CPD: Specifying tiling adhesives

There is a vast choice in terms of size and type of tile available on the market today. Large formats have become increasingly popular with panels over 3m available. In addition, the choice of substrate for tiled installation has grown and a number of proprietary materials and systems exist, complicating the choice of adhesive to enable it to bond correctly with both substrate and tile.

This CPD aims to provide guidance to the main contractor on how to choose and apply tile adhesive to ensure that your sub-contractors’ tile installations will be durable and problem-free. We concentrate on how to deal with large sizes, different substrates,
tile types and environments, explaining which adhesives to specify to avoid issues.

BS 5385 is the British Standard covering the design and installation of ceramic, mosaic, natural stone and terrazzo tiles. It previously had 5 parts, but Part 5 was amalgamated into Part 3 in 2024, leaving four parts covering internal, external and specific conditions for both floor and wall applications. 

Understanding the main types and sub-classifications of tile adhesive will enable you to specify the right one for the tile, substrate and service conditions. This is especially important for areas exposed to movement, temperature change or moisture.

Cementitious, ‘C’, are the most commonly used tile adhesives, suitable for most substrates and tile types. They are available in grey and white (preferred for light-coloured or translucent tiles). 

They subdivide into two main classes: C1 adhesives have a tensile adhesion strength ≤ 0.5 N/mm² and ≤ 1 N/mm²; C2 adhesives perform better with a tensile strength of  >1 N/mm².

Where present, additional performance characteristics are indicated by letter codes:

To give some real-world examples from Kerakoll’s range, H40 Gel is classified as C2 TE S1. This means that it is an advanced cementitious adhesive with reduced slip (‘T’), an extended open time (‘E’), and deformable (‘S1’). If we look at actual performance levels, its tensile adhesion after 28 days is ≥ 2.5 N/mm² compared to the minimum demands for C2 adhesive of  > 1 N/mm². It has vertical slip of ≤ 0.5 mm, an open time of around 16 hours at 23⁰C and transverse deformation of ≥ 2.5 mm < 5mm.

Kerakoll’s H40 Ultimate is classified C2F S2. This means that it is an advanced cementitious adhesive with a fast setting time and is highly deformable. The tensile adhesion is ≥ 2.5 N/mm²; it is ready for foot traffic and grouting after 3 hours at 23⁰C, and has transverse deformation of ≥ 5 mm.

Traditional cement-based adhesives have several technical shortfalls making life difficult for tilers and even leading to system failure in extreme cases. Their tendency to shrink means that tilers frequently mix them more stiffly than intended. As a consequence, the mix thickens quickly in the bucket and a film frequently forms preventing it from thoroughly wetting the tile. 

To complicate matters further, the wide choice of different formulations for different applications, and varying performance between adhesives make it hard to specify the correct adhesive.

Cementitious adhesives struggle to perform on more challenging substrates where a reactive resin adhesive (‘R’) should be used.

Typically epoxy, polyurethane, or hybrid formulations, these are used in more demanding environments. They are particularly suitable for difficult or non-porous substrates, such as GRP, metal or PVC, where cement-based adhesives may fail to bond effectively. 

Reactive resins offer exceptionally high chemical resistance and are ideal for heavy-duty applications, including commercial kitchens, industrial settings, or areas exposed to moisture and aggressive cleaning agents.

There are two classifications:

Some reactive adhesives, including Kerakoll’s H40 Extreme, also include the ‘T’ designation, indicating reduced slip. 

H40 Extreme is a hybrid reactive resin rated R2 T with a shear adhesion of  ≥ 7.5 N/mm². This remains at ≥ 5.5 N/mm² after thermal shock, and ≥ 3 N/mm² after immersion in chlorinated water. It has transverse deformation of ≥ 50 mm and can be used in temperatures ranging from -40°C to +90°C.

In 2017, the Kerakoll R&D team went back to the drawing board to reinvent their adhesive formulations, researching innovative materials and techniques. The end result was a range of Gel-Adhesives that perform above the parameters of the standard, with very low VOC emissions, a high level of recycled raw material, and non-toxic and non-hazardous ingredients.

The most obvious change from conventional cement- and epoxy resin-based adhesive ranges for the specifier is the fact that there are only four products to cover all possible standard- and rapid-set substrate and material combinations, whether flexible or not. H40 Gel (standard set C2 TE S1), H40 Advanced (rapid set C2 FTE S1), and H40 Ultimate (C2 F S2) all come in powder form and H40 Extreme (R 2T) is a two-part hybrid gel. 

Below we look at where you would specify which type of adhesive. 

Stone: the natural properties of stone vary greatly making adhesive selection critical. Choose a white adhesive for use with light coloured marble, natural stone, and grouts. This prevents the adhesive showing through translucent stones. The colour of the white versions of both H40 Gel and H40 Advanced comes from pure white cements, which also provide improved workability and increased coverage. For sensitive stones use a rapid-set (‘F’) adhesive as this locks in the moisture more quickly, preventing staining and curling. Always specify a reactive resin for extremely sensitive stone.

Large format tiles: the use of extra-large slabs makes it imperative to fully bed the tile, supporting its weight, and to use an ‘S1’ adhesive offering high resistance to shear stresses. An extended open time (‘E’) allows the tiler the time to place heavy tiles correctly.

Areas subject to movement: choose an ‘S1’ adhesive in areas where limited movement is expected. This includes timber floors, underfloor heating, and areas exposed to vibration from machinery. An ‘S2’ adhesive should be selected where the movement is greater such as very flexible or compressible substrates. The use of a decoupling membrane is strongly recommended where movement is expected. Decoupling membranes should also be used for cracked and green screeds. Kerakoll’s Aquastop Green membrane has been tested for use with H40 Gel-Adhesives. Together they give a resistance to strain parallel to the laying surface of 1.8 N/mm², and a shear adhesion of ≥ 0.8 N/mm².

Wet rooms and swimming pools: it is very important that there is a solid bed of adhesive with no voids in wet areas to ensure that there is no water ingress below the tile. Select an adhesive with good wettability, such as the H40 Gel-Adhesives. You should also choose an appropriate waterproofing membrane for the amount of water exposure. This can be a tanking system, such as Kerakoll’s Nanodefense Eco, or a waterproofing membrane, such as the Aquastop Green decoupling membrane mentioned above.

Demanding substrates: particularly demanding substrates such as metal, GRP, and PVC dictate the use of a reactive resin adhesive, often used for raised access floors, or in metal- and PVC-lined pools. This is where Kerakoll Extreme comes into its own with its extreme flexibility of  ≥ 50 mm and high shear adhesion of ≥ 7.5 N/mm².

As shown above, there are three things to consider when selecting tile adhesives: the substrate, the type of tile and the environment in which it will be placed. Once these are taken into account, the choice of adhesive becomes much simpler. 

If you would like to learn more about the other aspects of the installation including substrate selection and preparation, and common causes of failure, Kerakoll offers a RIBA-approved CPD seminar on tile installation. The Specification team is also available to support you with your project requirements.

Tim Boarer CSSW is head of specification at Kerakoll UK.