The purpose of an underlayment, from a ceramic tile perspective, is to provide a backing surface or other enhancement for direct bonding of ceramic tile. Underlayments can posses many properties, such as sound attenuation, waterproofing, crack isolation, vapor diffusion, or in wood structures, dimensional stability. Tile underlayments are not designed for structural value, nor will they compensate for a lack thereof. Sounds like an argument on the horizon doesn’t it? The argumentative portion would be related to wood structures so let’s start there.
In wood structures, there is no such thing as a single layer floor system. All wood structures require some type of underlayment to prevent structural movement from transferring through the finished tile work resulting in cracks at a minimum. There are a few thinset products on the market that claim they can be used in lieu of a separate underlayment layer. If the stars are in alignment and you have an exceptionally skilled carpenter using first-rate products under ideal conditions, maybe. In my experience, encountering such conditions is unlikely. More likely is that a pallet of plywood or OSB was dumped in the dirt on the jobsite followed by being placed on the deck in the rain with the air gun missing the fasteners on a third of the joists using a nailing pattern spaced too far apart. That is reality. Joist and subfloor construction are a very critical component when tile floors are selected. For any underlayment to be effective, the supporting structure must be properly prepared. A whole article could easily be devoted to proper preparation; however the Engineered Wood Association (also known as American Plywood Association or APA) has many excellent publications on wood construction available at their website available for free at www.apawood.org/publications. Many are available in both English and Spanish. They cover every conceivable issue you can imagine related to wood construction and also have some specific recommendations relative to preparation for ceramic tile installations.
The first backer board details published called for 3/4” plywood or 1” nominal lumber over 16” centers with 1/2” cement backer board underlayment which totals 1-1/4”, another often heard minimum. Methods continually evolve with both in experience and product capability. Most methods for use of underlayments currently call for a single layer of 3/4” tongue-and-groove underlayment grade-rated plywood or OSB on 16” centers. Some allow for a single layer of 5/8”, which is also required to be tongue-and-groove, a rare breed in most parts of the country. Spacing of the floor joists also dictates what type of supporting subfloor is required. All subfloor panels require a 1/8” gap to allow for acclimation and accommodate seasonal movement. Anybody who has been in flooring for any length of time knows that butt joint panels of either plywood or OSB have a strong likelihood of peaking and transferring through the finished floor installation if exposed to even seasonal moisture.
Unacclimated basements and improperly ventilated crawl spaces can also prove problematic. If the panels are butted tight, not able to accommodate this movement and joints must be cut open or standard square edge panels were used as a subfloor panel, the seams must be cut and/or blocked between the floor joists. None of these requirements are Dave’s rules or even TCA rules, but those of the organizations that publish guidelines for wood floor construction. They also serve as a basis for what will and won’t work once your underlayment selection is made. The TCA handbook contains numerous generic floor methods for various panel thicknesses and joist configurations that have been proven to perform as a result of joint studies conducted by the substrate trades.
When making your selection, you need to make absolutely sure your basic structure meets the requirements for a given underlayment product. People are often of the opinion that a tile underlayment adds some structural value to the floor; this is especially true in backer boards. A few manufacturers go so far as to tout the strengths of their products or assert they impart some strengthening ability. If backer boards were structural panels, they would not be called backer boards. Can you force a backer board product to add some structural value? Sure, bond it to the floor rather than bed it to the floor and nail it into the floor joist. Unfortunately, many do just that, contrary to recommendations of never nailing an underlayment into the floor joist. Forcing a backer board panel to function as a floor stiffener will likely result in cracks as the wood structure does what all wood does, move.
Membrane-type underlayments make absolutely no contribution whatsoever to stiffening a floor. However, membranes have been gaining in popularity for their waterproofing, crack isolation, and/or sound reduction attributes. When using a membrane, you must be certain that the supporting floor structure is up to the task of supporting the tile installation because you will receive no assistance from membrane underlayments. I often hear something to the effect that this or that was wrong so we used a membrane. Membranes are not designed to mask structural deficiencies. Whether wood or concrete, all floors can benefit from the use of a membrane and often, various other underlayments.
It’s important to understand that underlayments are not replacements for otherwise inadequate substrates. They should be used for the properties they offer, not as a substitute for lack of support.