Believe it or not this question comes up fairly often, and the answer is a bit more complex than you might think.
You can install tile over concrete, but it’s not recommended. You would need to first take several precautions since installing tile directly over concrete can lead to the tile cracking and other potential problems.
To properly install tile over concrete, you will need to start off with a sturdy flooring cover. Laminate flooring, hardwood flooring, and engineered wood tend to be semi-flexible, and luxury vinyl planks as well as sheet vinyl planks are extremely flexible.
However, ceramic and porcelain tile are not flexible at all, and they do not bend or shift. The more flexible floor coverings usually do not need a rock solid base, but tile (in any form) needs an extremely solid base.
Based on all this, if you want to install tile over concrete, what can you do?
Most architects suggest using an uncoupling membrane.
What is an Uncoupling Membrane?
An uncoupling membrane is a fancy term for what gets installed between the subfloor and your tile. They can absorb the movement of a subfloor before that movement results in loose or cracked tiles. Uncoupling membranes are not state-of-the-art, but they are still relatively new to the industry.
Some builders will try to tell you that you really do not need an uncoupling membrane when installing tile over concrete. While they could be right for your unique situation, more often than not, I have found that you really do need an uncoupling membrane.
An uncoupling membrane will unlock the tile from the subfloor, and it acts as a flexible buffer layer helping to reduce the flaws of concrete. The chief problem with concrete slabs is that they routinely develop cracks, and this is precisely where uncoupling membranes come in handy. They help absorb the movement of the subfloor before that movement causes cracked or tiles to get loose.
It happens to be a great investment for your concrete floor, especially because it prevents cracking from sudden movement. Uncoupling membranes are also waterproof and they can usually withstand a lot of pressure.
Worth noting, although they do an excellent job preventing problems with the tile, an uncoupling membrane will be of little value if your concrete cracks or tilts. They simply were not designed to handle that type of issue.
Why is Installing Tile Directly on Concrete a Bad Idea?
At first, you might think that installing tile directly onto concrete would be a great idea. After all, concrete is heavy and thought to be an uncompromising and unbending material. Concrete is far denser than plywood and weighs up to 75 pounds per square foot. Apart from being heavy, another advantage is that both concrete and tile are made up of mineral-based material. This means they both should be a perfect match.
Unfortunately, the reality is much different than the theory.
Although installing tile directly onto concrete works in concrete’s unchanged state, concrete responds terribly to foundation shifts. Tree routes often burrow under concrete slabs and crack or tilt them. Underground water pushing from below the surface can also crack concrete slabs. No matter how sturdy the concrete base is, at some point during your home ownership, it will almost certainly end up cracking.
When it does, the very foundation of your tile will be compromised, leading to shifts and cracking in your tile too.
If that is not bad enough, the only way to really fix a crack in concrete is by fixing it from below by filling them. No doubt, this fix would be a huge pain.
As you can start to see, installing tile directly over concrete would lead to a lot of problems. As soon as the concrete starts shifting or cracking, this directly affects the tile. Whatever movement the concrete slab endures gets directly transferred to the tile: a crack in the concrete would mean a crack in your tile.
Should I Install Cement Boards in Between Concrete and Tile?
If there are cracks in the concrete base, it is definitely not a good idea to install cement board underlayment (CBU) in between the tile and the concrete.
Cement boards like Durock, Wonderboard, and HardieBacker are made up of 100% inorganic materials that will not shrink, decompose, or rot. Consequently, putting a cement board on concrete would be a really foolish idea. All you would be doing is putting one cement product on top of another.
Especially with a middle layer of thinset, it would be very difficult (if not impossible) to properly screw the cement board into the concrete base. Not only that, but the nails also have no real holding power even if you could get them in, and the board seams would eventually crack through the tile when it loosens up.
Although it is possible to install CBUs in between tile and concrete, it would ultimately result in a really poor tile installation. Instead, like I said earlier, go with a trusted uncoupling membrane instead.
4 Simple Steps to Properly Lay Tile on Concrete
Installing tile is actually something that you can do on your own. If you can level the floor properly, install an uncoupling membrane, and layout the tile in the correct way, you can DIY a floor installation.
Here are four simple steps to lay tile perfectly on your concrete floor.
1 – Prepare the Concrete Base
First, clean the surface of the concrete. You can use a vacuum cleaner to do this. Next, check the level of the floor to see if the concrete is level or not. If the base is not level, put self-levelling underlayment down to create an even surface. If you find holes in your concrete, use a leveling compound or filler.
Next, roll a latex primer on the surface for the self-levelling underlayment. After smoothing down the edges of the floor with a roller, lay out an uncoupling membrane. Cut the sheets of the membrane to fit the area, apply thinset over the concrete, and smooth the sheets with the help of a trowel.
2 – Prepare the Layout
Use a try square to see if the center lines are exactly perpendicular or not. Draw chalk lines to measure the length and width of the room. Before laying out the tile, make sure none of them are broken and that the coloration matches perfectly.
Then, lay out the tile on the floor and adjust your center lines if the tiles near one edge are less than one-half tiles.
After that, for any tiles that you need to cut, carefully cut them with a tile cutter to fit the edges perfectly.
3 – Lay the Tile
Next, mix your thinset mortar, smooth it out over a three foot by three foot area, and comb it with a notched trowel before laying the tiles. While laying the tiles, push down gently to make sure the stick. Be sure to keep a bucket of water handy in case you need to use a wet sponge to wipe off the tiles if thinset comes out from the gaps.
4 – Grout the Tile
After the tiles are all placed, let the thinset sit for at least 24 hours. Mix your grout mixture with cool water. Using a grout float, lift some of it from the bucket and level it on the surface of the tiles. Let the grout sit for at least 20 minutes, and then clean the excess with the help of a damp sponge. The grout mixture needs to set completely, which usually takes up to three days. Lastly, apply a sealant to the grout with a sponge brush.
By now, you should better understand: 1) why it is a bad idea to install tile directly over concrete, 2) why CBUs will not help much in this particular circumstance, and 3) why the best method involves using an uncoupling membrane to avoid cracking the tile.
Wishing you a ton of success in your DIY tiling project!