Tile adhesive is often combed with a notched trowel before the tile is placed. The direction of those ridges may appear like a minor detail, but it affects how air can escape and how the adhesive bed collapses under the tile.
A controlled bed is easier to achieve
Parallel ridges provide a predictable pattern. When the tile is pressed and moved into position as specified, the ridges can collapse more consistently than a collection of random swirls and patches.
The exact trowel, bed and movement method depend on the adhesive, tile and substrate. The principle is controlled contact, not a particular trick applied to every project.
Random adhesive can leave random support
If adhesive is spread unevenly, left too long or applied with inconsistent pressure, some areas may remain unsupported. Large porcelain can bridge across those gaps while still appearing aligned from the front.
That is why manufacturers provide installation guidance and why large-format work needs more than a quick visual check after the tile is pressed into place.
The wall and tile work together
Where back-buttering is specified, the back of the tile and the prepared substrate need to meet in a way that supports the piece. The wall must already be flat enough for the intended adhesive bed. Trowel direction cannot correct a high spot or a hollow substrate.
The visible result depends on invisible discipline
Good tilework is often judged by straight lines and clean joints. Those visible qualities begin with the adhesive bed, the substrate and the handling method beneath the surface.
For large-format porcelain in a luxury bathroom, the installation method should be discussed before the first bucket of adhesive is opened.
Continue with why adhesive coverage matters, or read why levelling clips cannot level a substrate.