Tile Talk May 2019 | Spot-bonding: A Growing Problem

In my previous career as a tile contractor and quality control consultant, I’ve managed well over 1,500 projects in the DC/MD/VA area.  Spanning over a decade – working closely with labor, design and building professionals, I’d like to think I’ve seen just about everything in this industry.  The good and the bad. 

In the world of construction, evolution is a constant.  New products and improved technology help to promote increased efficiency and predictability of the finished system.  From early design to final closeout, every step of the process is measured and executed.  The construction process requires this level of detail because as with anything else, the process is only as strong as its weakest link.

We all know this, and obviously it isn’t anything new.  So, why am I standing on a soapbox and voicing this seemingly ubiquitous statement?  Well, to “know” something and to “do” something are completely different actions. With that being said, let’s bring this discussion to the world of tile.

The Evolution

Tile that we see in this day and age is not the same tile from generations past.  Purely from an aesthetic standpoint, today’s tile has a lot more range and versatility.  We see tile offered in a limitless assortment of geometric shapes, color, and design possibilities.  At the same time, we have seen tile perform like we’ve never seen.  Technology has allowed for tile to become denser, stronger, and less absorbent.  Nearly impervious to moisture and water, many tiles can withstand the annual freeze/thaw cycles. Tile has become the “go-to” hard finish for interior and exterior applications.  Which brings me to the topic of size.

Recent advancements in production and technology has allowed for tile to become larger and larger.  In my early years, 12” x 12” sizes were considered “large format”.  Yes, my crews would complain about wanting to install tiles they were comfortable installing – 8”x8″s!  But, as with all things, they adapted and accepted the fact that tiles were not getting any smaller.  Nowadays, 24”x24″s and 8”x48”s are the industry norm, and 12”x24”s are considered a commodity.

No Shortcuts 

So, what is the point of all this?  As I mentioned earlier, the system is only as strong as its weakest link.  Specific to tile, all the components that make up a finished tile system must be appropriate and installed in the correct manner – per industry standards and methods.  Any shortcut or misstep in this process will manifest into a faulty product.  As tile manufacturing has evolved, so has the means and methods of installation. 

In my experience, one of the main drivers that can make or break this system is the application of the mortar.  For the purposes of this conversation, lets stick to thinset mortar. 

The Problem

As tiles have become bigger and bigger, large segments of the labor force have struggled to keep up with the technical installation skills that are required.  Factor in the new generation of “user-friendly” thinset products that are widely available, and the chances of encountering improper installation methods skyrockets, either knowingly or unknowingly.  One of the most common issues that I have seen in improperly installing large format tiles is by way of spot-bonding

What is “Spot-bonding?”

Spot-bonding or “five spotting” is not a recognized method of installing any tile (no matter the size).  This method has become so prevalent, it’s considered one of the leading factors when large tiles crack, break, and de-bond.

Spot-bonding is placing globs or daubs of mortar on the back of the tile.  Typically, at the corners and the center of the tile – five spotting.  This process is a ticking time bomb for failure and no longer becomes a question of “if” it will fail, but rather “when” it will fail.  This method does not fully support or bond the mortar to the substrate.  Lastly, spot-bonding does not follow any of the established and proven methods of installation, nor is it acknowledged by the Tile Council of North America.

Mortar coverage in dry areas require a minimum 80%. These voids lead to structural damage and hollow sounds. This tile was damaged when an elementary student accidentally bumped their lunchbox on it.
A minimum 95% mortar coverage is required in this shower stall. Voids like this trap water and can lead to mold/mildew. In addition, the mechanical bond is weakened which lead to cracks in the tile.

Why do some installers choose a “spot-bonding” technique?

It’s way easier and faster.  Place daubs of mortar on the backside of the tile and push into position until it’s flush with the adjacent tiles.  This shortcut allows for the contractor to eliminate the substrate prep in most cases.  In addition, this method makes minimizing tile lippage a breeze.

Major Problems

  • Does not meet the mortar coverage that is required per Tile Council of North America and ANSI standards. If the spot-bonded tile receives 30% mortar coverage, that’s 70% of unsupported weight and will ultimately lead to damaged tiles.
  • Excessive voids under the tile reduces the overall modulus of rupture (strength of tile installed) thus lowering the overall breaking strength. Any concentrated impact loads will surely compromise the structural integrity.  Thinking about setting a 500lb refrigerator on this tile? 
  • Hollow tiles – reduced load resistance
  • Some lighter color tiles will discolor, and spot marks will show

The Correct Way

According to the ANSI Specifications, Section A108.5-2.2.2 describes the approved method for installing floor and wall tile as follows:

“Apply mortar with flat side of trowel over an area no greater than can be covered with tile before the mortar skins over. Using a notched trowel of type recommended by mortar manufacturer, comb mortar to obtain even setting bed without scraping backing material. Cover surface uniformly with no bare spots and with sufficient mortar to insure a minimum mortar thickness of 3/32” (2 mm) between tile and backing after tile has been beaten into place. Tile shall not be applied to skinned-over mortar.”

Using the above ANSI method, the mortar is keyed into the entire substrate resulting in a good mechanical bond with minimum voids.

Summary

The methods in which tile is installed will continue to evolve and change.  Newer technology in thinset, grout, and membranes have continued to make the installation process easier and better.  However, spot-bonding, in any form, is not a recognized method of tile installation.  Always follow the recommendations of ANSI A108 for all tile installation methods.

  • Properly clean the substrate and dampen if needed 
  • Properly key in your mortar to the substrate with the flat side of the trowel 
  • Comb the mortar in one direction – play it straight!
  • Place the tile into the mortar moving it in a back and forth motion, perpendicular to the trowel ridges.  This motion allows for the mortar to fully collapse on itself, eliminating voids, and transferring the right amount of mortar to the backside of the tile.

Lastly, put into your tile specification that spot-bonding is not an acceptable method on your next project.

As always, send any questions or comments to our QA&T Team – we are here to help.