Our friend Scott Carothers at Floor Trends & Installation wrote this important article to save installers from the mistakes that can lead to thin-set mortar failure.

One of the most common mistakes we see in the field is improper thin-set application, leading to poor tile adhesion and, ultimately, failed installations. Let’s clear up a common misconception: thin-set mortar is not one-sided. It must bond both to the substrate and to the back of the tile for a successful installation.

Understanding Thin-Set Mortar’s Role

Dry-set (thin-set) mortars are engineered to create a strong bond between the substrate and the tile. Manufacturers have fine-tuned these mortars for ease of mixing, workability, and strong adhesion. However, they do not make a mortar that adheres to only one surface. If the thin-set bonds well to the substrate but not to the tile, the issue often lies in site conditions or improper installation techniques.

What Went Wrong? A Case Study

Recently, we reviewed a case where a homeowner reported loose and breaking tiles in a four-year-old installation of large-format plank tile on a concrete slab. The direct bond method followed Tile Council of North America (TCNA) Handbook method F113, yet the tiles sounded hollow, grout joints were cracked and crumbling, and multiple tiles had fractured.

Upon removal of the failed tiles, we discovered that the thin-set mortar was well bonded to the concrete substrate, but not transferred to the back of the tiles. Why did this happen? There are three primary reasons this failure may have occurred:

1. Substrate Condition and Premature Drying

If the concrete slab was excessively dry and porous when the thin-set was applied, it may have absorbed moisture from the mortar too quickly, causing premature drying of the surface layer. When this happens, the top of the thin-set mortar can “skin over” before the tile is set, preventing a proper bond. The solution? Dampen the concrete substrate before applying thin-set to slow moisture absorption.

2. Environmental Factors and Air Movement

If the site was not properly climate-controlled—meaning windows and doors were open, HVAC systems were blowing air across the floor, or there were strong drafts—moisture could have evaporated too quickly from the thin-set. This can also contribute to skinning over, resulting in poor adhesion. Proper site preparation, including temperature control and draft prevention, is crucial for a lasting installation.

3. Extended Open Time Before Setting Tile

Thin-set mortar has a working window, typically 15 to 20 minutes, before it begins to lose its effectiveness. If an installer spreads mortar over too large an area without setting tiles promptly, the mortar can dry out before proper bonding occurs. The ANSI A108.5-2.2.2 standard is clear: “Tile shall not be applied to skinned-over mortar.” Always check the thin-set before placing tile. A simple test is to touch the mortar—if it transfers to your finger, it’s ready for tile. If not, it has skinned over and must be re-troweled or replaced with fresh mortar.

The Cost of a Failed Installation

In this case, the unfortunate reality is that the entire tile installation failed. The only solution is to remove the tile and mortar, properly prep the substrate, and reinstall with correct thin-set application methods. Poor bonding not only leads to costly repairs but also damages the reputation of the tile industry.

Tile America’s Best Practices for a Successful Installation

• Always prep the substrate—clean, level, and dampen if needed.
• Control the environment—avoid excessive airflow and maintain stable temperatures.
• Work efficiently—spread only as much thin-set as you can tile within 15-20 minutes.
• Check for transfer—ensure mortar is properly bonding to the tile’s back.
• Follow industry standards—stick to ANSI and TCNA guidelines for best results.

By following these steps, you can ensure that your tile installation stands the test of time. At Tile America, we are committed to helping our customers achieve beautiful, long-lasting tile installations. If you have any questions or need expert guidance, visit one of our showrooms and speak with our knowledgeable consultants. Because at Tile America, where inspiration surfaces, quality installation follows.