Ceramic or porcelain tile on your home's grade-level is typically installed on a cement board underlayment or directly on plywood. Given these underlayments, it seems almost a novel concept to install tile directly on concrete. Yet it is an application that does make some sense, as concrete is a heavy and solid material. Not only that, concrete and tile are both mineral-based materials. So it seems natural that the two would be a perfect match.
Or are they? As it turns out tile over concrete is a bit more difficult than it seems on the face of it.
Yes, you can install tile directly on concrete, though problems can happen when concrete cracks or shifts. All of the movement is transferred to the tile. Cracks in concrete immediately become tile cracks.
If you attach tile to concrete, you need to take special precautions for a flawless installation. You need to use an uncoupling membrane, such as Schluter Ditra or Redgard Uncoupling Mat. These membranes prevent concrete action from transferring to the tile.
No Need to Install Cement Board on Concrete
If the concrete floor is less than perfect--cracks, gaps, holes--does it make sense to lay down an entire underlayment of cement board, such as Hardiebacker or Durock instead of repairing each imperfection piecemeal?
Durock, Hardiebacker, and Wonderboard are all cementious backer units (CBUs), which means that they are 100% inorganic materials that will not rot, shrink, or decompose. Laying cement board on good concrete would be unnecessary--a cement product on a cement product.
Veteran tile installers have differing opinions, with a few saying that this can be done, especially if floor level needs to be raised significantly. Attaching a CBU to the concrete, they say, is preferable to floating an entire floor's worth of mortar bed.
Most concur that attaching a CBU to a concrete floor would be more trouble than it is worth. 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. Also, cement board is not considered an uncoupling membrane.
Option A: Install Tile Directly to Concrete
Of all the surfaces you can install tile on, concrete is about the most stable out there. Being heavy and solid, it will not warp or flex, thus cracking the tile. As long as the concrete never moves or cracks, tile can be installed directly on top.
But concrete is not permanent. It responds poorly to foundation shifts, and even ground water pushing up from below can crack it. The best mode of thought is not "if" concrete will crack but "when" it will.
Tile installer Bud Cline says that the best 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 hard sanding.
Option B: Install Tile With Uncoupling Membrane
An uncoupling material simply "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.
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 used by tile professionals, their true value for concrete bases is as an uncoupling material.
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, and tile damage will occur.