The Correct Distribution of Adhesive and The Double-Buttering Technique

In the era of large-size ceramic tiles (up to 320x160 cm), the correct distribution of adhesive has become a highly debated  issue. This concept was introduced  a number of years ago and is now mentioned  regularly  in  technical  specifications and guidelines from leading manufacturers of ceramic tiles and products used for their installation. While experts consider it a very  important  issue, operators  from  the sector have always been reluctant to fully accept it. Let’s try to shed some light on what we are talking about.

When installing ceramic tiles with adhesive  (classified  according to EN 12004 and  with  CE  marking), one of the main objectives  to  be  achieved is to make sure the adhesive covers as large an area of the tiles as possible, so that their performance characteristics are exploited as much as possible. In simple terms, it seems obvious to everybody that, 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.  Let’s  remember  that, with the term tile covering, we mean a system comprising ceramic tiles installed on a suitable substrate with their relative adhesive or mortar and grout for the joints – a definition that appears in Italian standard uNI 11493-1.

What has made this issue even more important as tile sizes have increased, has been the increasing 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 EN 1348), 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%).

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

What are the other main reasons that make the amount of contact surface between the adhesive and the tiles so important? First and foremost, having 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 (N.b. according to aforementioned uNI 11493-1 standards, thin tiles/slabs are those with a maximum thickness of 5 mm). 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 the specifications of the relative standards, 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 (if we consider that water is also a vehicle for transporting substances that  could potentially damage the adhesive used to install the tiles).

For all these reasons, it is clear that, 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), on the other hand, is a very important aspect when installing large-size tiles (according to the above-mentioned standard, large size tiles are those with one side measuring more than 60 cm) and/or thin tiles, and when installing tiles on external surfaces (including on facades) or on surfaces in constant contact with water.

Double-Buttering

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

The double-buttering method is recommended when the compactness of the layer of adhesive and the absence of air pockets under the tiles – that is, to have a “full” installation bed – are essential requirements, which is generally difficult to achieve using the conventional single buttering method, that is, by applying the adhesive only on the substrate. It is important to point out that the creation of a “full” installation bed can also be achieved, or made easier, by prescribing the use of adhesives with high wettability: adhesives 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 developing adhesives with modified rheology, that is, adhesives that 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.

The key aspects to take into consideration when prescribing the double-buttering method or, more in general, when the objective is to create a “full” installation bed, are the following:

â–  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;

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

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

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

â–  particular durability and safety requirements;

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

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.

Enrico Geronimi. Mapei SpA Techincal Services