Flatness tolerances for tiles

Hello,

I am currently preparing the surfaces for the installation of tiles in the bathroom. The walls have been skimmed (with Rotband, followed by sealing with a sealing slurry) and the difference over 1 m is about 1-2 mm. This means that if I place a spirit level, a 2-cent coin (1.7 mm) cannot be placed or just fits underneath.

On the floor, the difference is about 1 mm over 1 m.

Tiles of 60x60 cm (porcelain stoneware) 8 mm will be laid everywhere. The tiles will be installed using the buttering method (10 notch on the surface and 4 notch on the tiles).

Is further skimming/the improvement of the surfaces necessary here, or is it sufficient?

Hello questioner.
Your question probably targets the surface flatness of the installation substrate.
The usual limit values for flatness according to DIN 18202 Table 3 Row 3 are at [4mm/1m], the increased (and therefore especially to be compensated) flatness at [3mm/1m].
What you have measured or checked is therefore definitely sufficient as flatness!
What seems highly unusual to me, however, is that 8mm thick tiles are supposed to be laid using the buttering-floating method.
Only 8mm thick tiles are usually installed with the thin-bed method, assuming a sufficiently flat substrate.
In the procedure you mentioned, the tiles practically "float away" during installation.
Meaning: with, for example, natural stone or concrete stone with a relatively rough backside or very uneven substrates, the buttering-floating method is applied.. Backside application of the adhesive and also application of the adhesive on the installation substrate.
With only 8mm thick tiles, a questionable method!!
A thin-bed adhesive layer is sufficient there (after priming the substrate, otherwise the contained water is immediately rejected and the tiles form hollow spots). And it does not contribute to the questionably increased build-up height of the floor.
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Good luck anyway: KlaRa

Thank you. I checked the flatness as follows: a new 1-meter level was placed on the floor or wall, and then I tried to find the largest unevenness (largest gap with light) and put a 2-cent coin there. Everywhere the gap was therefore less than 1.7 mm.

Regarding tile installation: a tile installer told me he always applies 10 mm of adhesive to the surface (floor and wall) for 60x60 mm tiles, as well as 4 mm of adhesive on the tile itself. This ensures a contact layer and provides some leeway during installation.

10mm + 4mm = medium bed mortar (8mm - 15mm) as adhesive bed. The installer uses this to lay a medium bed as an adhesive layer in the buttering-floating method. However, there is now nothing to object to with tiles/slabs sized 60cm x 60cm! By the way, when measuring flatness, the straightedge (e.g. spirit level) should always be placed 10cm away from the wall to avoid including edge depressions or edge bulges of a cement screed in the assessment. But this is just mentioned in passing.

Thank you very much for the explanation. The ordered tiles are for the wall and floor (porcelain stoneware, thickness 8 mm, the back side is textured, rough). In any case, the surfaces are prepared for installation, I could actually have stopped correcting the flatness earlier :)

Due to edge bulging: exactly that has happened when the sealing was done with sealing slurry and sealing tape. Currently, there is a relatively large unevenness in the first 10 cm, as the sealing slurry and sealing tape are relatively thick. No idea if this will be a problem when laying the tiles. I estimate a difference of 5-6 mm or even more.