Regardless of your choice of underlayments, be they boards, sheets, liquids, or cements, when bonding the surface must be properly prepared. Many times a grinding device may be a good choice to aid prep even on wood.  Dust containment is always a concern but many tools offer collection systems.


Most self-leveling products and many other underlayments are moisture sensitive. As technology grows, knowing the conditions of the substrate has gained in importance.  If a high moisture vapor reading is obtained in a slab on grade application of fully cured concrete and there is no vapor barrier as shown here additional measures and/or products may be required.

Over the years we have spent a lot of time discussing various substrates for bonding tile but often we do not give enough consideration to the actual underlayment material. In this issue of FCI we focus on that wide array of products that fall in the underlayment category. Just what is an underlayment anyway? Underlayments can do many things but their purpose is to provide a solid base of support for the tile installation. While some types of underlayments have great strengths, the actual support must always be provided by the structure. High-strength underlayments will not compensate for structural deficiencies. Some products or installation methods may tend to mask the lack of structural support in the short term but that basic structural inadequacy typically surfaces at a later date.  Retailers, contractors, and installers, helped by marketing statements, tend to get way too caught up in the “strength” of an underlayment. Without question each type of underlayment product needs to have “strength” or attribute that provides a solid base of support for the tile installation. But, a product having a compressive strength of 30 p.s.i. can perform as well or in some cases better, than one with 7,000 p.s.i.

In this article we will be make references to products and methods as listed in the Tile Council of North America (TCNA) Handbook or under the American National Standards for Ceramic Tile (ANSI). The tile world revolves around both of these documents. They are not intended as market documents. As a matter of fact you cannot find a single manufacturer listed in them. To get an underlayment method in the handbook requires substantial independent testing and documentation plus an approval process. For a product to become an American National Standard, the requirements are even more rigorous. Does this mean products that are not included TCNA methods or not covered under ANSI performance criteria are inferior? Not necessarily.  It takes years of proven performance to get in either document. To give an example: Wonderboard was the original cement board patented in 1968 however; it did not become a method until competitive products were introduced to the market place and basic testing criteria were established. The first year you find the title CBU, Cementious Backer Unit (Wonderboard) in the TCA handbook is 1985. On the flip side of the coin, roofing felt has been used for a tile underlayment for many years. It has been tested for basic performance as an underlayment many times in all manner of conditions and consistently fails the basic performance tests of an underlayment.  There is also a substantial history of failure as a tile underlayment. For our readers, we will confine ourselves to proven products and methods.

While this floor appears very rough due to demolition of the previous floor covering it is in fact very flat and free of curing compounds. This makes it the perfect candidate for a patch and skim product rather than the more costly self-leveling alternative.



One product that seems to be increasing in popularity for both its code complying abilities (fireproofing) and other desirable characteristics such as sound attenuation, floor flattening, or radiant heat applications, is poured lightweight gypsum.  Poured Gypsum underlayments can provide a satisfactory surface to receive and ceramic tile installation system. In general, ceramic tile cannot be directly bonded to gypsum underlayments without use of a membrane system. The Tile Council of North America and all major manufacturers of lightweight underlayments recommend the use of a membrane system when installing ceramic tile over this type of product. The few exceptions to this recommendation are proprietary in nature and suitability for use rests solely with the setting material manufacturer. There are 4 industry approved methods for this product category. Very specific recommendations apply to each method as listed in the TCA Handbook and should be reviewed prior to installation. Anticipated service levels used in either commercial or residential applications should be met when tested under ASTM C627, the Robinson Floor Tester which provides guidance for floor traffic applications.  Lightweight gypsum products used in these TCA methods do share some basic recommendations. All lightweight product installer/applicators must be approved by the manufacturer. With only a single exception to my knowledge, these types of products are not available to non-approved or endorsed entities. As a licensed or approved applicator, they must be committed to performing a product installation per manufacturer’s instructions. All products used for tile underlayments should have a minimum of 2,000 pounds compressive strength when tested using ASTM C472, results of these tests should be available on request per manufacturer’s instructions from the lightweight installer. Some thinset manufacturers have higher compressive strength requirements for use of their bonding materials.

Lightweight Gypsum underlayments have very exacting requirements for proper installation. Adherence to these manufacturer recommendations are even more critical when ceramic tile is to be applied.



The most common issue seen in lightweight installations is low compressive strengths which can be an issue that will cause cracking and debonding of the tile. Floors also need to be dry prior to application of a membrane and should not exceed 5 pounds of moisture vapor emission (MVE) for fully adhered products and 10 pounds of MVE for uncoupling membranes. When using humidity probes the underlayment product should be in the 75-80% range using a Delmhorst meter. All of these products require a surface primer prior to application of the lightweight topping and an overspray (or sealer) post application after curing. This step is often eliminated due to the drying cycle required and can have disastrous results. Average drying times prior to application of sealer/primer and floor covering products can be from 5-7 days for a ¾” topping to 2 weeks or more for a 2” topping.  Unless specifically instructed it is very unwise to install tile over lightweight gypsum floors without verifying the primer/sealer was applied. The methods listed in the TCA Handbook apply to ceramic tile products. The Marble Institute of America makes recommendations for stone flooring products. Among those recommendations they note the lightweight topping have a depth of 2” above the heat tubes in radiant applications to aid in heat dissipation and avoid thermal stripping induced stress which may result in cracked stone

