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Do You Have Enough Mortar to Accommodate Tile Warpage?


When you install tile, do you ensure that you have enough mortar to accommodate inherent (actual) tile warpage associated with large format tile?

Tile sizes are changing; getting larger, longer, and proportionately having more inherent warpage. Likewise, changes are being seen in tile installation methods, using thin set and large and heavy tile mortars to accommodate tile warpage.

Let's review what's involved.

Accommodating Tile Warpage Article Sections

As Tile Sizes Get Larger, Inherent Warpage Increases

More Inherent Warpage Requires More and Different Mortar

A Skim Coat of Mortar on the Back of the Tile (a.k.a. Back-Buttering) Can Help

Occasionally Remove a Tile to Confirm Tile Industry Minimum Standards for Mortar Coverage

What About Lippage Control Devices?

Reader Question

As Tile Sizes Get Larger, Inherent Warpage Increases

Large format tile in the past was eight by eight and was, for the most part, installed using a ¼” x ¼” x ¼” trowel. The use of thin set mortars has met the challenge of properly installing ceramic and porcelain tile for years at a minimum thickness of 3/32” to a maximum of about ¼” (depending on the manufacturer’s recommendations) after the tile is embedded. 

However, the game has changed and continues to transform with tiles becoming larger and longer, and, proportionately, they can include more inherent warpage.

According to the Tile Council of North America (TCNA), the allowable warpage of a tile is as follows:

The ANSI A137.1 (The American National Standard Specifications for Ceramic Tile) defines allowable warpage according to the type of tile. For Pressed Floor Tile in the Calibrated category which is commonly used on floors, the allowable warpage is listed as follows:

Table 8: Pressed Floor Tile (does not include porcelain) – When measured as described in the ASTM C485, Warpage Edge, the warpage of each tile in the sample lot shall not exceed 0.75% (0.08 inch) along the edge nor 0.50% or 0,08 inch on the diagonal.

From this formula, the allowable warpage can be determined.

Today’s popular larger tile sizes can be 6” x 36”, one meter by one meter or larger, and include an industry-standard allowable amount of warpage (where the center of the tile is higher than the edges or ends).

More Inherent Warpage Requires More and Different Mortar

To accommodate and support this high spot in the center of the tile, mortars also needed to adapt.

What was formally known as a medium bed mortar is now known as a large and heavy tile or LHT mortar. Although these mortars do not yet have a standard established for the product or the installation method, one is on the way and should be in place soon. 

Selection of the appropriate trowel notch size is best determined by checking the mortar manufacturer’s data sheet or on the back of the bag.  Normally, LHT mortars allow for a ½” mortar thickness after the tile has been embedded. 

LHT mortars allow the mortar to be spread with a thicker configuration than conventional thin set mortars.

LHT mortars allow the mortar to be spread with a thicker configuration than conventional thin set mortars as seen in the attached photo. This additional amount of mortar, when troweled in a straight-line pattern, is designed to support the edges of the tile fully as well as the center which many times can be higher due to the tile warpage.

>> See How to Correctly Trowel Mortar When Installing Tile?

Without this needed support, the tile is very susceptible to breakage under a concentrated load point such as a woman’s high heel, which can exert over two thousand pounds of pressure per square inch. Additionally, this lack of mortar or empty space can produce a hollow sound under normal foot traffic which is unpleasant and most times, unacceptable.

>> See Why You Need to Eliminate Spot Bonding When Installing Ceramic Tile

A Skim Coat of Mortar on the Back of the Tile (a.k.a. Back-Buttering) Can Help

Even with these larger amounts of mortar being used, some ceramic tile products may necessitate the application of a skim coat of mortar on the back of the tile, commonly known as back-buttering. This action will ensure a good mechanical bond to the back of the tile and fill the warped area of the tile as needed. 

The TCNA Handbook further states,

“All corners and edges of the stone tiles must be fully supported, and back-parging, or back-buttering, is recommended in all areas.  Coating the back of the tile, however, does not constitute coverage, which is the area where the mortar makes contact with the tile and the substrate.” 

Using one or both of these techniques will normally meet or exceed the tile industry minimum standards of 80% mortar coverage on the back of the tile in dry areas with wet and exterior areas requiring 95% coverage. 

But, in coating the back of the tile and troweling mortar on the substrate, the maximum thickness of mortar permitted by the mortar manufacturer must not be exceeded.

>> See Back Buttering Tile: How Important Is It?

Occasionally Remove a Tile to Confirm Tile Industry Minimum Standards for Mortar Coverage

The best and most effective way to confirm that this minimum requirement has been met is to occasionally remove a tile. A good tile mechanic meeting the requirements of qualified labor, as recognized by the Tile Council of North America, will use this procedure regularly to ensure that the consumer gets what they deserve, a high-quality, long-lasting, and trouble-free installation.

>> Visit The Tile Installation Experience with Scott Carothers at Coverings.

>> Never Assume You Have Sufficient Mortar Coverage. Check It.

What About Lippage Control Devices?

The Lippage Control Devices (LCD) or Lippage Control Systems (LCS) (formerly known as edge leveling systems) are designed to do exactly as they describe; minimize lippage between two tiles. This is accomplished by raising the lower tile just enough to bring the face of a tile into a flat plane with the next tile. This slight upward movement should have no effect on the mortar coverage between the back of the tile and the substrate. Please understand that LCDs were never intended to increase or decrease mortar coverage.

It is crucial that an adequate amount of mortar is placed under the tile. Start with a clean substrate and determine if it is flat enough to receive tile.

  • For tiles less than 15" on any one side, the flatness requirement is 1/4" in 10'.
  • If the tile is 15" or longer on any one side, the requirement is 1/8" in 10'.

Once properly flattened with the proper patch or self-leveling product, key the mortar into the surface with the flat side of the trowel. Then experiment with various notch sizes and designs to determine which one will best achieve the American National Standards Institute (ANSI) requirement of 80% coverage in dry areas and 95% coverage in wet and exterior areas. Also, it is wise to occasionally lift an installed tile to ensure adequate coverage is being achieved. 

As long as there is enough mortar to meet the above ANSI requirements, the minor lifting of the tile by the LCDs should not lessen the mortar coverage. 

Reader Question About Tile Warpage

Gloria Yoder asks,

Is the minimum thickness after embedding still 3/32” to 1/4” for large format tiles that have no/little warpage?

Scott responds,

The minimum mortar thickness, after embedding, for conventional thin set mortars is 3/32” with the maximum thickness being 1/4” (depending on the mortar manufacturer’s recommendations). 

For large and heavy tile mortars, formerly known as medium bed products, the minimum thickness is also 3/32” while the maximum is 1/2” after the tile is embedded. 

How’s Your Mortar Coverage? Will it Accommodate Inherent Tile Warpage?

If you’re working with large format tile, you can’t ignore adequate mortar coverage. Be sure to follow these steps and let us know of any questions.

Thanks for reading.


 Learn More About Becoming a Certified Tile Installer Click Here to Download the CTI Kit. 

Note: this article was originally published on Nov 29, 2016, and has been updated.