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.

THE CORRECT APPROACH TO THE DOUBLE-BUTTERING TECHNIQUE

In order for the double-buttering method to be more effective it must be carried out correctly. Various test campaigns have been carried out in order to define the most effective method and to get the most benefit possible.

It was found that it is better to apply the adhesive parallel to the short side of the tile (for tiles that are not square) and in the same direction on the substrate and on the rear side of the tile (do not apply the adhesive in a criss-cross pattern between the substrate and the tile; air bubbles could become trapped in the criss-crossed areas).

The reason why it is better to apply the adhesive parallel to the short side is that it helps expel any air when tapping the tiles into position by creating a shorter escape route. It was also found that the latest generation of notched trowels (with diagonal notches) are better at distributing the adhesive and can spread more adhesive than traditional trowels with square, triangular or semi-circular notches, particularly when applying adhesive on substrates. The shape of the trowel to use depends on the factors mentioned previously (the flatness and characteristics of the substrate, the size of the tiles and the thickness required for the layer of adhesive).

For example, from a series of tests carried out in Mapei Research & Development laboratories, it was found that, to install tiles with a longest side of more than one metre, the best results (on a flat substrate) were obtained by using a trowel with 10 mm square notches for the substrate and a trowel with 3 mm square notches for the rear side of the tiles. The level of buttering was obviously verified after tapping the tiles into position with hand tools or power tools (such as a vibrating plate).

The concepts discussed are contained in national standards in many countries, such as the standards DIN 18157 Part 1 or DIN 18157 Part 3 in Germany.  

Things to know

• It is not always necessary to install tiles using the double-buttering method. In many cases, applying adhesive to the substrate only is enough to obtain sufficient covering of the rear side of the tiles, particularly when using adhesives with high wetting capacity (e.g. for small or medium size tiles, to install tiles on internal surfaces subjected to light traffic or to install tiles on internal vertical surfaces).
• Irrespective of any other factors that come into play, installation of large size tile is more effective when the double-buttering method is employed.
• It has been noticed that, if the adhesive is applied in a criss-cross pattern, air pockets are formed in the adhesive; this application method, therefore, should be avoided.

• Using the correct size trowel is very important in order to create a full bed of adhesive. A smaller trowel should be used for the rear side of the tiles, also to reduce their weight and make handling easier.

• Using a trowel smaller than required could leave areas without a proper amount of adhesive.
• Adhesives with improved rheology make it easier to create a full installation bed. Thanks to their particular consistency they have excellent rib stability and slip-resistance, they can be applied in thicker layers (up to 15 mm, depending on the type of adhesive), they have better wetting capacity and they have better performance properties and deformability (class S1 or S2 according to AS ISO 13007.1-2013).  
• Take the open-time of the adhesive into consideration in order to avoid “skin” forming before the tiles are installed, which would then have a negative effect on its final bonding strength (particularly in the case of large size tiles in hot and/or dry weather). It is better to use adhesives with extended open time (class E according to AS ISO 13007.1-2013), particularly in hot weather. During cold weather, on the other hand, and when installing tiles on non-absorbent substrates, it is better to use fast-setting adhesives (class F), otherwise it could take too long for the adhesive to dry out and set completely when installing large size tiles on non-absorbent substrates.