It’s not really fair to compare organic mastics to thinset mortars, because mastics occupy a particular niche in the industry and should be used only within very specific parameters. For example: for floor installations, organic mastics should only be used in residential applications. In contrast, thinset mortars can be used in commercial and residential environments.

Organic mastics are divided into two categories. “Type I” describes the most water-resistant mastics. These may be used in commercial and light-construction applications where there is some limited water exposure. Type I products also may be used for residential floors and countertops.

“Type II” mastics are restricted in terms of recommended use, because they do not have as high a level of water resistance as Type I products possess. Type II mastics may be used only for walls and ceilings in light-construction applications that are dry or subject to minimal water exposure.

At this point, it might be relevant to consider suitable applications for mastic use in light of ANSI 108 definitions:

Commercial construction (dry or limited water exposure) — dairies, breweries, commercial kitchens, laundries, etc.

Light construction (wet walls) — tub enclosures, showers and similar areas.

To illustrate the difference between Type I and Type II mastics, I’ll reference one important test: shear strength after water immersion. Type I products must meet a standard of 50 psi, whereas a Type II mastic is bound by a 20 psi requirement.

Dry-set mortars have strengths, expressed as “shear values,” that relate to the type of tile being installed. The values for seven-day water immersion testing are 150 psi for glazed wall tile, 100 psi for impervious tile and 50 psi for quarry tile.

As you can see, the requirements for mastic and dry-set mortars vary widely. Because they are packed ready to use in cans and pails, mastics are preferable to many users. Dry-set mortars must be mixed with water on the job, which creates a more labor-intensive situation for the installer. However, as I mentioned before, the use of mastics is limited in scope and these limits must be strictly observed.

Mastics will not withstand high-impact or wheel loads. And in cases where on-grade concrete slabs are subject to a great deal of moisture intrusion via water vapor transmission, there is increased potential for bonding problems over time.

Another small but vital point is that mastics cannot withstand exposure to temperatures in excess of 140º F. You may be wondering where such conditions would be prevalent. I’ve seen failures where ceramic tile has been installed with mastic behind the ovens in pizza kitchens.

Organic mastics, which are water-based, should not be used for installation of natural marble. There are two reasons for this. First, the water content of the mastic can cause “curling” of marble, especially the black and green varieties. And second, impurities in the water/mastic can migrate upward and stain the marble (because marble is very porous).

A good many mastics are used today and, not surprisingly, a large percentage of tile is being installed with mastic. If you opt to follow suit, make sure to use the proper type (I or II), refuse to use any non-spec mastics and be sure that the job site conditions are appropriate.

Dry-set mortar also has limitations. Tile Council of America (TCA) method F111 is preferred over precast and post-tensioned concrete floor systems, as well as other floors that are subject to movement of deflection. Any deflection should not exceed 1/360 for above-grade slabs.

As usual, I conclude with the advice that you observe all precautions and conform to applicable industry standards. Almost all of the failures I encounter are the result of some violation of the Standard of Care.