C2S1 tile adhesive, Class III membrane. Two ratings, two tests.

Data sheets are full of short codes. Two families of them turn up on every tiled wet area, and they are often mixed up.

The membrane: Class I, II or III

Class I

Stretches 10 to 59%

Class II

Stretches 60 to 299%

Class III

Stretches 300% or more

The class sets how wide the bond breaker has to be at a joint: the less a membrane stretches, the wider the gap it needs to bridge without tearing.

The adhesive: C2, S1, T, E

C

Cement-based

2

Improved: holds 1.0 N/mm² or more dry, after water, after heat and after freeze and thaw

S1

Deformable: bends 2.5 mm or more before it breaks

T

Does not slip on a wall

E

Extended open time: still bonds 30 minutes after spreading

The test that joins them

A membrane can be Class III and an adhesive can be C2 S1, and the tile can still come off, because nobody tested the adhesive on that membrane. The tiling standard closes the gap: a membrane under tiles has to pass a test with tiles bonded to it, and the adhesive has to be tested for compatibility with that membrane before use.