Warning: Do Not Use Mastic In Wet Areas
Ceramic tile adhesive, or mastic as it is commonly referred to, is one of the products that can be used to install certain types of tile. It is extremely popular because it is very easy to use with no mixing involved. It can be purchased at your local home improvement store and you can store it for a relatively long time without it going bad and becoming unusable.
Where It Can Be Used Safely:
Mastic can be safely and effectively used to install many types of backsplash materials. If you are having a new backsplash installed in your kitchen, as an example, mastic would be preferred over thin-set mortar for a variety of reasons, including ease of use (no mixing of powder and water), which saves time before, during and after the job. It's quite easy to pop open the lid of the adhesive and start troweling it on the wall, being able to simply close it up later on when you are done for the day, and be able to continue using it the next day with no wasted material if there is more left to do. With thin-set, you need a mixing drill, mixing paddle, buckets, and a space to mix outside of the immediate work area to avoid mess. You also need to ensure that you don't mix too much at a time, or else it may skin over and lose important adhesion characteristics. A tub of mastic won't dry up or skin over anywhere near as quickly as a bucket of mixed thinset mortar. This is why mastic is preferred in many cases where a backsplash is being installed; it allows for the installer to make the minute and detailed adjustments necessary when working with the backsplash tile, without having to worry about the adhesive drying up.
There are a few cases in backsplash installations where mastic might not be appropriate. One would be clear glass tile. Some glass tile has a white backing that will prevent the passing through of the colour of the adhesive. This is important because mastic has a tendency to dry to a yellow colour after a period of time has passed. If you use this to set a clear glass tile without an opaque backing, you run the risk of this showing through the tile after a period of time has passed. While this is not guaranteed to occur or even be noticeable, it is something to beware of, especially if you are a detail oriented individual. In this particular case, a white polymer modified thin-set mortar would be a better solution. Of course, your personal situation may differ; consult your general contractor or tile installer on what adhesive would be best for your application.
Where Mastic (And other premixed products) Should Not Be Used:
Mastics, or premixed thinset mortars, should never be used in wet areas. Even if the manufacturer suggests that this is an acceptable practice, you should avoid it.
Mastic or premixed thinset mortar being used to tile a shower or bathtub surround is used only for one purpose: to save time. General contractors who don't care about quality and durability won't hesitate to use premixed adhesives in wet areas because they (and their employees) can save time and get the work done very quickly, and be on to the next job. Why is this not a good idea? Unlike thinset, which when cured is not affected by water and moisture, cured mastic can actually emulsify when put into contact with water. This is bad because moisture in a bathroom is easily able to penetrate through grout and behind porous tiles. With thinset, you do not have to worry about this at all. Once cured, the thinset mortar behind the tiles in your shower or tub surround will not de-bond from the backerboard in a way that is highly probable to occur with premixed thinset or mastic.
Here is something you should also know: A good contractor will always spec out a finished product without shortcuts or cost cutting measures that negatively affect the long term performance. While it is by no means guaranteed that a shower or tub surround finished in tile that was installed using mastic will fail, it is certainly more likely to do so over a lifetime of regular usage. Your contractor should know this. Good contractors know that products have to stand the test of real life over a period of twenty years, not just 12 or 24 months. Moreover, when you compound the effect of real daily living over such a long time period, you begin to realize that the true test of quality shows itself long after the contractor is long gone.
A bathroom with a tiled tub surround that was installed using mastic might not see regular usage for any number of reasons. Maybe the homeowners are retired and it isn't the main bathroom. In this case, there may never be a problem. However, consider the same bathroom in a rental property with multiple tenants; it may be used multiple times every day for hot showers. What if the exhaust fan isn't being used to expel moisture? This would only compound the problem further. A reputable contractor will always assume the latter case and therefore build your bathroom to that usage standard, and not cut corners simply to save you some money initially.
The bottom line: Always be sure to use mastic only in areas that do not see constant water exposure. Other limitations in dry areas may apply based on material choices; contact your local tile contractor for details.