Choosing tile adhesive: C1, C2, S1, S2 and T explained (EN 12004)

Short answer: EN 12004 classes cementitious tile adhesives. C1 is standard bond strength, C2 is improved. S1 and S2 describe deformability (flexibility) — needed with underfloor heating, wood-based substrates and large formats. T means reduced slip (useful on walls). Choose substrate and tile first, then the class — not the other way round.

Also available in Dutch: Tegellijm kiezen: C1, C2, S1, S2 en T uitgelegd (EN 12004).

Why the class matters

The wrong adhesive class shows up later: hollow sounds, cracks when heating starts, or tiles sliding down a wall. For professionals, EN 12004 is the shared language on site and in specifications. General product choice is also in our blog on choosing tile adhesive and grout and our blog on the Kerakoll tile system; below we focus on the letters.

C1 versus C2

• C1: cementitious adhesive with standard bond strength. Usable on stable, dry mineral substrates with smaller tiles and moderate loads.
• C2: improved bond strength. Safer with larger tiles, low-absorption porcelain and heavier loads.

In practice many tilers default to C2 for bathrooms and floors. Range examples: Kerakoll Bioflex tile adhesive 25 kg, Kerakoll Bioflex S1 tile adhesive, Kerakoll H40 white gel 25 kg, Kerakoll H40 grey gel 25 kg and Kerakoll Superflex tile adhesive. Always check the technical data sheet for the exact class.

S1 and S2: flexibility

The S class is about deformability:

• S1: deformable — typical with underfloor heating, slight substrate movement or mid-size formats.
• S2: highly deformable — for more critical substrates or very large formats.

💡 Underfloor heating without a flexible adhesive invites cracks. Think at least S1 unless the manufacturer specifies otherwise.

Extra letters: T, E, F

• T (reduced slip): tiles stay on the wall more reliably.
• E (extended open time): longer working window — useful with large formats or warm rooms.
• F (fast setting): shorter wait before grouting; shorter pot life, so keep the pace.

A typical bathroom wall adhesive might read C2TE S1: improved bond, reduced slip, extended open time, flexible.

Paste or powder?

Powder adhesives are mixed with water and cure chemically — suitable for floors, wet zones and most serious jobs. Paste is ready to use but mainly for small wall tiles in dry areas; for floors and showers, powder is usually right. Mix with a Raimondi mortar mixer or mortar mixer Ø100 mm in a 12-litre bucket or construction bucket.

Substrate first

Adhesive class will not rescue a poor substrate. Assess flatness, moisture and strength before you glue. In wet zones a waterproofing system belongs under the tiles. Tools: 6 mm glue comb, Raimondi notched trowel or 10 mm glue comb matched to tile size; levelling aids such as levelling wedges reduce lippage on large formats. See our blog on tiling a floor, our blog on tiling a wall and our blog on Raimondi tile tools.

Work safely and cleanly

• Wear work gloves with nitrile coating, safety goggles and knee pads or Raimondi knee pads with gel; cement dust and splashes irritate skin and eyes.
• When sanding or cutting creates dust: FFP2 dust mask.
• Mix only what you can use within the open time; do not "refresh" dried adhesive with extra water.
• Clean comb and bucket immediately; cured adhesive is hard to remove.

👷 On critical substrates (timber, anhydrite, tiling over old tiles, large format with heating) a professional tiler is never unnecessary. The wrong class or build-up leads to more rework than the adhesive ever avoids.

Quick selection table

FAQ

Is C2 always better than C1?

C2 bonds more strongly, but is not automatically "better" if C1 matches the manufacturer and substrate. Over-specifying is fine; under-specifying is not.

Does S2 replace primer or levelling compound?

No. Flexibility handles movement; primers and levelling are still required when the substrate needs them.

Can I use paste adhesive on floors?

Usually not: paste dries by moisture loss and floor tiles hardly allow that. Choose powder unless the data sheet explicitly allows floors.

Read also: Choosing grout: cementitious or epoxy, CG1, CG2 and RG

Where can you find everything for this job?

At Saniso, the online store for DIYers and professionals, you will find tools, materials and personal protective equipment for your job.

• Browse our construction tools and our workwear and safety.
• Create a customer account and order easily.
• Working on a bigger project or want to hand over the job? Request a project quote.
• Book a showroom visit, at least 24 hours in advance.
• Not sure which material suits your job? Get in touch with us.

In doubt about the approach? Feel free to call in a professional.

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