I often get called into to figure out why a product installation failed and floor tiles are a common installation that fails. First I have bullet pointed a list of problems I commonly see and below that there are more some detailed points Here are some of the things I find and why I think they happen:
- Inadequate subfloor and improperly secured cement board substrate leads to cracks
- Tile choice can impact the final outcome. A better grade tile improves chances of success
- Choose the correct size tile for the existing floor conditions
- Failure of adhesive. I suggest a high quality thinset mortar instead of pre-mixed mastic
- Don't tile over a cracked concrete floor without using a crack isolation membrane
- Cracks are telegraphed through the tiles which indicate where the subfloor has movement (not a good thing). Usually when I see a linear crack that travels across several tiles it indicates that there is an improperly sized and/or secured plywood subfloor. If the plywood is too thin it will deflect under foot traffic and will transmit a crack through the tile. It all comes down to deflection and another way to prevent deflection is to use a cement board substrate and sandwich a thin layer of thinset mortar between the wood sheathing and the cement board. It needs to be screwed down but it is not critical that the screws hit the floor joists. The thinset mortar acts as a gap filler and an adhesive. I never see perfectly flat wood sheathing so thinset takes up the space under the cement board that would otherwise settle therefore creating movement up into the tiles. Thinset makes the whole floor much more stiff which means a less bouncy floor. Less bounce means less movement which also means less grout cracking. I would never adhere tiles directly to plywood or "luan" which was common back in the 80's and 90's. Although cement board is not waterproof, it does make a very good surface to adhere tile to if the cement board itself is properly secured to the subfloor and does not deflect too much. Cement board is very flexible so it will conform to a bouncy floor. Another reason a floor might be too bouncy is that the framing is under sized. You might need to stiffen up the framing before it is capable of being tiled. Staggering the seams of the cement board over the subfloor joints is also important.
- Often times the crack could have been avoided by simply choosing an appropriate floor tile. Most folks group all types of tile under the term "ceramic tile" but there are two types of commonly available tiles and they are Ceramic and Porcelain. They are made using a different combination of materials and Porcelain tends to be stronger and harder. Both types of tiles are Graded and come in Five different grades from 1-5 (or I,II,III,IV and V). You will find the grade of the tile on the bottom of the tile and it corresponds to its hardness or wear rating. Tiles with a V grading will be much more durable than a I but tiles with a V rating are usually reserved for high traffic commercial use and come in limited styles and colors. I would never choose a floor tile with a Grade less than III so it's important to look at this when buying the tiles. Grade I and some Grade II tiles are primarily for walls and backsplashes and are much easier to work with and cut. Porcelain tiles are harder to cut and require special tools.
- If you have a room with an uneven floor and a little bounce I would highly suggest you avoid large tiles. Selecting a large tile (12x12 or larger) will increase the odds of it cracking, especially with a weaker product like natural marble tile. Any deflection at all in the floor will also crack the larger tiles easier. Keeping the surface of all the adjacent tiles is difficult if you have an uneven floor and even more so with larger tiles. Marble tiles have a very small beveled edge so the margin for error is tiny as opposed to some ceramic or porcelain tiles with a larger beveled edge.
- Another thing I see is people using the premixed mastic on floor tiles. This is an absolute no-no and will almost guarantee a failure over time. I always use a thinset mortar. Thinset is available in many forms such a plain or modified and can contain additives to improve strength and elasticity. It is important to mix the mortar properly for it to work right. If it is too soupy it won't lay down right with the notched trowel. If it is too thick you won't get the adhesive properties you need to hold down the tile. If it is too dry the mortar won't stick to the back of the tile and will fail which will usually show itself by cracked grout lines all around the tile. You need to mix mortar in a bucket with a slow RPM drill and a proper mixing paddle. If you mix too fast you will introduce too much air and it will weaken the mix considerably. After the mortar is mixed it should sit for a few minutes and then give it a final mix just before you use it. This is considered allowing it to "Slake". Make sure there are no dry clumps in the mortar and it is best to put the water in the bucket first and then add dry mortar. Using the correct sized notched trowel is also important so you can refer to this chart for more info http://http//www.custombuildingproducts.com/HowToTile/notchTrowelSizeGuide.aspx?user=dis&lang=en
- Often times I see cracks in the tile where it is installed over a concrete slab. This is almost always because there was a crack in the slab before the tile was installed. The tiles are not strong enough to hold the slab together and prevent it from moving so a Crack Isolation Membrane should also be used when tiling over a cracked slab. The membrane only needs to be used over the cracked area if it is flexibly roll-on material. There is also a product made by Schluter which comes in a roll and serves a similar purpose but needs to cover the entire floor area http://www.schluter.com/6_1_ditra.aspx A crack isolation membrane is also critical when the tile will be installed over two adjacent and dissimilar materials such as concrete and wood subfloors. This is a crack waiting to happen because both materials move and flex at different rates.
I have seen many tile floor failures and I would be happy to help you troubleshoot a project. If you have any questions feel free to comment or find me at http://www.mvconstruction.com/ where you can email me any questions.