Tile Mortar Options That Minimize Hollow Spots

In commercial construction—whether it’s hospitality, healthcare, or retail—tile installations are expected to last decades. But one of the most persistent and costly failures in tile flooring and wall systems is hollow spots. These voids under the tile not only cause unsightly sounds when walked on but also lead to cracking, delamination, and expensive repairs.

For procurement heads, contractors, and facility managers, selecting the right tile mortar isn’t just about price—it’s about ensuring long-term integrity, load-bearing strength, and warranty compliance. The right mortar dramatically reduces the risk of hollow spots, preventing callbacks and protecting investments.

What Causes Hollow Spots in Tile Installations?

Hollow spots occur when mortar doesn’t fully bond with the back of the tile or the substrate, creating air pockets. Common causes include:

Insufficient mortar coverage

Poor substrate preparation (dust, contaminants, unevenness)

Mortar shrinkage during curing

Incompatible mortar for large-format or heavy tiles

Large-format tiles—those exceeding 15 inches on any side—are particularly vulnerable because even minor voids can compromise the tile’s structural support.

Mortar Types That Solve Hollow Spot Problems

Medium-Bed Mortars (Now Called Large and Heavy Tile Mortars)

Specifically designed for large-format tiles, these mortars are formulated to support tiles without sagging on walls or sinking on floors. Their thicker build prevents voids even when substrates aren’t perfectly flat.

High-Performance Modified Thinset Mortars

With higher polymer content, these mortars offer improved adhesion, flexibility, and bonding strength. They flow better, reducing the likelihood of air pockets and hollow spots.

Non-Sag Mortars (For Vertical Installations)

Essential for wall tiles, especially large panels. These mortars hold tiles in place immediately, preventing slippage and ensuring consistent coverage behind the tile.

Rapid-Set Mortars

Ideal for projects requiring fast turnaround, such as retail store renovations or healthcare facility repairs. Rapid set minimizes shrinkage-related hollow spots by curing faster.

Mortar Coverage: The Key to Success

Industry standards recommend 80% mortar coverage for interior walls and 95% for floors and exterior installations. Hollow spots usually occur when installers fail to achieve this level. Procurement teams can help prevent failures by specifying mortars that enhance workability, such as those with extended open times or self-leveling properties.

Trowel Size and Technique Matter

Even the best mortar fails if applied incorrectly. Large-format tiles require larger notched trowels—typically 1/2-inch or 3/4-inch U- or square-notched—to achieve the required coverage. Back-buttering (applying mortar to the back of the tile as well as the substrate) further ensures a full bond with no voids.

Substrate Preparation is Non-Negotiable

Uneven substrates create valleys that mortars can’t always fill. Leveling compounds should be used before tiling to provide a flat, stable surface. Moisture barriers or crack isolation membranes may also be necessary, depending on the substrate condition and project requirements.

Special Considerations for Heavy Traffic Areas

In commercial kitchens, airports, hospitals, and malls, tiles are subjected to extreme loads. Mortar systems designed for high-compression strength are critical to prevent deflection and subsequent hollow spots. For these environments, procurement should prioritize mortars rated for heavy commercial use, not just residential-grade products.

Moisture and Thermal Movement: Addressing Long-Term Risk

Hollow spots can also form years after installation due to moisture infiltration or thermal expansion and contraction. Choosing mortars with high flexibility (measured by ANSI A118.15 standards) mitigates this risk, particularly in environments exposed to temperature fluctuations, heavy cleaning processes, or moisture migration.

Sustainability and Low-VOC Mortars

Many commercial projects now require mortars that meet sustainability standards, such as LEED or WELL certification. Modern formulations offer low-VOC, environmentally friendly options without sacrificing performance. These choices are particularly relevant in schools, hospitals, and government buildings.

Conclusion: The Procurement Payoff in Choosing the Right Mortar

Minimizing hollow spots isn’t just an installer’s responsibility—it starts with procurement. Selecting the right tile mortar ensures the longevity, safety, and aesthetic integrity of any tiled surface. Medium-bed mortars, high-performance modified thinsets, and rapid-set formulations are investments that pay for themselves through reduced labor callbacks, extended tile lifespan, and client satisfaction.

For commercial projects where durability and performance are non-negotiable, the right mortar is not just a product—it’s a safeguard against failure.