So let’s just forget about all the “who is responsible for what and why” and just get down to the basics. Customers want flat floors free of any lippage. They want floors with very small grout joints perfectly aligned, straight as an arrow for long distances. The feeling is the tile is bigger so the work is less. There are a host of problems with that thought process. Anyone who has ever installed large tile knows the bigger the tile, the more work to get it installed properly. We can purchase high quality or “rectified” tile and eliminate size issues, but unless we have a perfectly flat floor there are going to be both alignment and height variation issues
We also get numerous calls and emails on a regular basis from both installers and end-users where standard thinset is used excessively thick to flatten the floor and lost bond under traffic. Once you get over 3/8 to 1/2” with a standard thinset, most have a very low bond and compressive strength. Medium beds, which can typically go up to 3/4”, are helpful but it can be very time consuming to set each piece flat with the next when you are completely bedding the tile. Often you will also find a substantial elevation change when you work your way across the room to another doorway that brings a new problem, been there myself.
With the dominance of big tile on today’s floors, using self-leveling products in conjunction with large tile truly does make sense. The steps required to make concrete flat enough to receive a 12” x 12” tile without excessive lippage would be roughly 3/16” in 10 ft. If you were installing an 18-inch tile, an 1/8” or less would be appropriate. In the world of concrete, these are considered “Super Flat” floors and the price to finish a slab to that profile would be at least double that of a normal slab. It is questionable that many concrete finishers even have the ability to finish a slab at 1/8” in 10-ft. tolerance, according to the American Concrete Institute. If the cost was not objectionable and qualified labor was available for such exacting tolerances, by the time the installer got on the job it is doubtful the slab would still be the same flatness. Flatness measurements to meet the specified requirements are taken shortly after the pour and do not extend across control or expansion joints in the slab. If the target was a 1/4” in 10” after 30 days of curing, then the slab would either need to be poured at a flatness of 1/8” in 10 feet or have substantial steel reinforcing to compensate for the natural warpage that occurs during the curing process. Complete details for the recommendations can be found in ACI 302.2-6.
Another part of the concrete industry recommendation in that document mentions because the high cost of such tight tolerances are not reasonable in most instances, the installer is perhaps better suited to bring the slab into compliance for his floor material. This is a departure from past recommendations that has yet to filter down into practice from a specification point of view. From the real world point of view for installers, it has always been that way but be aware, there used to be a defense when problems occurred that is no longer there.
All this is to say self-leveling products are here to stay, and we can count on their increased use for tile installation as tile continues to get larger. There are numerous types of products available and your choice needs to be appropriate for not only the level of service expected but the skills of the user. Personal experience tells me that product placement techniques and performance vary widely among both products and manufacturers. This is not to say some are bad or good, but rather some favor one placement technique that may not work well with another. If you don’t like your first or current experience with a product, try again or try another manufacturer.
Perhaps the biggest misconception of self-leveling products is that they actually level themselves. Not quite. Although they have the consistency of thick soup, they do require assistance in both spreading out and tying into successive pours. A pump is really the way to go if they are used on a project of substantial size.
Perhaps most important is to always have adequate manpower when using these types of products. The working time is very limited and there are few jobs that can be done with one person. Our practice was to never pour unless we had a minimum of three, though sometimes you could get by with two. If that was not possible, using a trowelable product would be much more appropriate and avoid flattening the leveler.
As wonderful as these products can be, getting paid appropriately for their use has always been a problem. It is very difficult to calculate the exact amount volume of material required to provide coverage for any given area. Making assumptions can be very hazardous to your financial well being when using a product with the consistency of soup. We have even gone as far as taking elevation readings all over the room using a laser and still been off on our calculation enough to hurt a little so always err on the side of caution. Once the skills and technique are mastered, I think you will find this is a much faster approach to correcting floor problems than squaring each tile up to the other. It is also much easier to get compensated for as the customer can see there is some corrective work going on, which can’t easily be perceived when adding additional thinset to make up for inconstancies in the floor. While this approach may not work for everyone, we found it to be very profitable, and that is why we were there; otherwise I would be motorcycling.