ᐅ Flatness Tolerances for Tiles

Hello,

I am currently preparing the surfaces for tiling in the bathroom. The walls have been skim-coated with Rotband (a finishing plaster), and the waterproofing with a sealing slurry will follow. The deviation over 1 meter is about 1-2 mm (0.04-0.08 inches). This means if I use a spirit level, a 2-cent coin (1.7 mm / 0.07 inches) either does not fit underneath or just barely does.

The floor has a deviation of about 1 mm (0.04 inches) over 1 meter.

Tiles of 60 x 60 cm (24 x 24 inches) (porcelain stoneware) with a thickness of 8 mm (0.3 inches) will be installed everywhere. The tiles will be laid using the buttering method (a notched trowel with 10 mm (0.4 inches) notches on the substrate and 4 mm (0.16 inches) notches on the back of the tiles).

Is further leveling or surface improvement necessary here, or is this sufficient?

Hello questioner.
Your question seems to focus more on the surface flatness of the installation substrate.
The usual flatness tolerances according to DIN 18202 Table 3 line 3 are [4mm/1m] (4mm per 1 meter), and the stricter (requiring special compensation) flatness is [3mm/1m] (3mm per 1 meter).
What you have measured or checked definitely meets the flatness requirements!
What seems very unusual to me, however, is that 8mm (0.3 inch) thick tiles are to be installed using the buttering-floating method.
Tiles of only 8mm (0.3 inch) thickness are typically installed with the thin-bed method, assuming a sufficiently even substrate.
With the method you mentioned, the tiles almost “float away” during installation.
This means: for example, with natural stone or concrete-based stone with a relatively rough back, or very uneven substrates, the buttering-floating method is used — applying adhesive to both the back of the tile and the substrate.
For tiles only 8mm (0.3 inch) thick, this is a questionable technique!
A thin-bed adhesive layer is completely sufficient here (after priming the substrate, otherwise the water contained in the adhesive will be rejected immediately and hollow spaces will form under the tiles). It also does not contribute to a questionable increased total flooring height.
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Good luck anyway: KlaRa

Thank you. I checked the flatness like this: I used a new 1 meter (3.3 feet) level on the floor and on the wall, then I tried to find the largest unevenness (largest gap with light) and placed a 2-cent coin there. Everywhere else the gap was under 1.7 millimeters (0.07 inches).

Regarding tile installation: a tile installer told me that for 60 x 60 millimeter (2.4 x 2.4 inch) tiles, he always applies 10 millimeters (0.4 inches) of mortar on the surface (floor and wall) and 4 millimeters (0.16 inches) of mortar on the tile itself. This ensures a proper bonding layer and allows some adjustment during installation.

10mm (0.4 inches) + 4mm (0.16 inches) = medium-bed mortar (8mm - 15mm / 0.3 inches - 0.6 inches) used as an adhesive layer.
The installer applies a medium bed as an adhesive layer using the buttering-floating method.
There is no objection to this for tiles or slabs sized 60cm x 60cm (24 inches x 24 inches)!
By the way, when measuring flatness, the straightedge (e.g., spirit level) should always be placed 10cm (4 inches) away from the wall to avoid including edge depressions or edge bows of a cement screed in the assessment.
Just a side note.

Thank you very much for the explanation. The ordered tiles are for the walls and floor (porcelain stoneware, 8 mm (0.3 inch) thick, with a textured, rough back). In any case, the surfaces are prepared for installation; I could have actually stopped correcting the flatness earlier 🙂

Regarding edge swelling: this is exactly what happened when the waterproofing was done with slurry and sealing tape. Currently, there is a fairly significant unevenness in the first 10 cm (4 inches), as the slurry and sealing tape are relatively thick. I have no idea if this will cause issues when laying the tiles. I estimate a difference of 5-6 mm (0.2-0.25 inches) or even more.