The most important thing when it comes to building a ceramic floor is the underlay. Consider it the foundation of your floor. Ok, perhaps the floor joists and subflooring would be better considered the foundation of your floor, so let me rephrase that… Consider it the foundation of your ceramic tiles. When dynamic loads are applied to a floor, that is, when you walk on a floor, the floor will have a tendency to deflect. It is very slight, and usually not noticeable to us, but there is some deflection that occurs. Tiles are very weak when it comes to shear and tensile forces, so they have a habit of cracking when only slightly deflected. This is why the underlay is so important. A proper underlay will provide the structural support needed to substantially reduce deflection when dynamic loads are applied. It will reduce the deflection to a point that is within the threshold that the tile can handle, and thus you don’t get tile cracks. The other common symptom of inadequate underlay is chipping grout. This is basically for the same reason. As the floor flexes, the grout breaks up and chips away.
So what is considered adequate? The minimum standard would be 3/8″ plywood. However, that tends to be thin, and I’ve seen quite a few ceramic floors damaged with 3/8″ ply. Personally, I like to see at least 1/2″ plywood, if not 5/8″. In all my years as a home inspector, I’ve never seen problems with a tiled floor built on 5/8″ plywood underlay. If you really want to be cautious, you could always step up to cement board. Probably about a quarter of the tiled floors that I inspect have cement board underlay. It’s more expensive, but you’re pretty much guaranteed that you won’t have any problems with the tiles or the grout. At least, not any problems related to floor deflection.
Some of you may have seen another type of underlay, that is used in some of the renovation shows on television. It is a yellow dimpled plastic product, called Ditra. Ditra is an engineered tile underlay product, and is substantially more expensive than standard plywood or even cement board. I’ve inspected over 5,000 homes, and I’ve only ever seen Ditra once in a home. Needless to say that it’s very rarely used, and frankly, it’s rather overkill. Of course, I’m not saying it’s not a good product. In fact, it’s an excellent underlay, better than the standard stuff. However, you should be just as fine with decent plywood.