MAPEI Explains 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 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 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 Express S1 and Keraflex Maxi S1.

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.

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).

• 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.

• 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 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 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.