Ceramic Tile and Subfloor Movement
There has to be nothing more misunderstood and underutilized in the whole tile industry than the need for movement accommodation joints in a tile installation. While manufacturers all rightly insist on their use in printed literature, they provide few if any options or guidelines on how to deal with them otherwise. The salesperson almost never makes mention of the fact that the 18-inch rectified tile they just sold with 1/8″ grout joint spacing needs a ¼″ expansion space every 20 to 25 feet. They either don’t want to jeopardize the sale or perhaps, as they are in sales, don’t know anything about their necessity. This leaves it up to the installer, the last and most important link in the process, to create awareness because it is he who stands to lose the greatest amount of cash if the floor fails. Then of course, there is always the guy who has never used them and never had a problem. Most often, this person’s place of business has four wheels and tires so he is not the easiest guy to find.
So just what necessitates this overwhelming need for movement (expansion) joints? It starts with the tile itself. Many people are quite surprised to hear that ceramic tile, a product made by manufactured by firing clay and other minerals at several thousand degrees, expands when exposed to direct sunlight. Porcelain tile, a very dense bodied product with a very low thermal expansion rate, expands approximately .000004 inches per degree Fahrenheit. Sounds minimal but that same porcelain tile covering 40 linear feet and exposed to a change in surface temperature of 50 degrees Fahrenheit has the potential to expand between a 1/16″ to 3/32″.
The adhesion of the bonding material to the tile and substrate restrains the actual amount of expansion but does not eliminate it. This is where coverage and type of thinset can make a big difference in whether the tile stays in place or not. Types of tile other than porcelain, especially glass, are much more expansive under heat. Thermal expansion is a two-way street. Tile both expands and contracts due to thermal variation. This is one reason for my personal preference in using the term movement joints. If I were only concerned about moisture, then I could use an expansion joint. Why? Moisture is an expansive force on ceramic tile. Once it expands, it does not contract. This growth rate is very slow and very minimal. Except for this minor amount of growth, ceramic tile is unaffected by moisture or water; it is an inert material. When someone says they have a tile failure due to excessive moisture what they should really be saying is due to defective workmanship. Either they used the wrong thinset, did not allow it fully cure prior to moisture exposure, or, the expansion joints are few to none and likely there are a few other ills.
Failures due to moisture expansion can happen in as little as a few years in areas of very high water exposure applications such as an exterior deck or restaurant kitchen with no movement joints. However, most often they take a number of years to occur. Coastal areas which have very high ground water tables show a disproportionate amount of slab-on-grade, moisture-related failures, most commonly due to lack of movement accommodation joints. These can occur in just a few years in very wet environments or may years later for those installations that are subjected to little if any moisture.
What are some other good reasons for movement/expansion joints? Wood structures; if it is made of wood, it is going to move. All wood-supported floor systems have a certain amount of deflection. Too much movement can cause a host of problems but even normal movement from bending stresses must be accommodated. Perimeter joints are critical in wood structures to accommodate normal movement of the structure and seasonal moisture changes in the wood itself. If there is a beam running down the center of the room or home, common sense tells us that is a pivot point from which the floor will deform on either side, like a board over a fulcrum. It would be prudent to put a soft joint or movement profile over that area to allow the floor to move without putting the tile assembly in stress.
Another area of consideration is doorways. Take the example of a kitchen with an adjoining dining room. If we have a 300-square-foot kitchen with a doorway to a 250-square-foot dining room, both installed over backer board, we have roughly 1,800 pounds on one side of the wall and 1,500 pounds on the other side, joined together at a 3-linear-foot doorway, like a bar bell. Again, common sense would say this is not a good idea and real world experience confirms it isn’t; you need a movement joint. Most areas of the country go through seasonal changes in humidity levels. These changes affect the dimensional stability of the supporting floor structure. This moisture induced movement is magnified several times if the method of construction utilizes a crawl space, which subjects the supporting structure to a much greater level of dimensional instability.
Concrete does not earn a free pass on stability either. Did you know that concrete spends its whole life moving? Concrete has a natural tendency to warp. Control joints are placed primarily for concrete shrinkage during the initial curing process. Because a concrete control joint is not cracked at the time of tile installation does not mean it will not crack at some point. In all likelihood, you can be fairly well assured it will. If there are no control joints in the slab this cracking will occur at random locations over a period of time which can be many years. Movement/expansion joints must be provided at all control joints. As the concrete continues to fully cure, which can take as long as a year, some additional shrinkage will occur; control joints provide the means for dictating where the separation will occur. Even after the concrete is fully cured there will be some minor warpage. If tile is installed without the proper movement accommodation joints, it is quite possible it may either crack or debond. Using a membrane does not eliminate the need for movement joints. For a membrane to function properly they must still be placed in the installation though some products and methods will allow you to select a more atheistically pleasing location. The physics of concrete is very complicated but it is not as unpredictable as many think. The cause and effect is actually rather well defined.