Stresses from drying shrinkage, deflection and moisture movement in the substrate, plus thermal and moisture changes affecting the flooring, can cause loss of adhesion, resulting in bulging or cracking of the floor.
In particular, deflections in suspended floors can induce high compressive stresses in rigid floor tiling, and may be the principal cause of "hollowness" in those situations. The shear stress resulting in the substrate and ceramic or stone surface moving differently from each other is often too great for the adhesive to hold - shown in the top picture.
Therefore, stress-relieving joints are an essential part of any tiling installation, and should be incorporated at the design stage.
There are different widths of pre-formed movement joints, and the correct width and material - brass, aluminium, stainless steel or PVC - must be specified to take thermal movement into account.
The amount of movement that can be absorbed - and therefore the degree of protection given by the joint - depends on the size of the profile and the compressible material used. Pre-formed surface joints will usually accommodate movement up to 20% of the movement zone width. For example, one of the larger stress-relieving joints at 15mm wide, with a movement zone of 11mm, will accommodate up to 2.5mm of tile movement.
However, as the majority of tiled installations involve the thin-bed fixing method, cracks in the substrate will readily be transferred to the surface, causing the tiles to crack. Where irregular hairline cracks in the screed or timber board joints are present, its not practical or possible to position movement joints over those. In this situation the best way of preventing damage is to incorporate movement joints with an uncoupling system, such as a polyethylene membrane, to separate the covering from the substrate, in order to guarantee the long-lasting integrity of the installation.