Why are movement joints needed?

Ian Knifton, Technical Manager at Schluter-Systems, explains why movement joints are needed.

This is the first part of an in-depth series look at movement joints - why they're needed, where they're needed, and the technical aspects that have to be considered regarding their installation.

Ian Knifton, Technical Manager at Schluter-Systems, explains that a ceramic or stone covering can be compared to a sheet of glass, in that each is rigid by nature.

Movement joints must be installed in certain areas and positions to prevent tiles or grout from cracking - and in some cases prevent the tiles from tenting and becoming debonded from the substrate.

Question:.

What is a movement joint?.

Answer:.

A movement joint is the interruption of the surface to allow for movement.

Common terms are:.

* Movement joint.

* Expansion joint.

* Stress relieving joint.

Question:.

Why are they needed?.

Answer:.

All tiles expand and contract with temperature and moisture changes.

In almost every case the substrate will move differently to the covering material.

The larger the tile field, the more it will expand and contract, and be vulnerable to failure.

In 95 per cent of today's tile installations they will be fixed using the thin-bed method.

This means the tile is adhered directly to the substrate with an appropriate adhesive.

Movement joints accommodate the differential stresses within each "field" of tiling, so they don't build up to a level which would cause shearing stresses at the bonded interface, protecting the tiles from cracking, tenting and debonding.

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.

Therefore, stress-relieving joints are an essential part of any tiling installation, and should be incorporated at the design stage.

Question:.

Will any one type of movement joint be adequate for all types of installation?.

Answer:.

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, it's 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.

* Next time - now we've identified why we need movement joints, we look at the technical aspects of exactly where they need to be laid.

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