Tile adhesive – which one to choose and how to prepare the substrate?

Choosing the right ceramic tiles is usually the most enjoyable part of a renovation, but the ultimate visual and technical success depends on the materials that are hidden beneath the surface of the tiles and not visible on a day-to-day basis. Proper preparation of the substrate and an informed decision on which tile adhesive to use are extremely important factors in ensuring the durability of surfaces in bathrooms, kitchens or on terraces for many years to come. Many homeowners make the mistake of focusing solely on the aesthetics of porcelain stoneware, whilst it is actually the physicochemical properties of the adhesive mortar that are responsible for stress transfer and moisture resistance. It is important to understand that modern ceramic tiles, particularly large-format ones, place entirely different demands on the installer than the materials used even a decade ago – the incorrect choice of construction chemicals can lead to cracking of the grout lines, „hollow” sounds when walking, and in extreme cases, to the cladding becoming completely detached from the wall. In this guide, we’ll analyse in detail which tile adhesive is best suited to your project and how to prepare the substrate for its application. We’ll also answer the most common questions about product performance and installation techniques on challenging substrates, such as drywall systems. Let’s get started!

Classification of adhesive mortars – what do the symbols on the packaging mean?

Before you decide which is the best tile adhesive for your flat, you need to make sense of the technical markings on the packaging – and these may seem incomprehensible to the layperson. The basic classification divides adhesives into the following classes:

• C1 (normally binding),
• C2 (with enhanced grip), 

which is an absolute must when choosing a product. The letter S (standing for „Special” or ‘S-class’) is also important, as it indicates how much the dried adhesive can ‘bend’ before it cracks. In today’s construction industry, where deformable substrates are the norm, tile adhesive marked with the symbol S1 is becoming the standard, as it has the ability to compensate for stresses caused by temperature changes or building settlement. Additional letters, such as T (reduced flow), allow tiles to be laid safely from the top of the wall without the risk of them slipping, whilst E (extended open time) gives the tiler more time to precisely adjust the position of the tile. 

Understanding these parameters enables you to make an informed choice of product for a specific task, rather than relying on chance or the lowest price. Investing in the C2TE S1 class guarantees that the adhesive will hold heavy tiles in place even under the most demanding operating conditions.

Preparing the substrate. When is a tile levelling compound needed?

The biggest installation error, which affects the durability of the entire cladding, is attempting to compensate for significant wall curvature using adhesive mortar alone. Thin-bed adhesives, even those of the highest quality, shrink as they set, which, if the layer is too thick (over 10–15 mm), can lead to the formation of voids beneath the tiles and cause them to crack. Therefore, a professional preparation process should always include levelling the surface, for which a specialised tile filler or a specialised levelling mortar. This allows you to achieve perfectly vertical lines and angles even before you start tiling, which significantly reduces the amount of more expensive adhesive used and improves the appearance of the grout lines. A properly prepared substrate should be clean, load-bearing and, above all, primed, which will reduce its absorbency and increase the efficiency of the tile adhesive. Ignoring the skimming stage is a sure-fire way to run into problems with cutting tiles to size in corners and risks of the cladding lifting in places where the adhesive layer was too thick!

What type of tile adhesive should I use for bathroom tiles and plasterboard?

The bathroom is a unique environment in which building materials must cope with the constant presence of moisture and sudden changes in water temperature. You should therefore look for products with low water absorption and high elasticity, particularly if you are planning to install a walk-in shower enclosure with a linear drain. In such areas, the adhesive must work in close conjunction with the waterproofing membrane (known as ‘liquid membrane’) to form a watertight barrier that protects the building’s structure from flooding. 

Another challenge is fitting ceramic tiles to drywall systems – which tile adhesive is suitable for use on plasterboard? Plasterboard is a „flexible” substrate, which means that it may undergo slight deformation under the weight of the tiles or due to changes in humidity. In this case, the only safe choice is a high-quality flexible tile adhesive, which will allow you to work without damaging the grout between the tiles.

Why is the flexibility of the adhesive so important?

Modern adhesive mortars are advanced mineral mixtures which, in addition to cement and aggregates, contain a range of polymer additives; it is these that are responsible for their elasticity, adhesion and setting time. In areas such as terraces, balconies or rooms with underfloor heating, the substrate is subject to constant thermal stress – it expands and contracts. If a rigid adhesive is used, these movements will cause stresses to be transferred to the tile, resulting in it cracking or becoming detached from the substrate along with the mortar layer. Therefore, in such conditions, a flexible S1-grade tile adhesive is an absolute must, as it is capable of accommodating these micro-movements. It is also worth paying attention to the colour of the adhesive: for light-coloured tiles, marble or glass mosaics, white adhesives must be used to avoid discolouration, which may be visible through the semi-transparent material.

How do you lay tiles without wasting adhesive?

The question of how much tile adhesive is required comes up whenever a renovation is being costed, and depends on several factors: the size of the trowel teeth, the level of evenness of the substrate, and the application technique. On average, consumption is estimated at 1.5–3.0 kg of dry mix per 1 m² for a 1 mm-thick layer; however, actual figures are usually higher due to the need to use the combined method. This involves applying a thin layer of adhesive to both the wall and the underside of the tile (known as ‘buttering’), which ensures the space beneath the tile is fully filled and eliminates air bubbles. Knowing how to lay tiles using this method is particularly important with large-format tiles and technical porcelain tiles – here, any empty space beneath the tile risks damaging it when subjected to localised pressure. The correct working technique, combined with a perfectly prepared substrate, will allow the mortar’s properties to be utilised to the full and, ultimately, reduce material costs and speed up the work.

In summary, the answer to the question of which good tile adhesive to choose depends on a detailed analysis of the conditions in the room and the type of tiling itself. Investing in a modern, flexible C2 S1-grade tile adhesive and carefully preparing the substrate is the only way to avoid costly complaints. Remember that even the most expensive tiles from the latest collection will not be immune to cracking if they are laid on an unstable, uneven or improperly primed substrate. By choosing Knauf construction chemicals available at the PK Complex shop, you’re opting for tried-and-tested system solutions that have been trusted by professionals for years. We encourage you to explore our range of tile adhesives and levelling compounds, which will ensure your new floor or wall remains a pleasure to look at for many years to come, without any technical surprises.