The biggest problem in the ceramic tile industry today is cracked tile and cracked joints. The single biggest cause of this is deflection; Deflection in the floor joists and between the floor joists.
Today's installers not only have to be skilled craftsman but also knowledgeable about the structural requirements needed to make sure their floors will perform long after they have been installed. The homes today are much bigger than the ones we grew up in. The spans are longer and the distances between the joists go from the traditional sixteen inches on center to 24 inches on center. Installers should inspect the floor joists along with the sub floor to make sure there is enough structural support. Whenever there is a failure, the first guy usually blamed is the installer.
With fully adhered vinyl floors, movement in the floor joists can cause the seams of the vinyl to telegraph. If a vinyl floor has to be replaced it isn't easy, but nowhere near as hard as a ceramic tile floor. With ceramic floors, if the floor joists bounce, or deflection between the floor joists occur; the tile and grout will crack. Even with hardwood flooring, if there is deflection in the floor, the fasteners will loosen and then squeak.
Most engineered wood manufacturers recommend an l/480 rating for the floor system to insure a strong solid floor. Many building codes accept an l/360 for compliance. Most people don't understand what an l/480 or l/360 means but simply put, it's a way of creating a common denominator to measure stiffness. The manufacturers of engineered wood and solid lumber rate their products based on these standard test methods. For l/360 it means the deflection in the floor cannot exceed 1/360 of the span (one inch over 360 inches). So on a 30-foot span, the maximum deflection is one inch. On a fifteen-foot span, the maximum deflection is a half-inch.
Most people when asked if they would be comfortable with a half inch of deflection in their own floor typically feel that is too extreme, yet many building codes allow it. From a practical matter, carpeting along with the padding, typically has enough elasticity to take up some deflection. If you ever wonder why that seam in the berber that looked perfect three months ago is now separating, it's probably not due to the installer or the seam tape but due to deflection.
The Tile Council of North America has recommended an l/360 with a 300 pound concentrated load for years as a measure of minimum stiffness. The confusing part about this is none of the wood tables reference a 300-pound concentrated load. More often the load tables are designed with a 40-pound live load and a ten-pound dead load. Now the Tile Council of North America is suggesting that one follows (IRC) which stands for "International Residential Code." For many people they have no idea what this means. By simply following the recommended methods in the Tile Council of North America handbook, one should be able to have a trouble free floor.
The fact there are various methods in The Tile Council of North America (TCA) handbook that have proven to work gives people choices to determine which method works better than the others or which one is perhaps easier to install. All of the methods are based on structurally strong floors meeting the minimum requirement of l/360 with a 300 pound concentrated load.
If a floor has a ¾ inch sub floor, it obviously performs much better than a 5/8th sub floor.
If the floor is spaced 16 inches on center it is going to have less deflection than a floor that is 24 inches on center. The following illustration by the American Plywood Association highlights the difference in strength between plywood and their spacing.
For an installer not to inspect how the floor was constructed makes the installation a gamble. For an installer to say I don't have time, or it's not my job, they should remember their reputation is what will be remembered long after the job is finished.
There are several unscientific methods to check for deflection. Some installers fill a glass with water and walk across the floor. If the water is moving they know they have a problem. Some take a string and nail it to the bottom of a floor joist, and let it hang to the floor. Then a quarter inch of the string is cut off at the bottom. Next, bags of thin set and tile are put on the floor to give it some weight. If the string touches the floor you know you have too much deflection.
An easier and faster way of checking deflection is with a laser. The laser is shot across a wall and then weight applied to the floor. If there is a difference in where the laser line was before the weight was applied, you know you have deflection.
The only way to correct for deflection is to strengthen the sub floor or underlayment and increase the floor joists with either more joists or a load-bearing beam. Some of the engineered wood manufactures make special bridging products that go between the joists. The fact is the deflection can be corrected and should be corrected before the floor is installed.
One advantage plywood underlayments offer is their higher strength values compared to cementitious underlayments. Plywood inherently has more strength than cementitious panels and that is one reason you never see homes built with cementitious products as sub floors. With cementitious panels, thin set is required underneath the panels to help prevent them from cracking and to give them extra strength. With the plywood underlayment, thinset is not required and it still provides substantially more strength than cementitious panels. Without having to use a leveling bed of thin set, the plywood underlayment is faster, easier, and takes less labor to install.
In the recent Tile Council of North America handbook committee meeting, a seven ply Birch panel was approved for 24-inch on center applications with a 3/4inch plywood subfloor.