Natural stone tile and 24″ joists
Hi there, we are building a new house and the second floor is on 24″ joists. We have put down thin set mortar, backerboard, followed by electric tile heat and then the leveler and now have cracks in the leveler- many of which run parallel to the joists and a few are at the seams of the backerboard. Our tile installer fears that the floor is moving too much since it is on 24″ and does not recommend installing a natural stone product because it will crack. Unfortunately, we have already purchased and planned our dream bathroom. How do you all handle the 24″ gap if the client wants a natural stone? Is there something else that can be done?
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Are the joists 24" deep or the spacing 24"? Either way, the deflection for natural stone should be L/720. This should have been designed into the floor system during the design of your home. You may want to talk to your designer/architect.
Last tile project for me had 24" spacing for joists. I used Schluter Ditra-xl made for 24" spacing instead of backerboard.
If your joists are 24 inches on center, you need to be sure that you've properly planned for deflection. Tile expert Michael Byrne has written, "I won't install a tile floor unless the subfloor and the underlayment are at least 1 1/4 in. thick," but he was talking about rooms with joists that are 16 inches on center. If your joists are 24 in. on center, you'll need to talk to an engineer or tile expert.
Ref John Zito. He is correct in the minimum deflection criteria. You need to go back and double the joists under the tile area - extending the double joists to a bearing wall. This will guarantee achieving l/720 as I am sure the span is not greater than 8 or 10 feet. If the required stiffness is not achieved you will have a continuous problem of the grout popping out of the tile joint. In reference to Martin Holladay, defelction must be achieved between the joists as well hence his recommendation on a minimim 1-1/4" floor sheathing, glued and screwed at 6" spaceing along the doubled joists. Use a reliable tile company and they will ensure that the job is done correctly as they will warrante their workmanship and material performance.
So just to give an update in case anyone is curious on how this played out. The builder did not believe we could meet the deflection criteria for natural stone tile. We asked the home designer, who is a green guru and contributor to this site, and he confirmed that using a natural stone tile product on 24" joist spacing could be problematic.
Also, at our stage of construction it would have been problematic to work on the joists and/or subfloor beneath the bathroom because drywall was already installed which would have led to extra costs.
So on top of the floor leveler the builder and plumber decided to use the Schluter Ditra product and then we will use a 24 x 24" porcelain tile product instead of natural stone on top of that.
We think basically the moral of the story is that if a customer wants to use a natural stone tile product for a bathroom floor on 24' spacing the conversation on how to plan for that needs to occur very very early in the process.