneven surface, but you can fully skim-coat the Ditra to create a flat surface. Smaller tiles are more forgiving of structural movement than larger tiles so even if your framing isn't at L/720 I bet you would be fine. But of course, internet advice is worth what you pay for it and it's always best to follow manufacturer recommendations if you're worried about things like warranties.