This defect represents differences in the height of adjacent tiles (“small steps” to make it clearer, although in most cases this is an exaggeration), thus the surface does not meet the requirements of flatness, continuity and uniformity. The consequences for the aesthetic qualities of the tile are obvious: such defects irritate the eye, especially in rooms where the tiled surface occupies large areas, or under certain lighting (at a low angle of incidence of light, shadows emphasize the relief of the surface and highlight irregularities). If this is a floor surface, then such defects make walking difficult. Over time and during operation, other defects arise. Dirt begins to accumulate in such places and is quite difficult to remove. These areas are most susceptible to wear and mechanical stress, resulting in chipping, surface abrasion, etc.
Cause
There are two main reasons for this defect to occur. The first is incorrect laying of the tiles, in particular the laying process itself (for example, tapping the tiles too weakly when laying on cement-lime mortar) or even, when using thin-layer mortars, an insufficiently flat supporting surface or poor distribution of the adhesive mortar. The second possible reason is the quality of the tile, that is, the tile may have dimensional defects, for example, a difference in flatness that is greater than the difference established by the standards. This can be quickly determined by carrying out the tests specified in the Standards. The study, of course, should be carried out on a sample of tiles (i.e. not laid); this is why it is always a good idea to save tile samples.
Prevention
First of all, a smooth surface depends on the laying of the tiles, and, consequently, on the professionalism of the layer; here, not only the laying process itself is taken into account, but also the preparation of the base, the distribution of the cement-lime or adhesive mortar (it is necessary to correctly immerse the tiles in the cement-lime mortar ). If, while laying tiles, the installer has difficulty positioning the tiles in one line or achieving an even surface, he should ask to check the dimensional characteristics of the tiles, and in any case, refuse to work with the supposedly defective material. It should be noted that laying tiles is more difficult the lower the water absorption and the larger the size of the tiles. In any case, it is known that highly vitrified tiles (tiles that belong to the BI group, especially non-permeable tiles) are more susceptible to deformation during firing. Regarding the size of the tile, it must be remembered that the Standards indicate permissible percentage deviations in relation to the working size of the tile, thus, the absolute value of the permissible deviations is higher, the larger the size of the tile.
This very important point must be taken into account at the planning and design stage. A convenient way to reduce the risk of such a defect is to lay tiles with open joints. Indeed, with open joints, the edges of the tiles are located at a large distance from each other, and filled joints provide a smooth connection between the tiles, such results are difficult to achieve when laying with closed joints.
The article was based on the following materials:
“How and Why” by Timellini Giorgio and Palmonari Carlo.