Organic Mastic Woes -- Cracks in this tile installation developed because an organic adhesive was used improperly as a leveler.
The shear values of Type I and Type II ceramic tile adhesives remain the same in the ANSI A-108-99 handbook. The differences in values between Type I and Type II mastics vary according to their intended uses.

Type I mastics, which have higher values, can be used in all Tile Council of America (TCA) details that cover organic mastics. Type II products, on the other hand, may be used only in a limited manner as described in the TCA Handbook.

The basic water-immersion and shear values illustrate this. Type I mastics, after being immersed in water for seven days, must have a shear value of 50 psi. In comparison, Type II, after water immersion (four cycles at four hours each), needs a shear value of only 20 psi.

Another limitation applied to organic mastics in ANSI-AN 2.3.2 states that slabs on-grade subject to moisture transmission are not suitable for ceramic tile set with organic mastic. (So, get your test meters going.)

And there's another point to remember. You cannot use mastic for an installation where the temperature may reach 140¼. Where might such conditions be prevalent? This kind of heat would not be unusual in a commercial kitchen - especially, for instance, behind pizza ovens.

The TCA Handbook lists various DOs and DON'Ts that apply to the use of organic mastics for installation of ceramic tile. The following are some of these.

Walls. Where the backing units are cement backerboards/underlayments in wet areas, organic mastics should not be used.

Where the backing units are coated glass mat backerboards, as shown in detail N245.01, only an organic mastic rated Type I should be used. If these backerboard units are in a dry area, it is possible to use a Type II product.

When installing ceramic tile over gypsum board that's been screwed to metal studs in dry areas, as in ASTM C-36 (not water resistant), you may use a Type II mastic. And, of course, Type I is okay as well.

When installing ceramic tile in interior areas, mastics may be used for gypsum board, plaster and cementitious backer units. Type I mastics are recommended for tub surrounds. Type II is acceptable in dry areas only. When installing tile over cementitious backerboard, use only a Type I product that's been approved by the adhesive manufacturer for that use.

Detail courtesy Tile Council of America. Used with permission.
Bathtub walls.When installing ceramic tile over a water-resistant (WR) gypsum board, a Type I mastic should be used.

Showers. Some shower walls now use coated glass mat backerboards as the substrate. Here, again, a Type I mastic is required.

Shower receptor walls. A Type I mastic is recommended for this wall usage.

Countertops. In the thin-bed method TCA C512-01, Type I mastic may be used when the installation calls for a plywood substrate.

It's wise to determine whether a proper substrate is being used for the installtion. ANSI A 108 (99) states that gypsum wallboard (ASTM C-36) or gypsum plaster must not be used in wet areas. Hence, they are not to be used in showers, saunas or steam rooms.

Another important item to remember is that, when installing with organic mastics, you must wait 24 hours prior to grouting. Proceeding sooner than that may cause grout staining or discoloration. The mastic must be allowed to lose its moisture via evaporation, or it may carry contaminants to the grout.

Organic mastics are for interior use only. And, as a word of caution, do not use them over wallpaper or as a means to set bathroom fixtures.

Marble and stone may present problems when organic mastics are used. Specifically, these materials may stain or curl due to water transmission through the tile. I recommend that you do a test section to determine whether there will be adverse effects.

Do not use a trowel larger than 1/4 inch. When applying the mastic, use the flat side of the trowel to create a uniform "bed." Then use the notched side of the trowel to fashion beads in the bed.

It is most important that the tile be beaten into the mastic to ensure the required coverage and allow for shrinkage.

Organic mastics are easy to work with - just take the lid off the can. In addition, they are VOC compliant and less labor intensive than other types of bonding materials. To ensure installation success and longevity, however, it's vital that you understand the limitations of the product.