S1 or S2 Tile Adhesive for Underfloor Heating?
10/09/2025
Although an underfloor heating system creates a warm and cosy home, we all know that the tiling laid on top can make all the difference. To perfect this final piece of your project, ensure your tiles are laid securely to prevent movement and cracking, and to make grouting much easier. The best way to achieve this is by choosing the best tile adhesive for your type of underfloor heating system.
S1 and S2 tile adhesives are two of the most popular choices of floor tile adhesive for underfloor heating. But what’s the difference between them? In this guide, we explain what S1 and S2 mean, how they benefit your project, and which one is right for you.
Why Do I Need a Tile Adhesive for Underfloor Heating?
Whether you have a water underfloor heating system or an electric underfloor heating system, both require suitable tile adhesion. As underfloor heating fluctuates in temperature and distributes heat across a room, it causes tiles and substrates to expand and contract. This movement can potentially lead to cracking or detachment. By applying the correct type of tile adhesive, you can prevent these issues from occurring.
What are S1 and S2 Tile Adhesives?
S1 and S2 refer to the flexibility ratings (as defined by the European Standard EN 12004) given to a tile adhesive, indicating its level of deformability. Deformability is noted by the letter ‘S’, which refers to the extent to which the product can compress and absorb movement without compromising its binding capabilities.
The numbers (1 and 2) refer to the level of deformability (flexibility) it has. For example, S1 is given to tile adhesives with a standard deformable flex of 2.5 to 5mm. Whereas S2 is given to tile adhesives with a higher deformable flex of 5mm or more. The higher the deformable level, the greater the flexibility and resistance to external factors, such as temperature and moisture levels.
For an in-depth guide on each tile adhesive classification, take a look at this article: Which Tile Adhesive Is Best? A Guide To Classification Of Adhesives And Grouts
What is S1 Tile Adhesive?
S1 tile adhesive is a cement-based adhesive, making it an excellent and durable bonding adhesive for tiles on underfloor heating projects. It has a flex of 2.5 to 5mm, making it ideal for smaller areas and residential projects. It’s incredibly versatile, suitable for porcelain, ceramic, glass, mosaic, marble, slate, and stone tiles, whether used internally or externally.
S1 Product | Image | Features | Coverage | Tile Type |
 | Flexible. Cement-based. Rapid setting. Interior & exterior use. Grey colour. | 1.5kg per m2 for every 1mm of bed thickness. | Ceramic. Porcelain. Natural Stone. Low porosity tiles. Substrates. | |
 | Highly flexible. Low dust. Interior & exterior use. Frost-resistant. Grey colour. | 3.5-6 m² for thickness up to 20mm. | Ceramic. Porcelain. Glass tile. Non-moisture-sensitive stone. Substrates. | |
 | Flexible. Cement-based. Slump resistant. Internal & external use. White colour. | 5m2 per 20kg using an 8mm x 8mm notched trowel. | ||
BAL SPF S1 Standard Setting Flexible Wall & Floor Adhesive 20kg |  | Highly flexible. Internal & external use. Water/frost resistant. White colour. | 0.5 m² per kg with a 6mm notched trowel. | Porcelain. |
 | Accelerated gel-adhesive. Rapid setting. High deformability. Grey colour. | 1.3kg/m2 per mm thickness. | Suitable for all types of tiles & stone. | |
Bostik MC250 Vitri-Flex Polymer Modified Rapid Set Flexible S1 20kg |  | Rapid setting. Excellent adhesion. Grey colour.. | 1.8kg/m2 per mm thickness. | Ceramic. Quarry tiles. Porcelain. Vitrified tiles. Natural stone. Mosaic. |
Larsen Professional FibreFlex Semi-Rapid Set+ S1 Tile Adhesive 20kg |  | Gel-technology. Cement-based. Microfibre-reinforced. 3-20mm depth. Grey colour. | 4m2/20kg at 3mm bed. | Large format tiles. |
Mapei Keraquick Easy S1 Setting High Performance Adhesive 20g |  | Fast setting. Internal & external use. Grey colour. | 2-5kg per m2, depending on tile size. | Ceramic. Porcelain. Stone. Mosaics. |
Opera SPM Professional Standard Set High Performance Flexible S1 Tile Adhesive 20kg |  | Cement-based. Internal & external use. 15mm max bed thickness. White colour. | 4-5m2 for a 10mm bed. | Porcelain. Low-thickness stone. Glass mosaic. Ceramic mosaic. Natural stone. Cured cement-based screed. |
What is S2 Tile Adhesive?
S2 tile adhesive is a cement-based tile adhesive with a higher level of flexibility, often exceeding 5mm. This makes it a stronger and superior adhesive compared to S1, and is ideal for larger underfloor heating projects that may be prone to bigger movements. S2 is also better suited for underfloor heating systems with a higher vibration level.
S2 Product | Image | Features | Coverage | Tile Type |
 | Fast setting. Interior & exterior use. Grey colour Grout after 3 hours | Approx 4kg/m2 @ 3mm bed thickness. | Ceramic. Marble. Natural stone. Porcelain. Glass. Mosaic. Terrazzo. | |
 | Rapid setting. Fibre-reinforced. Grey Colour. | 4.5kg per m3. | Mosaic tiles. Ceramic. Porcelain. Glass. Terracotta. Limestone. Granite. | |
 | Rapid setting. Ultra flexible. White colour. Internal & external use. | 1.4kg per m2. | Tiles & stone of all types & formats. |
S1 vs S2 Tile Adhesive for Underfloor Heating?
S1 Tiling Adhesive | S2 Tiling Adhesive |
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Knowing which floor tile adhesive to use for your underfloor heating project, either S1 or S2, will save you a lot of time, stress, and money. With the correct bonding, you can ensure that the flooring above your underfloor heating is secure, durable, and can withstand the heat. If you have any more questions about tiling or flooring over underfloor heating, get in touch with our team via email at [email protected] or call 01604 859800.