Q: How should I reinforce my floor before installing new ceramic tiles? A written quote I received from a contractor includes 3/8” plywood fastened on top of the existing subfloor, but my friend says it should be 5/8”. What do you think?
A: The challenge with ceramic tile installations is that you can’t go back and make it better if things start to fail. And adding 3/8″-thick plywood to an ordinary subfloor before tile installation will probably result in loose grout and failed tiles in time. You’ll find 5/8″ plywood is better, but not sufficient on its own in my experience. I’ve seen too many ceramic tile floors fail, even with lots of added wood underneath. Differential movement is the reason why, and an uncoupling membrane is the solution.
No matter how much wood you have directly underneath tiles, this wood will expand and contract at different rates than the tile for a given change in moisture and temperature. This movement causes horizontal pressures to develop where the tiles meet the wood, often leading to cracks, loose tiles and failed grout. And once this defect shows up in a completed installation, there’s nothing you can do to stop it permanently.
All this is why uncoupling membranes were developed. They’re a dimpled plastic membrane that goes down against the subfloor, allowing a small amount of side-to-side movement of tiles while also supporting tiles and grout fully. A German company named Schluter invented modern uncoupling membranes, and their DITRA product is still the best I know. A contractor friend of mine used DITRA over half of his own kitchen floor, then cheapened out and bought a less expensive uncoupling membrane for the other half. The DITRA half is still performing well today, while the other half of the floor failed within a year and needed to be torn up and replaced. Even for tiles installed over concrete, I consider an uncoupling membrane essential.
Watch the video up next to see a proper ceramic tile installation completed by a certified installer using uncoupling membrane.