Thinset is an expensive and inappropriate material used for this purpose more often than we would like to see. Thinset products are designed as bonding materials. With a standard thinset the maximum bonding thickness should never exceed 3/8 inch. If something is classified a medium bed mortar it is typically good for a maximum bonding thickness of ¾ inch. The problem with applying thinset that thick from an installer perspective is that you are going to be using a lot of mid-price-range bonding material. This is a great money maker for setting material manufacturers, but bad for the profits of the tile person, as typically you don’t know in advance what the flatness conditions are going to be. Self-leveling products, a misnomer if there ever was one, are also a possibility. However they tend to be very finicky about prep prior to application and cost wise, in our shop we referred to it as pouring dollars out of a bag. Most are also adverse to moisture so damp floors and wet areas are a further consideration. When possible, sometimes the best thing to do is also the least expensive: just mud it!
The best deck mud is the result of three things: the right combination or ratio of ingredients, proper mixing and compaction. For all mortar applications the sand should be clean, sharp and slightly damp. Rounded sand grains offer only a fraction of the compressive strength achieved with sharp sand. Think of trying to pack a bunch of round golf balls in a box as opposed to a bunch of diamonds with their flat surfaces. Flat surfaces on the sand offer much more compactness and greater strength as well as easier handling. The proper mix ratio is determined by the installation application.
In some instances a latex additive is used in a mortar bed when low porosity or high strength is required. Use of latex additives in mortar beds can produce a dense material much harder than most concrete. However, when using a latex additive, the mortar bed must be protected from exposure to additional moisture and allowed to thoroughly cure before water emersion. In the case of swimming pools or other equally wet environments this can be 14 to 21 days. When mixing with plain water, dependent on application, a sand/cement ratio between 6/1 and 4/1 is typically recommended for floors. Mixtures too “rich” in cement are prone to cracking and excessive shrinkage. On the other hand not enough cement yields a soft deck mud with very little compressive strength. My experience, as echoed by any other tile setter I have ever worked with or spoken to, says you will always need to tune the mix ratio based on the cement and sand available in your area; materials vary greatly in a given geographic area. There are many ways to mix mortar and none are right or wrong, as long at it is thoroughly and properly mixed. On large installations, transit trucks, gas and electric powered mixers and even roto-tillers have been used to mix both floor and wall mud. Regardless of the method used, mixing must be complete enough for all the ingredients to become homogenized. One type of mixer that does not work is a concrete mixer. Mortar mixing needs a blade or paddle type configuration.
All of this is good information but still no mention of exactly how to do it. Well, the skills of working with mortar work are similar to riding a bicycle; once you master it, it comes naturally. What does not come so easily is all the little knowledge-based nuances like: acceptable wire reinforcing; how the wire is placed and/or attached; what is the right mix; and how to get it flat. A good source of information on varied mortar related subjects is the Ceramic Tile Institute of America Field Reports available for review at www.CTIOA.org. You can find more information on such subjects as Portland Cement, Aggregates, and Water; Portland Cements, Lime and Sand; Reinforcing Wires for Mortar; Expanded Metal Lath and Wire; and Working With Portland Cement. The Tile Council of America Installation Handbook (www.tileusa.com) has numerous details for both interior and exterior mortar applications such as decks, showers, countertops and swimming pools. Material requirements for mortar beds are contained in the American National Standards for Ceramic Tile Installation (ANSI A-108) where you will find reference to the types of wire acceptable, sand to cement ratios, cleavage membranes, and other mortar work related items. None of these documents serve as instruction manuals, but with common sense and a little help from someone experienced, a dedicated tile setter can figure it out.
Mortar work is not difficult to master; it just takes some basic knowledge of sand and cement and a lot of time to practice. Those who do learn tend to have a life-long preference to do mud work. Given the shortage of mud mechanics (yes, a technically knowledgeable person in a manual trade makes you a mechanic), wages tend to be higher than those who do thinset methods exclusively. The success stories of those who attended the CTEF mortar shower courses and mortar floor and wall basics are numerous and in some instances amazing to me. A long time mud man, John Bridge at johnbridge.com, has also put together a mortar course to be held mid September in Dallas, TX, and hosted by Laticrete for those interested. The opportunities are there and the money is pretty good; maybe you should give CTEF or John Bridge a try and broaden your horizons a little.