Ceramic Tile & Wood Subfloors
The feeling is unmistakable. Your body tells you that you are on a stiff trampoline but your eyes tell you that you are not. The wood floor you are walking across is bouncing. Has this ever happened to you? It has happened to me when I walk across the floor systems of many a new home. The floor joist systems are built to code, however they have considerable bounce to them. If you have ever lived in an older home, the feeling is quite unsettling.
Springy Floors Hurt Tile
Wood floor systems with a bounce are fine if you decide to carpet. Sheet vinyl products can also tolerate substantial movement. Hardwood flooring will also fair well on a somewhat bouncy floor. Ceramic tile, on the other hand, will suffer drastically if a wood floor system is too bouncy. Tiles will crack, grout will crumble, and floor tile will sometimes actually detach. The reason is simple. Ceramic flooring materials are rigid. As such, they require a rigid base to stay in one piece. Wood floor systems can meet the necessary rigidity requirements. However, certain guidelines must be followed.
I Beams - Height is Everything
Steel I beams have certain strength or load bearing characteristics. Wood floor joists are no different. Their strength is a function of the thickness of the joist, its height, the type of wood and the distance between supports. Many people realize some of these factors, however, not everybody recognizes that all of them come into play. The species of wood is very important. For example, a 2x10 floor joist made from southern yellow pine is significantly stronger than a similar sized joist made from spruce-pine-fir.
Subflooring is Critical
The flooring material installed on top of the floor joists is also important. If you are using plywood, this material also can flex. Thicker plywood is almost always more rigid than thinner plywood. The number of plys or layers in the sheet is also important. Many people do not realize that plywood of the same thickness can have a different number of plys. More plys is almost always stronger.
Tile floors need additional support other than a thick single piece of plywood. Three quarter inch tongue and groove plywood, believe it or not, will not suffice! An additional layer of 1/2 inch plywood or 1/2 inch cement board is usually required.
As for installing this extra material, you just don't lay it down and bang it in with a bunch of nails. It is vitally important that the seams of both materials are staggered from that of the plywood subfloor. This offset should be a minimum of two inches. Edges of the materials are most secure when screwed.
If you use cement board as an underlayment, it is necessary to spread wet thinset on the plywood subfloor! This thinset will fill any void spots that may exist between the two layers. These void spots often are caused by floor joists that are crowned differently.
What do you do if you have an existing floor that is bouncy? This is a tough call. There is no miracle fix, I can assure you of that. Adding additional beams under the floor in the basement or crawl space is the easiest. This solution immediately stiffens the floor because you significantly decrease the span of the joists. If it is possible to do this, always make it your first choice.
Another solution is much more extensive and not always successful. You can install a "stress skin" on the underside of the troubled floor joists. A stress skin is a layer of plywood that is glued and screwed to the bottom of the floor joists while they are temporarily jacked up. The plywood, once the glue is dried, acts to stiffen the floor joists.
This solution must be engineered. In other words, you must hire a structural engineer to develop a plan as to how to install the 'skin'. Remember, this solution is not always worth the effort.
Wood floor joist systems will work with ceramic tile. I have used ceramic and slate extensively in my own new home. I only have one cracked floor tile in my entire house. This tile crack, I feel, occurred as a result of lumber shrinkage.
Read my "Floor Joists Materials-Span Table" article for the allowable span limits for your wood floors by two to three feet and you will be blessed with stiff wood floors. Or, use as tall a joist as you can afford.