Adhesive contact beneath a tile: what can you check while laying?

Illustrative cutaway, not to scale. The removed section reveals the layer normally hidden by the tile.

You can see adhesive at the tile's edge. Does that mean it also makes good contact beneath the centre? The edge alone does not show the whole area under the tile. During laying, though, you can look at the contact before the next rows conceal it.

The edge shows only part of the picture. Checking means exposing the contact

A photo beside the wall shows the accessible edge, not a map of contact beneath the whole tile. It cannot, by itself, settle whether gaps remain under the centre. CTEF recommends lifting tiles periodically during installation to assess the contact being achieved. That is a different observation from looking at adhesive squeezed out at the side. Source: CTEF.

Lift a freshly bedded tile and inspect both surfaces

The installer checks at the start of laying, then periodically as work continues. After lifting, they inspect the tile's underside and the corresponding adhesive bed. The point is to see where the layers actually met, rather than just where adhesive was spread beforehand. Source: CTEF.

What should you look for after lifting?

Have the ridges flattened into the valleys, or are separate high ridges and gaps still visible? Are there areas without mortar, including at the edges and corners? CTEF describes combing in one direction and moving the tile across the ridges to flatten them. Remaining ridges and gaps call for correction. Source: CTEF.

A thin coat on the tile's underside does not settle this check. As CTEF explains, coating the back, or back-buttering, is not itself contact between the tile, mortar and substrate. Source: CTEF.

Contact is insufficient — what should you change?

MAPEI recommends adjusting the trowel notch size or technique when transfer is insufficient. Instead of laying the next tiles in exactly the same way, the installer changes the application and checks its effect on a freshly bedded tile. A larger notch is not automatically the answer in every case. Source: MAPEI.

Match a trowel change to the mortar's permitted layer thickness and use, as specified by its manufacturer. Source: CTEF.

That is why inspection matters before work continues: it lets you change the method and see whether the change helped. Adding adhesive at the visible edge alone does not show what happened further beneath the tile.

One lifted tile shows only that spot

Repeat the check during laying. A good result on one tile does not automatically show the contact across the whole floor. Nor does it measure final strength after curing. Source: MAPEI.

If you are the client watching the work, ask the installer to show you this check during laying and explain what is visible. A useful question to ask: “Where did the mortar meet the tile, where are the gaps, and what will we change before the next one?” That tells you more than judging the edge alone.

Scope: this describes an installer’s check on a freshly bedded tile, not removing cured tilework. We draw on technical explanations published by MAPEI and CTEF; we do not apply any percentage thresholds from these sources to a Polish project. Required contact, trowel selection and permitted layer thickness depend on the particular material, location and mortar instructions.

Technical sources: MAPEI, “How to properly use a trowel for perfect flooring results”, 23 July 2025; CTEF, “Do You Have Enough Mortar to Accommodate Tile Warpage?”, 10 October 2023; CTEF, “How to Correctly Trowel Mortar When Installing Tile?”, 13 December 2022.

Still planning before you bond the tiles? See how to plan the layout and cuts at the wall.