Movement Joints - Part of a Complete Systems Solution
Figure 1 shows a familiar sight of a cracked tile. While the installation of movement joints will normally prevent such cracking, a particular problem can result from the tiles bridging a board joint over a timber substrate.

In some cases it is simply just not practicable to insert a movement joint over every joint in the floor, especially if the joint or crack is irregular.

Adhesives used to adhere tiles rely on a strong bond to keep the tile firmly fixed to the substrate. But this type of bonding can pose problems with the transmission of shear stresses to the surface covering, particularly where there are board joints in the substrate. A solution would be to remove the shear stresses between the substrate and tile using an uncoupling system.

The British Standards recommend isolating a rigid covering, such as ceramic tile or stone, from the substrate, which is dealt with in BS 5385 Part 3. This states that failures arising from variable stresses can be avoided by isolating the tile bed from the base by using a separating layer which prevents the two elements from adhering to each other, and thus allows each to move independently.

In order to explain how this isolation works we need to go back in time. The sketch in Figure 2 represents tile fixing methods before the introduction of thin-bed adhesives. The floor assembly with ceramic tiles could be compared to a sandwich consisting of a load-bearing substrate, a filler layer of sand, the setting mortar, and finally the tiles on top.

The ceramic tile or stone surface covering was the strongest and dominating element compared to the mortar and fill layer. But the stresses, deformations and cracks within the substrate could not be transferred to the surface covering, and floor assemblies like these have lasted for many centuries without damage occurring.

In order to isolate the covering from the substrate with modern thin-bed methods, if we take this type of tiled application and apply state-of-the-art technology, an uncoupling system can be produced that works in a similar way. A membrane type uncoupling system, such as Schlüter-DITRA, which is illustrated in Figure 3, prevents damage to the ceramic or stone surface, even when tiles bridge a board joint as in Figure 1.

Schlüter-DITRA has a uniform three-millimetre-deep grid structure of square cavities cut back in a dovetail configuration. An anchoring fleece is laminated to the underside, and the mat is installed by bonding this anchoring fleece onto the load-bearing substrate. A tile adhesive suitable for the format and application of the tile is then applied on top of the mat, and the tiles are set in the thin-bed method. Because the adhesive is mechanically anchored into the cut-back grid cavities, there is no direct bond between the covering and the substrate. Consequently, deformation stresses originating in the substrate are not transferred to the surface. The result: no surface damage.

Although this uncoupling system protects the surface, movement joints are still required. The two work in conjunction with each other to produce a complete systems solution. Stress-relieving movement joints are still needed within the surface to absorb thermal expansion and contraction of the tile.

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