Natural Stone Installations over Wood Sub-Floors
Did you know that natural stone installations over wood sub-floors require a mandatory second layer of subfloor before a backer-board or DITRA can be installed? The detail from the Schluter Systems DITRA installation handbook (D-W-S and DPS-W-S page 6) shows this requirement for this application. We emphasize this detail as often as opportunity allows, specifically during our Innovation Workshops. We make all handbook details and information readily available in both printed and digital formats.
Schluter has adopted this second layer requirement from TCNA (Tile Council of North America), F141-25 and F250-25 details. The two accepted assemblies are either (1) a wire reinforced mortar bed over a slip sheet membrane (minimum 1 1/4” & max 2” reinforced mud bed), or (2) a second layer of OSB minimum 3/8” thick before (a) approved tile backer-board or (b) Schluter DITRA Uncoupling Membrane (as noted in our detail).
It does not matter how firm you feel the floor is.
It does not matter what type of stone is being installed.
It does not matter how large the stone is.
It does not matter what type of mortar bond coat is used.
It does not matter what your past success rate may be.
If God made the tile that is to be installed - this pertains to you.
Included below are a couple of photos from a smaller Carrera Hex Mosaic tile installed over 1/4” backer-board. The contractor was not aware of the second layer requirement. After the homeowner reported visible fishers in the floor, and the TCNA/Schluter assembly requirements were brought to the contractors attention, it was argued that (1) they have never had a floor fail in their past, (2) the mosaic is small and should flex with any possible deflection in the floors, and (3) backer-board should firm up any deflection prior to installing stone.
The truth is (1) many times when a floor has failed the original contractor does not get a call back. Someone else gets called in to resolve the issue. (2) Clearly these small mosaics were not exempt to deflection stresses in the floors (3) despite the use of a backboard.
Upon removing tile, it was clearly evident that the fisher cracks were directly over the perimeter/joints of the 4’ x 8’ sheets of 3/4” T&G OSB plywood subfloor.
In my most recent experience, a great personal friend and long time tile contractor installed large format Carrera Marble (app. 3,000 SF) in his attorney’s home. He was not aware of the 2nd layer sub-floor requirement, and the floor has failed. Fortunately, his attorney has helped him with a plan to resolve his floor issue.
If you would like a copy of the most recent TCNA handbook, please email our customer service team and request either a hard copy or a digital copy. CSRRENO@schluter.com. You can download our DITRA installation Handbook from our website, or screenshots of the TCNA detail here.
