First and foremost, in new construction especially, we all have jobs to do. Concrete guys have an industry association, the largest and most active being the American Concrete Institute (www.aci.org). Wood guys have the America Plywood Association (www.apawood.org), which has spent years working with Tile Council of America, at their expense, developing recommendations for wood floor construction and ceramic tile.
Even the drywallers have their own organization, the Gypsum Association (www.gypsum.org). Before you open your wallet, you should know something about the responsibility of other trades. It is very difficult to argue just because "you know it isn't right." You need to put some teeth in that argument if you want to keep your money. Now all this does not apply if you are all by yourself in a remodeling project; however, if you are doing more than just tile, it is just as important to understand the reasons why, even though you have accepted the responsibility of preparing the structure. All of these groups specifically make recommendations where the surface is to be covered with ceramic tile. These recommendations are part of their trade, not ours. In the tile industry, with rare exception, the industry guidelines are based on and a part of the recommendations of the substrate trade organizations listed above. People doing their jobs and some common courtesy go a long way towards the satisfaction of everybody. That is the perfect world picture of the way things are. Now let's get to some real world floor prep issues we left above.
You have heard time and time again, clean flat floors. But for some reason, many of the problems bonding tile come from poor surface preparation. The success of any installation where tiles are directly bonded in a thin-set application begins with identifying potential bond breaking contaminants and successfully removing them.
Once we can get the floor clean, for a small fee, we can cure the other ills such as flatness. While the industry tolerance is and remains 1/4-inch variation in 10 feet, we all know that does not work well with large unit tile; we often need a much flatter floor. You need to be very careful here about your choice of methods. In the perfect world of open wallets and price is no object, there is nothing better than a self-leveling compound. However, the only requirement for tile floors is flat, not level. If all you have is a control joint peaked up off the floor it is much less expensive to grind the joint down than level the whole floor. If you do the math you will find that you could buy a whole bunch of tools for what it costs to level the average floor. If a few hills and valleys are your only concern use a good cement-based floor patching compound. Wood floors and floor joists can certainly be more of a challenge when it comes to leveling. The options here can be the same but must be taken with much greater care. Given the flexible nature of wood construction, much more consideration needs to be given to both product and application.
If you are going to use a membrane system of some type, all of the prep needs to occur under the membrane. Membrane use has increased rapidly as more people become aware of the benefits an industry tested system can provide. What is important for the user to understand is that crack isolation and sound attenuation membranes are designed to reduce problems, while waterproofing membranes are expected to eliminate them. Cracks can be suppressed, noise can be reduced, but waterproofing must be waterproof. I mentioned industry tested. There is a performance standard for waterproof membranes. In June of this year a crack suppression standard was also submitted and will in all likelihood be approved by the time your read this. While the proposed standard is less than perfect, the intention is to make people aware of what may and may not work. Uses of roofing felt, sheet vinyl, scribing paper, or scrim reinforced Kraft paper with unknown values, glued or unglued to concrete slabs and in some cases surprisingly, wood, have a long history of failure. The products typically lack the performance features and criteria that would allow effective control of concrete fractures or wood structure movement without transmission through the finished tile surface; they have little if any elastomeric properties. Relative to concrete, they lack properties required, and they are not designed for prolonged moisture exposure common in concrete applications.
It would not be a substrate article if we did not mention backerboards. Given a choice, they are a great improvement over plywood because they are dimensionally stable. However, if you do not have a good solid subfloor there are instances where I would select plywood if I needed the structural support value. That being said, there are many substrates on the market for use under tile. The premium underlayment for tile is and always has been, thick-bed cement mortar, which provides a solid base that stands up to use for the life of the building. Most people cannot afford thick-bed mortar, and it also takes years of experience to install correctly. Affordable backerboards are often chosen as a substrate alternative for tiled floors, walls, back splashes and countertops. There are many choices in this category. Glass-mat backer boards feature a gypsum core treated with silicone and surrounded by an inorganic glass mat. The surface on one side of the panel is then treated with an acrylic coating that is waterproof.
Cement board offers many of the advantages of thick-bed cement mortar without the expense and lengthy installation process. It provides a strong, water tolerant base for tiled surfaces. Those that are directly and repeatedly exposed to moisture will not swell, soften, decay, delaminate or disintegrate when exposed to water. Cement board is a cementitious substrate made from portland cement and sandwiched between layers of inorganic fiberglass mat. The panels come 1/2-inch thick for use in bathtub and shower surrounds, walls, ceilings and floors, as well as 5/16-inch thick for countertops. Due to their alkaline nature, the product does not support mold growth, an increasingly important issue. Fibered cement board is a very popular choice for its ease of handling and lack of abrasiveness. Originally designed for harsh exterior siding, it has been widely used in backerboard applications. One of the newest backerboard products on the market is cement coated foam boards. The properties of these panels vary with manufacturer, but they are all very light, water resistant, and provide an excellent support base for ceramic tile. And no, I would not have said this 4 or 5 years ago; you had to prove it to me first and it has been proven over and over again. The one complaint we hear over and over again with backerboards is no thinset under the panel or tape on the joints. I think I have had maybe 5-6 calls this week, all the same; I can read the 3-by-5 sheets or my tile is cracking. This goes on every week. The thinset is to provide a supporting plane for the panel, it really is important.
Hope this gives you a little food for thought. With the assistance of Tile Council of America, I have started a series of tests to provide some real numbers on just what effect paint overspray, curing compound, excessively dry surfaces and any number of other scenarios have on the ability to bond ceramic tile to a surface under those conditions. We have heard the stories time and time again but nobody has some hard numbers on exactly what it does though we know it has an effect. Understandably, this is a major project that will take months to complete, but it is in the works and I can promise the outcome will provide some interesting reading.