Problem
1. Voids behind the tiles allow access to any water that penetrates the grout
This can cause a damp, musty smell in showers.
Any water leaks are made worse by having channels available for water to run through.
Externally, any trapped water can freeze and the associated expansion can push tiles off.
2. Voids leave floors susceptible to point loads
Ceramic tiles are inherently brittle and therefore any area of the tile that is not fully supported is vulnerable when subjected to a localised load. Examples could be: a chair leg, something being dropped, or any heavy item on small feet such as a kitchen appliance.
3. The thickness of the adhesive bed is also important
If the bed is too thin a cement-based adhesive may dry out very quickly and not gain strength.
In the case of floors where some movement is expected the adhesive is required to give and its ability to do this is severely limited if it is applied too thinly.
4. Variations in the adhesive bed will show through some tiles
Some tiles and natural stones (e.g. limestone and marble) will show a shadow on the face where the adhesive is in contact behind. Voids will be visible from the front of the tiles.
Areas where there is no contact between the tile and the adhesive bed will not be bonded. A 50% contact will therefore result in only 50% of the overall bond strength across the tile.
Solution
Use a suitable notched trowel
Using the appropriate trowel helps to ensure that an even layer of adhesive is achieved and leaves ribs that allow for adjustment of the tile height. For wet areas, exterior areas and all floors, it is necessary to achieve a solid bed of adhesive under the tile. The adhesive should be applied in a consistent manner using the correct type of trowel. On walls the adhesive should be applied in horizontal parallel strokes for dry areas and vertical strokes for wet areas. Do not apply in swirls as this causes air pockets to form. The tile should be pressed into place with a twisting and/or sliding action to ensure a good contact with the adhesive. It is recommended that during fixing, an occasional tile is lifted back up, to verify that the desired contact is being achieved.
