Tile mastic or thinset mortar:  both are sticky, both are used for tile.  What's the difference?  

When laying ceramic or porcelain tile, you need some type of wet adhesive to make the tile stick to its cement backerboard base.  Though they are a great idea, dry adhesive tile mats like Musselbound unfortunately haven't quite taken hold yet, as stickability is dicey and they tend to be expensive.  So the job falls to mastic or mortar.

What They Are

Thinset is inorganic, composed of materials mined from the earth.  It is cheap and comes either pre-mixed (about $13/gallon) or in dry form.  Tile mastic is organic, a plant-based resin.  

Tile Adhesives vs. Water:  Will The Water Win Out?

Tile is installed in a range of moisture conditions, from high amounts of water to zero-water bone dry places.

Typical residential places with high and prolonged amounts of water:  shower pans; shower walls; tub surrounds; hot tubs; swimming pools.

Typical places on the other end of the wetness spectrum:  any normal room (i.e., not a sauna) that is not a bathroom or kitchen; bathroom wall wainscots; kitchen heat guards behind stoves; kitchen backsplashes; bathroom backsplashes.

Due to their organic nature, tile mastics  are not good for area with prolonged exposure to water.  Organic materials love to provide a ready-and-willing environment for mold and mildew to develop.  And organic materials will rot away.

True, it is possible to do such a great job of tiling that you believe that water will never get underneath.

 But no matter how well installed, tile is still considered a potential victim of water infiltration.  This is because even the smallest crack in tile or grout can lead to water wicking through, which can work its way around to the back of the tile.  Here the water remains and begins to create mold and mildew.

Where You Can Use Them

Tile Mastic

• Kitchen walls
• Kitchen backsplashes*
• Bathroom backsplashes*
• Bathroom walls, if not adjoining a shower or tub
• Any walls where moisture is not prevalent

Thinset Mortar

• All of the above
• Shower pans (floors)
• Shower walls
• Bathroom floors
• Bathroom walls
• Kitchen floors
• Kitchen countertops
• All floors

Mastic's Stickability vs. Thinset's Sliding, Sagging Problem

If you can use thinset wherever you use mastic--and more--why even consider mastic in the first place?  As opposed to thinset, mastics are easier to work with; stick better initially; and set faster.

One of the great downfalls of thinsets, at least for the amateur tile setter, is that it is slow to set, during which time the tile can move out of place. The problem is compounded when working with vertical spaces such as bathroom walls and kitchen tile backslashes.

It's that dreaded "sag" that new tile-setters experience after laying tile on a vertical surface with thinset. The tile looks perfect at first; but an hour later the tiler comes back to find their beautiful tile sliding downward like houses in a mudslide.  By that time, it's too late.

In terms of low-impact vertical surfaces, both mastics and thinsets hold equally well.

 

Thinset Is Your Material of Choice For Floors

When push comes to shove, thinset is a stronger material than mastic, making it the only choice for horizontal applications such as flooring that get a lot of pounding.

* = While mastic is often classed as "OK" for backsplashes, I prefer thinset mortar, as the name "backsplash" does imply water.  Exceptions are those backsplashes that are more like short counter backs than true sink surrounds that receive lots of water contact.