Stop your installer from proceeding if you  suspect spot bonding. Spot bonding is easy to see during installation, but after the install is complete this method is harder to determine without removing some of the tiles. Evidence can sometimes be gleamed by looking at exposed edges of the installation or by determining that the height of the tile surface is excessive in depth or height in relation to the substrate. A thermal imaging camera can also bring a questionable installation into the light.

If you are an installer that uses spot bonding as an installation method, potential legal action could be taken against you. There is no benefit to this method and there can be serious consequences for those that do.

 This article will touch on;

  • bond strength
  • mortar coverage
  • thermal stress
  • impact resistance
  • moisture retention
  • safety

We cannot be sure when
spot bonding came into use but through my experience I have removed installations that were done in the late nineties and discovered the method during demolition. That was around the time tile exceeding 16 inches was becoming the trend.

Thin-set mortar, the adhesive used to adhere most tile, needs to be defined. A Thin-set installation is a method, not a material. When a bag of mortar reads thin-set mortar, the bag is indicating that this mortar may be installed using the thin-set method, i.e. mortar that is applied in a thin layer to set tile.

Every type of mortar has limitations in regards to how and where it is used.

When thin-set mortar exceeds the recommended thickness, it will not achieve a bond strength anywhere near as intended. It may also attribute to an unevenness in the finished tile work.  A good bond begins with a proper mix, applied to the appropriate substrate with the tile firmly embedded. The amount of mortar coverage on the back of the tile must meet the requirement for the given area to receive tile.

Imagine a spot of thin-set mortar on the back of a tile.
This spot might resemble the size of a large golf ball or more. Let’s say that the spot, when compressed behind the tile, forms the shape of a hamburger patty. As the mortar cures, it will begin to dry starting from the outside with the center of this mortar spot drying much slower. When a cementitious material dries it is actually growing microscopic roots. These roots are attempting to penetrate and grab the substrate as well as the tile. When cured, you will have a root system that is very hard to break, however consider what happens to mortar when it is applied too thick.

What is tile spot bonding?

 Written by Michael Weaver

Typical modified and unmodified mortars can be applied with a ½ inch trowel and when compressed during installation will be approximately 3/16ths to a ¼ inch thick. The mortar should be uniform or consistent in thickness when the tile is embedded. Mortars designated as LHT (large heavy tile) mortar can be applied slightly thicker and this thickness is specific to each manufacturers requirements. Consistency during application is very important. Thin-set mortar regardless as to whether it is LHT or not, must not be used to true or level an irregular substrate. The best method for truing a wall should be done prior to the installation of the wallboard. If this is not possible or practical, proper patching compounds should be used to correct deviations or imperfections in the substrate prior to the application of thin-set mortar.

A condition that often effects a spot bonding install is when a shower expands and contracts daily. Thermal cycling that a shower goes through can easily de-bond tile. Tile installations should always be regarded as dynamic surfaces. Tile surfaces move either independently or in harmony with dissimilar materials, but they always move.

Thermal cycling also happens when an interior area receives direct sunlight through a window or glass sliding door. In Florida, due to spot bonding failures, we have many instances of tiles de-bonding from exterior surfaces when a cold snap occurs. A cold 45 degree night followed by direct sunlight are a recipe for disaster, compounded if proper expansion joints were not used. They usually go hand in hand; an installer that installs with spots, will neglect joints to accommodate movement.

 A situation can immediately effect the integrity of a spot bonding install is when the shower doors are being installed. Impact and vibration can de-bond or further weaken a poor installation.

Bond strength as we can see is weakened due to mortar applied in the spot method but the installation is also weak because of a lack of mortar coverage. There is simply not enough mortar in contact with the back of the tile to achieve an appropriate bond or support even the mildest of foot traffic.

Interior spaces require 80% coverage on the back of the tile. Exterior, as well as any area receiving moisture like showers or steam rooms require 95% coverage. Maximum coverage is important in terms of bond strength but especially in impact resistance. Most tile can easily be broken or cracked prior to installation. Manufacturers are always looking for better ways to package tile as it is easy to damage during shipping. However once tile is installed properly, it can withstand many forms of abuse. The mortar bedding is what reinforces the tiles durability. Without proper coverage tile cannot be expected to deal with impact or point loading.

Another problem spot bonding may introduce is moisture buildup. Between each spot, where mortar is lacking, a cavity is formed. These cavities will retain moisture either from liquid build up or water vapor condensing. A shower receptor is an area that is meant to dry out between uses. A lack of drying out can weaken assemblies and discolor grout and stone permanently.

Besides the aesthetic and functional shortfalls, safety is a serious concern. I can tell you that as a professional, removing spot bond installations, even when you’re ready for them to fall are still dangerous. Imagine a tile falling on someone unaware of the danger.  

Is there any spot bonding method that is acceptable? Yes, TCNA method W215 and W260. This method can only be done with epoxy adhesive and is only to be used on dry interior wall applications. It has no impact resistance and the walls must be masonry or concrete for method W215 and cement backer board on wood or metal framing for method W260. The Epoxy must be approved by the manufacturer for spot bonding. This method is specified by an architect or design professional and the epoxy cannot be substituted with a cementitious mortar.

If you suspect your current installation is being spot bonded or you are concerned about the safety regarding an existing installation, please send us an email. We can have an installer evaluate the installation and compile a report on the methods that were used and what the accepted methods from the TCNA and ANSI are for the area in question. Spot bonding has damaged consumer faith in the tile industry. We hope to restore that faith one project at a time.

Tile spot bonding is a failure waiting to happen. This unapproved method uses spots of thin-set mortar on the back of the tile in an attempt to correct wall or floor irregularities and adhere the tile to the substrate. This method has a high failure rate and poses a very serious safety risk.

Mortar moves while it’s curing. You can’t see it happening, but mortar shrinks and even deforms as it loses moisture during the curing process. Especially when it contains too much water or exceeds its recommended maximum thickness. As the outer portion of the spot dries the initial set has begun. However the interior of the spot is still early on in the curing process. As the center begins to dry, the mortar shrinks and curls. Now the bond that was initial achieved on the outer most part of the spot is compromised. As the mortar curls and shrinks, it destroys the initial root system that it created early on in the process. The tile is barely hanging on, waiting for the right conditions or situation to completely de-bond the tile from the substrate.

 Often the thin-set mortar used incorrectly is a modified mortar.  This is because when spot bonding is used, it’s usually used on a large format tile and the majority of large format tile is porcelain. Porcelain tile requires modified mortars. Modified mortars require air to dry, and cure at a much slower rate compared to un-modified mortars. Since the curing takes longer, the effect of the mortar warpage destroying the earlier established roots is extended, and further contributes to a weak bond. This is contrary to the fact that a slow cure increases bond strength when a cementitious material is curing. Slow cure= strong bond only applies if every stage from mortar mixing to application is done as the manufacturer intended.


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