The correct distribution of adhesive and the double-buttering technique

Tiles are getting bigger and installing them is becoming increasingly complex.

In a sector that is evolving according to the latest trends choosing the right adhesive is fundamental in achieving the quality finish you desire.

There are a number of factors that contribute to a successful finish, including, how the adhesive is distributed, the double-buttering method, choosing the correct notched trowel, employing products that are non-hazardous for tile layers, end users and the environment, not to mention compliance with international standards.

The correct distribution of adhesive has become a highly debated issue for large ceramic tiles (up to 320x160cm). Let’s try to shed some light on the concept.

THE RELATIONSHIP BETWEEN WETTING AND PERFORMANCE PROPERTIES

When installing ceramic tiles with adhesive like Mapei's Ultralite S1 or Ultralite S2 (classified according to AS ISO 13007.1-2013), one of the main objectives to make sure the adhesive covers as large an area of the tiles as possible, to achieve the best performance. The higher the amount of surface covered by the adhesive (that is, the contact area between the adhesive and the rear side of the tiles), the higher the performance characteristics and properties of the finished covering. When referencing tile covering, we mean the system comprising of ceramic tiles installed on a suitable substrate with their relative adhesive or mortar and grout for the joints.

LARGE FORMATS AND CORRECT WETTING PRACTICE

As tile sizes have increased, so has the difficulty in fulfilling this requirement, particularly with tiles that have an increasingly large surface area which, at times, is not perfectly flat. If we consider that adhesion (bonding strength) is one of the most important tests adhesives have to undergo when they are going through the classification process to comply with international standards (which is measured through direct tensile pull-off tests according to the method described in AS ISO 13007.2-2013), it becomes clear that, in order to exploit the performance properties of the adhesive declared by the manufacturer and tested according to the relative standards, it is important that as much of the surface as possible of the tile is covered by the adhesive (ideally 100%). Use products such as Keraflex Maxi S1, Ultralite S1 and Ultralite S2 adhesives.

The final performance of the tiled surface, therefore, is directly proportional to the area of contact surface between the adhesive and the tiles.

THE IMPORTANCE OF BUTTERING TILES

Correct application of adhesive has long lasting impact. Ensuring there are no air pockets in the layer of adhesive can prevent tiles breaking if they are subjected to concentrated loads, especially in the case of thin tiles that have a lower level of flexural strength compared with traditional thicker tiles. A full layer of adhesive (which means a layer with no air pockets) guarantees that loads are distributed evenly throughout the support layers which, if compliant with specifications, have the capacity to withstand such loads.

Buttering tiles correctly is also significant when they are installed on external surfaces or on surfaces constantly in contact with water. The buttering of the tiles with the adhesive is inversely proportional to the amount of air pockets; the fewer air pockets in the surface, the lower the amount of space where standing water could collect and lead to problems/risks connected with freeze-thaw cycles, in the case of external tiling. Leaching and aggression to the layer of adhesive is also problematic if we consider that water is also a vehicle for transporting substances that could potentially damage the adhesive used to install the tiles.

The higher the percentage of the rear side of the tile/slab covered by the adhesive, the better the end result. A full installation bed (100% buttering), is a very important aspect when installing large-size tiles measuring more than 60 cm, thin tiles, installation of tiles on external surfaces (including on facades), or on surfaces in constant contact with water.

ACHIEVING A FULL INSTALLATION BED – THE DOUBLE-BUTTERING TECHNIQUE

In order to achieve a full installation bed, the double-buttering method is recommended to install tiles by applying the adhesive on both the substrate and on the rear side of the tiles with a suitable notched trowel.

A “full” installation bed can also be achieved, or made easier, by using adhesives with high wettability: that are specially formulated to increase their buttering capacity and, as a result, increase the amount of contact area, under the same conditions, compared with traditional adhesives. This result is obtained by using adhesives with modified rheology like Keraflex Easy S1 . These adhesives are distributed over the rear side of the tiles more easily during application and then when the tiles are tapped into position after being installed, without affecting either their rib stability or the thickness of the layer of adhesive, including when installing heavy tiles.

Key considerations to take into account when using the double-buttering method or, more in general, when the objective is to create a “full” installation bed, are the following:

- the type and size of the tiles (length of the sides, thickness, the profile of the rear side): a full bed needs to be prescribed for large size tiles, tiles with uneven rear side and for thin tiles;

- characteristics of the installation surface (a full bed should be prescribed, for example, when installing tiles on facades);

- high mechanical loads and stresses acting on the tiles (areas subjected to heavy and/or intense traffic);

- high thermal-hygrometric loads and stresses acting on the tiles;

- particular durability and safety requirements;

- heated substrates: heat is transmitted more efficiently through a full bed.