Problem
1. Substrates move slightly with changes in temperature and moisture
When a substrate expands or contracts slightly due to changes in temperature or water content, the tile being of a different material will change by a different amount.
This can arise for a number of reasons, for example:
- Shrinkage in a screed as it dries
- Thermal expansion in hot weather
- Moisture-induced expansion on getting waterlogged
- Thermal cycling of under-floor/ under-tile warming systems
2. Larger tiles suffer higher strain levels than smaller tiles
The increase in strain is cumulative across the width of the tile i.e. the further a contact point is from the centre of a tile the more adjacent points in the tile and the substrate will be stretched away from each other.
So for a given set of circumstances, a smaller tile will be less likely to delaminate than a larger one.
3. Large tiles are placed under more stress by any deflection in the floor
When a non-rigid substrate deflects, the rigid tile cannot.
For a given deflection in the substrate, the chord height that the tile tries to bridge will be higher for larger tiles.
This results in higher stresses and therefore requires greater flexibility in the adhesive to prevent failure when fixing larger tiles.
Solution
Use the appropriate flexible adhesive
It is necessary to select an adhesive with the appropriate level of flexibility to accommodate the expected strains from either differential thermal or moisture movement and/or deflection of the substrate under load. There are many possible permutations, so we have indicated some guidelines below.
Weber products
Main products
weber.setWF21
A two-part, dark grey, flexible, ceramic wall and floor tile adhesive, for interior and exterior use on backgrounds where movement is likely
weber.setrapid
A rapid-setting, cement-based wall and floor tile adhesive, available in light grey or natural white, for interior and exterior use