Q: I have a kitchen remodel with a dining room addition in an older home with board sheathing over 2x8 joists. The client wants uninterrupted tile throughout both rooms. The floor of the addition will be I-joists, but how do I join the two floors without putting down a mud layer?
A: Michael Byrne, a veteran tile installer and consultant, and the moderator of JLC’s Ceramic Tile Forum, responds: I don’t recommend installing tile over any floor with 2x8 joists, and the 1-inch diagonal subflooring is not much help.
The whole purpose of beefing up the floor is to eliminate flex, which can crack or loosen the tile over time. If you want to avoid spreading a mud layer over the entire area, here is what I would recommend. First remove the existing board sheathing and then screw and glue down 3/4-inch plywood or engineered-lumber subflooring to the joists, with the face grain of the subflooring running perpendicular to the joists. When you build your addition, install the floor so that the 3/4-inch subflooring over the I-joists abuts the new kitchen subfloor at the same level.
Now instead of 3/8-inch or 1/2-inch plywood underlayment just screwed to the subfloor, glue and screw 5/8-inch or 3/4-inch plywood underlayment to the subfloor. To fasten the underlayment, first cover the entire subflooring surface with Type I or II glue using a toothed trowel. Then screw down the underlayment every 6 inches around the perimeter and every 8 inches in the field. Run the underlayment perpendicular to the joists, but stagger the end seams by 4 feet and the side seams by at least 8 inches from the subfloor layer. If the kitchen and dining room subfloors are the same level, you should be able to run the underlayment across both floors uninterrupted. You now have 1 1/2 inches of consistent underlayment to support the tile, which is the bare minimum I’d recommend for the kitchen floor.
With any substrate assembly like this, install the tile with a latex-modified thinset mortar, but I’d suggest first installing a load-bearing, crack-isolation/waterproofing membrane over the underlayment. The membrane minimizes problems related to the expansion differences between the subfloor and the tile, and it protects the substructure from water damage in normally wet areas such as a kitchen. If the new floor is already installed, you can adjust the thickness of the underlayment to give you some flexibility when trying to match the height of the two floors. But keep in mind that 5/8-inch is the absolute minimum required thickness for plywood underlayment for ceramic tile installations—any less and the installation will fail.
Also realize that these configurations may only apply to ceramic tile, and not to stone or glass, which usually require a thicker and stiffer substrate. For best results, consult the tile manufacturer’s substrate requirements, as well as the requirements for all of the materials that you use.
As a final point: Depending on the overall size of the floor, you may have to install a network of movement joints. In section EJ171 of the TCNA handbook (tcnatile.com), movement joint requirements are spelled out and must be followed for both tile and membrane installations. Without the correct network of movement joints, any tile installation may fail even if the best materials and practices are used
