Floor coverings need a solid base for installation. But if the base is not solid, some compensation can be found with the floor covering itself.
Laminate flooring, engineered wood, and even solid hardwood are moderately flexible. As the house expands and contracts, the flooring does too. Luxury vinyl plank and tile, along with sheet vinyl flooring, are all supremely flexible floor coverings.
Ceramic and porcelain tile, by contrast, do not compensate. Tile cannot bend, flex, or shift. Compensation needs to be made with the substrate, not the tile.
Complicating matters, the material that fills tile seams, tile grout, cannot flex or shift. More than almost any other type of floor covering, tile needs a rock-solid base.
3 Methods of Installing Tile on Concrete
If you attach the tile to concrete, there are three ways to do it, with the third way being preferred. You can install the tile directly on the concrete. You can install a CBU or cement board on the concrete, then the tile on top of that. Finally, you can use an uncoupling membrane between the tile and the concrete.
Install Tile Directly on Concrete
Ceramic and porcelain tile are so frequently installed at or above grade level on a cement board underlayment or directly on plywood that it almost seems novel to install tile directly on concrete.
Requires no extra materials
Often can work for years, until concrete begins to crack
Substrate is not raised
Concrete cracks transfer to tile
Thinset remains on concrete if tile is removed
This application does make sense, since concrete is heavy, solid, and is typically thought of as an unbending, uncompromising material. Far denser than plywood and weighing in at a hefty 75 pounds per square foot (at a six-inch depth), concrete is heavy. Not only that, concrete and tile are both mineral-based materials, so it seems natural that the two would be a perfect match.
But that only describes concrete in its perfect, unchanged state. Concrete responds poorly to foundation shifts. Groundwater pushing up from below can crack it. Tree roots routinely burrow under concrete slabs, then lift and crack them. The best mode of thought is to assume that your concrete will crack at some point in its lifespan.
While you can install tile directly on concrete, problems may erupt when the concrete cracks or shifts. All of the movement in concrete is transferred to the tile. Cracks in concrete immediately become cracks in the tile. If you were to remove a cracked tile from concrete, undoubtedly you would see the same crack pattern below.
That said, installing tile directly on concrete is not always disastrous. Stable concrete can act as a substrate for many years.
Install Tile on Concrete With a Cement Board
If the concrete floor exhibits cracks, gaps, holes, or other imperfections, does it make sense to put down an entire underlayment of cement board, such as HardieBacker or Durock, instead of repairing each imperfection piecemeal?
Preferable to direct-to-concrete installation
Helps smooth out bumpy, uneven concrete
Raises substrate level
Might end up transferring cracks anyway
Durock, HardieBacker, and WonderBoard are all cement backer boards and are 100-percent inorganic materials that will not rot, shrink, or decompose.
In one sense, laying cement board on good concrete would be unnecessary and redundant: a cement product on a cement product. Veteran tile installers have differing opinions, with some saying that this can be done, especially if floor level needs to be raised significantly. In this type of application, attaching a CBU to the concrete is preferable to floating an entire floor's worth of mortar bed.
Most tile professionals agree that attaching a CBU to a concrete floor would be more trouble than it is worth. If anything, it would be difficult to screw the CBU into the concrete, especially with the middle layer of the thinset.
In short, installing cement board between concrete and tile is possible. But generally, it is not worth the effort and it may even result in a poor tile installation. Most importantly, the cement board is not considered to be an effective uncoupling material. While you may gain some benefits from using cement board as an uncoupling surface, a true uncoupling membrane's benefits far exceed this.
Install Tile on Concrete With an Uncoupling Membrane
It's best to take special precautions by uncoupling the tile from the concrete. Rather than using cement boards (CBUs), the favored method is to use an uncoupling membrane.
Effective at uncoupling tile bond from concrete
Helps smooth minor imperfections in concrete
Better suited for tile professionals
Schluter's Ditra and Redgard Uncoupling Mat are brands of polyethylene membranes with a grid of squares or circles embossed in the face. These are often used as waterproofing elements for building shower pans. Widely installed by tile professionals, their true value for concrete bases is as an uncoupling material.
An uncoupling material does just that: it uncouples one thing from another. In this case, it unlocks tile from its subfloor. Acting as a buffer layer, it is flexible and does not mimic the actions of the concrete. Because subfloors can move and crack, they transmit the same to the tile above. Uncoupling material breaks this chain of transmission.
Uncoupling membranes are invaluable if you anticipate any movement or cracking from the concrete floor. Generally, it is well worth the cost and the time to purchase and install an uncoupling membrane.
Uncoupling membranes are not perfect, though. When the concrete dramatically tilts or cracks, no membrane can uncouple the two surfaces enough to prevent tile damage from happening.
The Ultimate Guide to Underlayment for Tile. Ceramic Tile Foundation.