Understanding The Tiling Shift

[Dave]

Historical methods of laying floor tiles of thousands of years ago have influenced modern methods. However, there are occasions where the installation goes wrong. Understanding of the causes of floor tiling problems and the implimentation of correct preparation and laying procedures, should result in a satisfactory, problem-free floor.
Understanding The Tiling Shift Today's installation systems allow the incorporation of ceramic and stone tile on virtually any surface. In addition to traditional projects such as bathrooms and kitchens, other tile applications which are becoming increasingly popular include worktops, exterior balconies, and even bedrooms.
However, the development of these contemporary systems highlights a shift in common knowledge about the physical dynamics of the floor assembly regarding the relationship between the covering and the substrate.
Thousands of years ago European builders developed a successful means of installing tiles which virtually eliminated the transfer of stresses within the various layers of the overall assembly, thereby eliminating failures.
These builders understood that a forgiving shear interface between the building structure and the tiled surface was necessary in order to allow the two elements to move independently.
The installation method used by these European builders was a "sandwich" comprising a structural base and a layer of sand.

Historical method of laying tiles using sand as a debonding membrane

Then a mortar bed - a mixture of sand, cement or other binder, and water - was laid, with the tile adhered on top. The critical component in this sandwich was the layer of sand, which uncoupled the tile covering from the structure, allowing the two to move independently. The layer of sand supported the tile covering under heavy loads, while, at the same time providing a shear interface that effectively prevented movement from the structural base being transmitted into the tiled surface.

The use of this "sand strata" method is virtually extinct today, largely due to the introduction of thin-set mortar in the 1960s, which allowed the tile to be bonded directly to the building structure - i.e. to concrete or plywood, etc.
It can be argued that the most monumental change the tiling industry has undergone in the last millennium was a shift away from uncoupling the surface covering from the building structure, to a technique where the tile was bonded directly to the substrate.

A direct bond to the substate

It's based on the theory that the principle element needed in a direct-bond system to achieve a problem-free tile installation, is an extremely strong bond between the tile and the substrate, and the mortar bed was eventually perceived as unnecessary. It quickly became popular because the ease of installation allowed anyone - not just trained installers - to fix tiles.
However, this created a problem in many cases, with tiles eventually cracking, splitting or becoming debonded from the substrate.

A typical debonding problem

Bonding the tiles directly to the building structure creates what is known as a "force-transfer assembly," because the physical dynamics of each layer in a tile assembly are vastly different from each other. The substrate, whether it be timber, concrete, or gypsum, etc., expands and contracts due to changes in temperature and moisture levels. The tiled surface also expands and contracts, but at a different rate.
As the surface is directly -- and firmly -- bonded to the substrate, naturally-occurring stress cracks, and movement in the substrate, both manifest themselves in the tiles as cracks, splits, tenting, or debonding.