Mastic vs. Thinset Mortar: Which to Use When Tiling?

Tiled bathroom
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Tile mastic or thinset mortar:  both are adhesive in nature, both are used for tile.  What is the difference and what are each used for?  

What They Are

When laying ceramic or porcelain tile, you need a wet adhesive to make the tile stick to its cement backerboard base.  Theoretically a great idea, dry adhesive tile mats like Bondera and Musselbound unfortunately have not been widely adopted, as adhesion is marginal and they tend to be expensive.

 So what's the solution?

The job falls to wet adhesives, either tile thinset or mastic:

  • Tile Thinset:  Inorganic, composed of materials mined from the earth.  Thinset is cheap and comes either in pre-mixed or dry form.
  • Tile Mastic:  Traditionally, mastic has been an organic plant-based resin from the Pistacia lentiscus shrub, though now more inorganic ingredients are finding their way into mastic.  It is available in wet form in tubs only.

The Basics

 ThinsetMastic
Moisture FactorCan be used in very wet areas, even those you expect to be submergedDry or damp areas only, not in areas that will be submerged in water (such as a shower pan or swimming pool)
Best AreasShower, tub, backsplashes--the entire range of tiling areasWalls, wainscot, dry backsplashes
You May LoveThinset is cheap and it fills in gaps and depressionsMastic is very sticky and quick setting
You May Have Issues WithThinset is slow to set, which has several implications, mainly that it produces "tile sag" on vertical applications.Mastic can give off a sharp, strong odor that takes time to dissipate
Cost Difference$23 of pre-mixed thinset will cover 40 square feet, using a V-notch trowel$15 of mastic will cover about 60 square feet

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.

Areas 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 is the 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 is too late.

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

Summary:  Thinset Is Your Material of Choice For Floors

Thinset is a stronger material than mastic, making it your go-to choice for horizontal applications such as flooring that get a lot of pounding.

One downside sometimes mentioned in the mastic vs. thinset debate:  thinset, being a bit like concrete, cannot be saved.  Excess thinset must be thrown out, as opposed to mastic, which can be reused.

* = While mastic is often classed as "OK" for backsplashes, you may wish to use 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.