While mastic does have strong points, such as great adhesive properties and adaptability to many substrata, high performance in wet areas is not one of its high points.
Bottom Line
Mastic is porcelain, glass, or ceramic tile adhesive. Mastic is a largely outdated term. Ceramic tile adhesive is more appropriate. Mastic is not recommended for high moisture areas: “intermittent water exposure” areas only. Mastic adhesive is very sticky, making it well suited for vertical applications where tile may slide. Does not fill gaps in a substrate or have “building” characteristics, as it is more watery than thinset. Best used for tiles in the 8-inch range or smaller. Not recommended for tiles over 15 inches. Older tile mastics were derived from the resin of the Pistacia lentiscus shrub. Today, mastics are inorganic, containing acrylic copolymers and calcium carbonate. These organic properties are one reason why older mastics tend to break down over time.
Mastic Can Be Used for Areas of Light Moisture
Tile mastic is not known for its high resistance to moisture. Some tilers claim that tile mastic is fine in wet areas, as long as the grout is properly sealed—and kept sealed. Other tilers say that tile mastic should be confined to dry areas. Custom Building Products, a major manufacturer of tile adhesives (AcrylPro), thinsets, and grout, states that the product can be used in “Interior wet areas with intermittent water exposure such as tub surrounds and shower walls.” The “intermittent” designation is important because it distinguishes shower walls from high-moisture sections of the shower, such as floor pans.
Where You Can Use Mastic
Wall tiling Kitchen or bathroom backsplashes Bathroom (or any room) wall wainscot Glass tiles Interiors Residential and another low-traffic flooring (though drying time is longer than with thinsets)
Where You Cannot Use Mastic or Not Recommended
Steam rooms, underwater, or shower roomsOver radiant heating systemsTiling swimming poolsTiling in or around water featuresHigh traffic flooring
Common Substrata Approved for Mastic
Concrete Cement board such as HardiBacker, Wonderboard, or Durock Exterior grade or better plywood Interior drywall (if the surface is painted you must roughen the surface with a sander or apply a layer of special bonding primer).
Common Substrata Unapproved for Mastic
HardwoodLuan plywood or other types of veneered plywood, where the veneer is in danger of delaminatingParticleboardParquet
When to Use Mastic vs. Thinset Mortar
Mastic and thin-set mortar mainly differ in terms of adhesion, ability to fill gaps, and the conditions under which they can be used.
When Immediate Adhesion Is an Issue: Thinset is a mortar, made of Portland cement, silica sand, and moisture-retaining agents. In fact, you can even buy sanded or non-sanded thin-set mortar. It has no odor (or trace odors) upon installation. It is recommended in most installations. However, although thinset’s curing time is shorter than it is with mastic, it has less grip while drying, which can be an issue when you are installing tile vertically. Tile mastic has better adhesion properties for vertical surfaces. It has great “grab,” and this is important on walls.Filling and Building Properties: Sometimes when tiling you need to build up depressions a bit with your thin-set mortar. Thinset works well for this because it has excellent “build” properties as well as adhesive properties. While thin-set is not meant to be a leveler (there are compounds that will do this, though), it can take care of minor leveling problems. Filling, leveling, and acting as a “builder” is impossible to do with the tile mastic because it is more syrupy and has no filling capabilities. Mastic tends to spread out quickly, having a consistency resembling pancake batter.Moisture: As noted earlier, tile mastic cannot be used in high moisture areas, such as shower floors, swimming pools, or bathtubs. Mastics can be used on shower and tub surrounds, though, because these are rated as “intermittent” moisture areas. Thinset can be used in all of these areas, including shower pans.Size Issues: One interesting aspect of tile adhesives such as mastic, is that they are best used for smaller tiles in the 8 inch or smaller range. In a pinch, it can be used with tile up to 15 inches, but this is not recommended, and drying time will significantly increase.
