Above 60 × 60, everything changes

A 30 × 60 tile forgives a lot. A 120 × 280 slab forgives nothing, and the three things that go wrong are always the same three.

One: the adhesive class

Large formats need C2TE S1 as a floor, not a preference. The S1 is deformability — the bed can move a little without transferring the whole strain into the tile. Over underfloor heating, where the screed expands and contracts through every cycle, a rigid C1 bed is what cracks tiles at the corners six months after handover.

Atlas Plus and Ceresit CM 17 are both C2TE S1 and both work. Mapei Keraflex Maxi S1 is the low-shrinkage option when the bed has to be thick and even.

Two: the substrate tolerance

A large tile is a straight edge. Lay it on a floor that is 4 mm out over two metres and the tile will show you, because one corner will sit proud and the whole piece will rock. The tolerance a 120 cm tile needs is roughly 2 mm over 2 m, and almost no raw screed arrives like that.

That is why a self-levelling compound is not an optional extra on a large-format job. Atlas SAM 200 or Ceresit CN 76 across the whole floor costs less than lifting and relaying forty square metres of slab.

Three: getting it onto the wall in one piece

A 120 × 280 × 6 mm slab weighs around 45 kg and will snap under its own weight if it is picked up flat. It needs a handling frame and suction cups, and two people who have used them before. We lend the frame and the rig with the order — ask for it when you place the line, not on the morning of the fix.

Back-buttering and coverage

Comb the bed in one direction, back-butter the tile in the same direction, and aim for full coverage. Voids under a large tile are where it will crack when something heavy is dropped on it. On a floor, 100 % coverage is the target; on a wall, 90 % with no voids at the edges.

None of this is exotic. It is just that the margin for doing it approximately disappears somewhere around 80 cm.