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. 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.
If you attach tile to concrete, you need 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. Schluter Ditra and Redgard Uncoupling Mat are two popular brands of tiling membranes that lessen the chance of concrete events becoming tile events.
Installing 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. Yet 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.
The safest way to approach questionable concrete is not to cover it over with CBUs but to fix the concrete. Cracks and gaps can be filled with Portland cement-based fillers. Tile cannot be attached directly to painted concrete, as the thinset will not adhere well to the paint. Painted concrete can be made porous with sandblasting or other hard abrasive actions.
While you can install tile directly on concrete, problems 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.
Installing on 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?
Durock, HardieBacker, and Wonderboard are all cement backer boards and are 100-percent inorganic materials that will not rot, shrink, or decompose. Laying cement board on good concrete would be unnecessary and redundant: a cement product on a cement product. Yet veteran tile installers have differing opinions, with a few 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 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.
Installing on Uncoupling Membrane
Schluter's Ditra and Redgard Uncoupling Mat are brands of polyethylene membranes with a square grid 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.