These conditions can contribute to hollow-sounding tile either by themselves or in combination with other factors. We will explore each of these potential situations and determine if a failure is around the corner.

Mortar skinning Allowing tile setting mortar to skin over can be a result of several sources. Troweling out more mortar than can be covered in 15 or 20 minutes (depending on the manufacturer’s recommendations) will dry out on the surface – or skin over – which will inhibit proper transfer to the back of the tile.

Bond breaker on tile/wet cuts Any time tile is being installed, it is wise to wipe your hand across the back of the tile to determine if a contaminant is present. This list could include:

• The kiln release dust, when properly applied during production, allows the tile to move freely from the conveyor after traveling through the kiln firing process. If excessive kiln release dust is applied to the back of the tile, the bond will be compromised.
• A chalky residue consisting of water and tile or natural stone dust resulting from cutting on a wet saw or dust from using a dry-cutting method.
• Dust (other than kiln release) from the manufacturing process, transportation, or on-site conditions.
• Water on the back of the tile from cutting on a wet saw can act as a bond-breaker. This situation is sometimes known as the disappearing bond breaker. The water on the back of the tile is absorbed by the mortar, leaving the tile looking as if it was fresh out of the box. This circumstance is like a vehicle tire hydroplaning on a wet highway.

Regardless of the cause, the contaminant must be completely removed from the back of the tile prior to installation.

Bond breaker on substrate Likewise, the substrate, whether concrete or wood, must be clean. For tile installed over a concrete substrate, ANSI A108.01-2.6.2.2 states in part: “Surfaces must be clean, free of wax, curing compound, and other coatings.” ANSI A108.01-3.2.1.2; states: “Cure concrete slabs that are to receive tile before tile application. Do not use liquid curing compounds or other coatings that may prevent bonding of tile setting materials to slabs.” On a plywood substrate, ANSI A108.01-2.6.2.4 states: “Surfaces must be structurally sound, dry, and free of contaminants such as sealers, cleaning compound, coatings, oil, dust, dirt, etc.”

An additional contaminant found on almost every job is paint overspray from priming the walls and ceilings. Initially, the dry-set mortar may bond to the paint, but over time the mortar may lift the paint from the substrate, resulting in a loss of bond.

Wrong trowel Using the wrong trowel that does not provide enough mortar to contact the back of the tile will also yield poor results.

ANSI A108.5-2.2.3 states: “Contact area shall not be less than 80%. Contact area on all exterior installations or interior wet installations, shall not be less 95%. The 80% or 95% coverage shall be sufficiently distributed to give full support of the tile with particular attention to all corners and edges of the tile. To achieve the 80% or 95% coverage required in the project specifications, select a notched trowel sized to facilitate the proper coverage. It may be necessary to flat back trowel individual tile with bond coat to achieve the proper coverage.”

Substrate preparation Substrate preparation is critical to eliminating hollow-sounding tile. If the prep is not completed, creating a substrate that is flat enough to provide an appropriate bond to the tile, the area in question should be corrected by using a flash patch in smaller areas or using a flowable hydraulic cement underlayment (formerly SLU) with the same manufacturer’s primer.

Spot bonding The trick of spot bonding tile continues to be the nemesis of the tile industry. The voids left between the dabs of mortar provide both a failure point and hollow-sounding echo chamber. For more information on spot bonding, please follow these links.