Problem
Newly-laid screeds and concrete contain relatively large amounts of water and as this evaporates, the screed will shrink to compensate for the resulting volume loss.
All substrates will shrink and expand naturally due to humidity and/or temperature fluctuation.
Any movement, whether shrinkage or expansion will cause stresses to form between the substrate and the tiling layer as both move at a different rate.
These stresses can be enough to either fracture or delaminate the tiles.
Nearly all substrates are affected by water in some way but certain substrates such as plaster, anhydrite or plasterboard will lose nearly all its cohesive strength if it gets wet.
Most wooden substrates, when wet, will expand/warp causing large stresses to form between the substrate and the tile, which can cause tiles to fracture or delaminate. Wooden substrates will also become weak and rot if continually wet.
Certain substrates can be very difficult to reliably tile in a conventional manner. Existing asphalt contains oils which make it very hard to adhere to, whilst metal substrates are very smooth allowing virtually no mechanical key.
Solution
Using uncoupling membranes
Uncoupling membranes are usually used to uncouple the tiling layer from the substrate and thus reduce the stresses built up between substrate and tile. Modern uncoupling membranes can also offer more features including waterproofing and channels for evaporation or heat exchange. It is advised to contact the supplier of the membrane prior to application to make sure that the correct uncoupling membrane/ technique is being used.
