Uncoupling membranes and movement joints

Managing movement: the benefits of employing a complete system.

By Carl Stokes Head of Marketing Schlüter-Systems Ltd.

Within every tile or stone installation, the issue of movement is a key consideration; tile, stone, and the substrates above which they are fixed, can all be considered to be ‘living’ materials in that they respond to their environment.

Installations must be considered both in terms of differential movement between the substrate and the covering, and also in terms of the interactions between tile ‘fields’ across the area of the surface covering. The ideal solution to accommodating both these forms of movement is the pairing of uncoupling membranes and movement joints.

Uncoupling membranes
The role of uncoupling membranes is to accommodate the different movement dynamics of substrate and covering, thereby neutralising lateral stresses.

Before the introduction of thin-bed adhesives, a floor assembly could be compared to a sandwich consisting of a load-bearing substrate, a filler layer of sand, the setting mortar and, finally, the covering layer of tile or stone. This ‘sand strata’ method is virtually extinct today but is the reason why you’ll see tile and stone installations from many hundreds of years ago still intact; the loose layer ‘uncouples’ the covering from the structure, allowing for independent movement of the materials within the installation.

The arrival of thin-bed adhesives in the 1960s allowed the tile to be bonded directly to the building structure. This method was based on the theory that the principal element needed in a direct bond system is an extremely strong bond between the tile and substrate. The thin-bed method of fixing became popular as it allowed anyone – not just trained installers – to fix tiles. However, the action of fixing directly to the substrate caused problems, with tile and stone cracking, splitting or becoming debonded from the substrate.

It became clear that a modern equivalent to the sand-strata layer was needed; one that avoided excessive build-up but provided the same uncoupling functionality. In 1987, Schlüter introduced a product that has become familiar the world over. Schlüter-DITRA 25 is a supremely versatile uncoupling membrane that can be used with any suitable C2-graded adhesive, as well as tiles as small as 50 by 50mm, in thicknesses from 5.5mm. Schlüter-DITRA 25 can be used in both interior and exterior areas making it the ideal companion for all manner of tiling projects.

Schlüter-DITRA 25 is a patented polyethylene membrane with a grid structure of square cavities, each cut back in a dovetail configuration, and an anchoring fleece laminated to its underside. The geometry of the product allows for the absorption of lateral movement in each direction. Through its unique two-layer design, this uncoupling mat truly separates the substrate and tile or stone covering by creating essential free space between the layers, in which the matting can move safely.

Why use an uncoupling mat with cavities?
The tile adhesive mechanically locks into the cut-back indentations on the topside of the matting, eliminating the need for additional anchoring mesh. This mechanical lock means that the adhesive does not adhere to the mat, avoiding the formation of a direct bond and allowing for independent movement.

While the tile adhesive locks into the cut-back indentations on the membrane, the channels serve as crush zones that can change shape during deformation processes. This neutralises any stresses that may lead to cracks, tearing and detachment in bonded assemblies. A grid structure of cavities gives a network of interconnected air channels, allowing for controlled release of residual moisture from the substrate.

Movement joints
Movement joints are incorporated at certain intervals and at restraining structures to allow for the relief of tension across a surface covering.

The larger the field of tile, the greater the degree of stress that accumulates and, therefore, the higher the chance of failure. To relieve tension, the covering surface needs ‘interruption’ at appropriate points and positions. Movement joint provision is particularly vital given the swing towards trends for larger tile formats: grout lines provide a certain degree of tension relief but there are far fewer of them in an installation of large format tiles. Inadequate provision of movement joints can result in a number of different issues, including cracked grout joints, cracked tiles and debonding.

It is a specifier’s job to design in the appropriate movement joint provision across an installation. The choice broadly comes down to one of two options:
Silicone joints: Silicone joints are suitable for a wide variety of applications and are the only method for some. However, in many circumstances there are distinct disadvantages to using a silicone movement joint. Not least of these is that a silicone joint retains its memory, hence it will stick to the surface with which it has a better bond. Placed under the stress of differential movement, this can result in the tearing of joints.

The problem of torn joints runs deeper than merely aesthetics; once a joint has torn it opens the floodgates for moisture ingress into the tiling assembly. This can cause damage beneath the surface, out of sight, and the existence of an issue only usually becomes apparent at a later stage, such as on leakage through to another room or on the failure of the covering assembly.

Not only can silicone joints tear, they are also difficult to maintain in the long-run. Although many silicones now have additives in them to try and prevent the onset of mould and mildew, they inevitably still succumb to this at some point during the life of the installation.

Not only is this mould growth unsightly, but it also may cause problems in areas with strict hygiene requirements, such as hospitals and food preparation areas. Once this point is reached, the silicone has to be dug out and replaced; the process of stripping the silicone out, cleaning the void, and replacing like-for-like can be lengthy.

Prefabricated movement joints: In the majority of cases and, in particular, when we consider high-end applications, prefabricated movement joints are a preferential choice to silicone. Prefabricated movement joints offer edge protection for the tile and stone either side of them and a uniformly neat and straight joint. They are maintenance-free and offer a one-step installation process, being fixed at the same time as the tile or stone covering. Larger amounts of movement can be absorbed within a smaller width, making prefabricated movement joints an obvious choice when selecting inconspicuous joints.

Schlüter-DILEX is a comprehensive range of movement joints. The movement, expansion, and control joint profiles of the Schlüter-DILEX series offer a maintenance-free and functional solution, controlling movement in the substrate and surface covering when installed in line with the recommendations of British Standards (BSI) BS 5385.