Self-leveling compounds are also seeing increasing use as underlayments. This can be a sore spot for many installers, as getting paid appropriately for use of these types of products can be very challenging. The term self leveling is a bit of a misnomer. A more accurate description would be floor flattening underlayment, but there is nothing catchy about that term. It also is indeed possible that a floor may be made perfectly level by their use. Before you and your customer commit to using these types of products make sure everyone understands what the term flat or level means and establish your goal accordingly. Growth in this product category is driven by big tile which demands flat floors, flatter than possible to achieve with the skill level of many concrete contractors. Still, it is possible achieve the desired floor flatness. Typically you only have to visit your local big box or Sam’s Club to see what “super flat” floors look like. In new construction it is possible to specify flatter floors than the typical ¼” in 10’ no more than a 1/16” variation in 12” called for when tile floors are specified under the recommendations of the concrete industry. However, the cost of doing a floor with a 1/8” in 10’ would double the cost of the concrete work at a minimum. There is also the concrete curing process during which concrete slightly warps which takes place after the flatness measurements are taken under concrete industry standards. Use of self leveling toppings is also gaining popularity for installation in residential construction. When it comes to wood structures, the saying is crown up but any crown is a potential flatness problem for tile floors.  We are also seeing an increasing use of mortar beds used in residential applications with large size tile. Both would be subjects of a whole different article. With either concrete or wood structures, before committing to using self-leveling products take the time to make sure that the problem exceeds grinding a few ridges and filling a few low spots. In many instances it is much cheaper and money ahead to do some repair work.

Most poured underlayments require a primer prior to application of product. In the case of lightweight gypsum, another primer or sealer is also applied to the top after curing prior to installation of a membrane.



Selecting the self-leveling product with appropriate performance characteristics and floor prep is also very important. Proper preparation of any substrate is the most important step in achieving a successful installation of all self leveling products. Whatever product is being used to restore or repair a damaged surface it will only be as strong as the surface over which it is bonded. This surface must not have any foreign material that act as a bond breaker at the interface between the self leveling system and the substrate. Therefore it must be sound, clean, dust free and not contaminated with any oil, grease, paint, wax, curing compounds, asphalt residue or any other material that may act as a bond breaker. Curing and Sealing Compounds are especially problematic. Curing compounds stop the rapid evaporation of water which would cause concrete to curl. The issue when using ANY bonding material over a slab with curing compound is it also prevents anything from going in. Curing compounds are used in the overwhelming majority of floor slab construction. The chemistry of curing and sealing compounds varies considerably. Typically and often mysteriously the concrete contractor suffers memory loss and does not know or remember what was used. There are a few curing compounds that under specific circumstances may not pose a problem. However, unless you are absolutely sure what was used and they are warranting the bonding abilities going over their product, complete mechanical removal is a prudent choice prior to proceeding with the installation of the underlayment, topping or any other bonding product. Typical mechanical substrate preparation methods are: sandblasting, bush hammering, grinding, sanding, shot blasting, or in some instances high pressure water blasting, each of these methods depending upon situations, job conditions and environmental restrictions have their advantages. Shotblasting the substrate has long been the preferred method and has become more cost effective with increased popularity. Never use chemical methods such as acid washing/ etching, solvents or adhesive removers. All of these methods, especially on porous concrete, will either not completely remove the contaminant or force it deeper into the substrate resulting migration at a later date causing further problems such as bond failure. When using a self leveling topping, all movement expansion, control, and isolation joints in the slab must be maintained and should not covered by the underlayment or topping. They are integral to the needs of the construction material and movement requirements of the structure.

Quite often a cement patching compound may be more appropriate than the other two products mentioned. Tile setters tend to use thinset to “true-up” the floor which is a bad practice and often the cause of bond failure at a later date. I can hear the “I been doing it that way for 30 years” going off in the background as I type this. This subject has come up countless times over the years at technical meetings and seminars and not once have I ever heard of a single manufacture promoting it. Often the discussion is occurring because some good customer finally had a floor failure caused by the practice and they were now swearing off and at the manufacturer’s product and going with a better product in the future.  I guess we all have that natural instinct of denial when we mess up to some degree. When using a patching compound get the right product for the job. I research products on a regular basis trying to keep up with the latest and the greatest. My two greatest product category challenges are keeping up with recommendations and limitations of thinset mortars followed by patching compounds. Doing a little research for this article I found recommendations when using various products for slab moisture to be as little as 3 pounds per 1,000 square feet to a recommendation of fully saturating the slab to a surface dry condition. Thickness limitations ran from featheredge to 4” for a floor patch. Cement floor patch is a product that is impossible to generalize the dos and don’ts. Read the instructions thoroughly of your selected product. If you think you know what it says, read it again anyway. Something else I have found consistently in my research is rarely do the instructions and/or recommendations stay the same for more than a year or two. Whether due to a failure claim on a job done at the edge of acceptability or some chemical innovation or revocation, the instructions do change.

Floor prep has always been the most time consuming area of any installation. Increased use of tile in ever growing sizes and patterns have served to increase an area we have always had the greatest difficulty in receiving proper compensation, floor prep. There are many practical ways to demonstrate the need for proper prep and underlayments when given the opportunity. It has to start with the sales process by creating awareness that the big tile of the customer’s dreams has its needs and limitations. When it comes to floor flattening, the bigger the tile, the tighter the grout joint, the flatter the floor. If you do not have the ability to influence creating this awareness perhaps it is time to adjust the installation time required along with your prices to reflect the needs of a professional installation. That should get someone’s attention.