What type of movement joint do I need to specify?
To answer this question, it helps to understand the types of movement that can occur. These generally fall into one of the following six categories:

Drying shrinkage: Contraction and shrinking causing an increase in tensile stress
Differential movement: Different parts of the structure moving at different rates
Deflection movement: The degree of movement when a structural element is placed under a load

Structural movement: This occurs frequently within buildings and can include expansion and contraction of the structure materials due to subsidence, settlement, or sway, etc.
Moisture movement: Moisture enters buildings through porous surfaces as liquid or as vapour, causing movement

Thermal movement: Changes in the shape, area and/or volume of materials due to temperature changes and fluctuations

Each of the above influences stress on the structure, leading to movement in the material surfaces and substrates. There are different types of movement joint to deal with different areas of a building project.

Joint options
Connection joints: Connection joints are movement joints placed in the screed and the covering at construction elements such as window openings, door frames, shower trays and bath tubs.

Perimeter joints: Perimeter joints are movement joints placed in the screed and the covering along walls and construction elements that penetrate the screed, such as columns. They reduce impact sound transmission and absorb the movements of the floor assembly. Edge joints must not be rigidly closed, since this may lead to the formation of sound bridges and tensions in the covering construction.

Intermediate joints: Intermediate joint profiles in screeds create a pattern of limited fields in large areas of screed and covering. They must be continued from the surface of the covering to the separating layer under the screed or to the covering of the insulation or waterproofing layer. In door transition areas, the screed should contain movement joints which are continued in the covering, to reduce stresses occurring at these locations and to prevent the transmission of impact sound. Movement joints in the substrate must not be closed or covered with flooring materials.

Structural joints: Structural joints, often called expansion joints, are joints required for static or engineering reasons, which divide a building in various movement segments. They run through all load bearing and non-load bearing parts of a building and must be continued in the screed construction and the floor covering at the identical location and in the specified width.

Where do I place movement joints?
Movement joints must be installed in certain locations and positions to prevent cracking, tenting and debonding of the tile, stone, or grout. In placing movement joints, the idea is to create tile or stone ‘fields’ large enough to absorb the anticipated movement between the substrate and the tile or stone covering.

Industry guidelines for unheated screeds suggest that the maximum tile or stone field should be no more than 10 metres in each direction, but in practice – depending on the individual applications – it tends to be more in the region of between five and eight metres.

British Standards (BSI) 5385 covers the requirements and methods for movement joint applications. BS 5385 states that the building designer should assess the magnitude of any stresses and decide where movement joints should be located, considering all relevant factors, including the type of flooring, bed and substrate.

A circle provides the best configuration for movement joints, because the forces from the centre are equal in each direction. However, in practice you will more likely be dealing with square and rectangular floors than circular ones, so these provide the best basis. In a square configuration, the ideal field size is where the ratio of the shortest to the longest distance from the centre of the force is approximately 1:1.5. Generally, the tile or stone field should be kept as square as possible and, where underfloor heating is present, the field should not exceed 40 sq. metres.

However, most floors tend to be rectangular rather than square; rectangular shapes are not usually the best configuration, as the ratio of the shortest to the longest distance exceeds 1:1.5.

On suspended floors, stress-relieving joints should be inserted where flexing is likely to occur; for instance, over supporting walls or beams. Movement joints must be carried through and situated directly over any joints in the substrate, and at any changes in the substrate; such as timber to screed, new to old screed, and heated to unheated.

In areas less than two metres wide, perimeter joints are not normally required, unless conditions, such as temperature changes, generate stresses which are likely to become extreme.

What should my movement joints be made from?
Prefabricated movement joints are available in a number of different materials. Each material is suited to different types of applications: whether a heavy duty or lighter duty profile is required depends on how strong the joint needs to be. This depends on what is being asked of it; namely, it has to respond to the expected mechanical or chemical stresses that the tiled surface will be exposed to.

Generally aluminium is an ideal choice for commercial areas, with brass and stainless steel needed for heavy-duty commercial and industrial projects, such as warehouses, production facilities, and airports, and where the tiled surface is cleaned by a scrubbing machine, or where there are rolling loads, such as pallet trucks and metal-rimmed trolleys.

Stainless steel is ideal in places with high levels of chemical exposure, such as laboratories, food processing plants, and leisure centres. PVC can be used for residential and medium-duty commercial applications including offices and swimming pools, and areas subject to light mechanical loading such as showrooms and car dealerships.

Schlüter-DILEX and Schlüter-DITRA: A secure system
Movement joints and uncoupling membranes perform complementary but distinctly different functions within a tile or stone assembly. As such, one is not a substitute for the other: each plays its own role. These two esteemed product ranges from Schlüter-Systems, each boasting decades of proven use, combine to offer specifiers a guaranteed system solution for managing movement and keeping coverings crack-free.

Learn more about specifying movement joints
Host the CPD seminar ‘Specifying Solutions for Crack-Free Tile and Stone Coverings’ to gain knowledge and confidence in specifying appropriate movement joints and uncoupling membranes. In this 45 to 60 minute presentation, held at your offices, you’ll learn how to counteract stresses in the substrate, such as drying shrinkage, deflection and thermal movement, with the aim of preventing cracked tiles, stone and joints etc.

For support across all elements of your specification journey – from learning about the fundamentals of tiling systems, through to project-specific advice and detailing – call 01530 813396 or email specifiers@schluter.co.uk.

Schlüter-Systems Ltd’s newly launched specification website is a helpful companion for architects and specifiers tasked with achieving first-class installations of tile and stone; head to www.schluterspecifier.co.uk.

T: 01530 813396
E: specifiers@schluter.co.uk
W: www.schluterspecifier.co.uk

Want an interactive introduction?
Take a Google 360 Tour of Schlüter-Systems’ UK premises, which are full of its products in situ. Visit www.exploreschluter.co.uk to view now or request your free VR headset.