Specifying movement joints into your flooring project.
By Carl Stokes Head of Marketing SCHLüTER-SYSTEMS ltd.
THE designer specifying a flooring installation always needs to leave room for expansion and contraction. With so many different materials and variants of the same material being used in a building, leaving them to figure out their differences by themselves isn’t an option.
Movement joints accommodate the differing properties for peace of mind and provide a cost-effective insurance against floor installation failure.
So how do you go about provisioning for movement joints within your flooring projects? Let’s work our way up layer-by-layer, starting with the most fundamental considerations:
1. The wider building picture
Movement joints in flooring assemblies shouldn’t be an afterthought – this is a sure-fire recipe for a result that is at best an eyesore and at worst an outright failure. Movement joints need to be considered as an integral part of a building, hence BS 5385 states that the appropriate provision of movement joints falls to the building designer.
Structural movement joints – that is those separating different sections of a building – will provide the basis for positioning of any further movement joints. These joints allow for vertical and horizontal movement in the context of the whole building, so must be able to move around freely. Wherever there is a structural movement joint in the subfloor, it should be reflected in the screed and covering; structural joints mustn’t be bridged at any point by the flooring installation above.
2. Screed considerations
Achieving an appropriate standard of surface
Screeds are generally laid over the top of a concrete floor base to create a smooth surface ready to receive the final floorcovering. In order to receive rigid floorcoverings, screeds should meet the SR1 classification of the Surface Regularity of Screeds for Normal Accuracy Flooring. This is the highest standard and specifies that a 2m straightedge laid in contact with the screed should demonstrate deviance of 3mm or less at any point across its length.
If correctly laid, a dry-pack screed can achieve the SR1 standard without the need for additional products. A self-levelling screed will usually require the addition of a self-levelling compound before it meets the mark.
Accommodating screed behaviours
As they dry and shrink, levelling screeds laid over large areas tend to develop random cracks; not good news for the final flooring assembly. Whilst a logical solution to this may seem to be laying screeds in small bays from the start, this can cause problems which are harder to rectify, such as curling at vertical butt joints (daywork joints). It’s best practice therefore to lay the screed over the entire area and subsequently introduce movement joints at appropriate points.
Where to introduce intermediate movement joints into screeds
The screed should be divided into bays in accordance with the requirements for the type of floor covering being used, to minimise the risk of shrinkage cracks transferring through to and being reflected in the covering surface.
BS 8204 Part 1 sets out the code of practice for screeds, including advice on how to form movement joints. Screed movement joint positioning will either guide or be guided by the type of floorcovering used. For instance, in rigid tiled coverings screed joints should follow in-line with the requirements for positioning in the floor finish as set out in BS 5385, in intervals of up to 10m.
How to introduce intermediate movement joints into screeds
Joints should be formed in one of two ways:
- Cutting through with a trowel during laying (for dry pack screeds only)
- Cutting through with a saw after hardening (generally within 3-4 weeks, for dry pack and flowing screeds)
Joints should be straight, vertical and at least to mid-depth of the screed height.
Certain situations will require special consideration when addressing the placement of movement joints.
Buildings subjected to vibration
If a building is subject to higher than average levels of vibration, either from external influences (such as being situated next to a railway line) or internal factors (such as being home to heavy machinery), additional movement joint provision should be considered.
Flooring subject to a lot of solar gain
Flooring in rooms and spaces with large expanses of glazing, such as atriums, conservatories etc, absorbs a lot of heat from the sun shining on to it. Installations of this ilk therefore experience greater levels of thermal expansion.
If a floor is to be heated, more frequent intervals for movement joints are required. The reason for this is that a heated floor will experience an even greater range of temperatures than a normal floor and therefore more noticeable dynamics of movement.
Additional movement joints accommodate the greater expectations of movement. Again, exact intervals will be determined by guidelines relating to the type of rigid flooring to be received. In order to minimise the possibility of random cracks appearing in heated screeds, screed reinforcement measures may be necessary, using wire mesh or fibres.
Alternatives to screed joints
It’s possible to eliminate the need for screed movement joints by employing a modular screed system. Modular screed systems are designed to systematically induce micro-cracks in a screed via a studded polystyrene panel. During curing, the studded screed panel absorbs contraction, resulting in a screed free of inherent stresses. The result is continuous screed surfaces that are free from wire reinforcement and movement joints.
A thinner and lighter screed can be constructed using a modular screed system than can be created using traditional screed, resulting in a positive impact on the static calculations of a building. The estimated volume of a conventional screed is 61l, or 122kg/sq m. Modular screed systems can take this down to figures of between 40-56kg/sq m, a reduction of two-thirds in some instances.
Structural joints still need to be carried through the modular screed system – a common way to do this is to install the movement joint profile first and then butt the panels up to it.
When screeds are laid, even gaps should be left between the screed and all fixed structures, such as walls, plinths, columns, stairs, steps etc; these gaps provide space for the screed to expand. In theory, these gaps could be left open, but doing so allows the entry of debris that could get caught between the screed and fixed structures and interfere with expansion.
No matter the screed system used, perimeter joints are recommended, primarily for the purpose of infilling these gaps and producing an aesthetically pleasing appearance.
Movement joints should be included in areas where stresses are most concentrated, therefore provision is always recommended at transitions between rooms and is even more important when transitioning between different materials.
3. Covering considerations
Whereas a modular screed system can negate the need for intermediate movement joints in the screed, in most rigid floorcoverings, intermediate movement joints are a necessity.
Below, we run through a selection of the most common types of flooring that Schlüter is approached about.
Ceramic tile and natural stone
For tile and stone, movement joints can be created from sealant or a pre-fabricated joint can be used. Though sealant joints are a popular choice and the only method for some applications, in many cases they present distinct disadvantages in comparison to pre-fabricated joints, particularly in terms of serviceability and longevity.
Modern prefabricated movement joints for coverings are simultaneously designed to provide edge protection and accommodate movement. They provide a neat and uniformly straight joint and a one-step installation process. Various colours are available in the form of inserts/options for the visible surface area of the joint, meaning that movement joints are evermore inconspicuous within an installation. Joint profiles are maintenance-free and inserts are more than often replaceable.
One appeal of resin flooring is its seamless look – as it’s poured or spread onto an area, there are no interruptions owing to grout joints. This being said, movement joints are still necessary and can come in the form of proprietary movement joints or sealant joints.
Choice as to which one to use comes down to service conditions, the type of finish and aesthetic preference.
As wood is a natural material there are of course more opportunities for variance in properties, owing to knots, grain etc. Wood’s propensity to absorb water can be tempered with use of engineered wood, which involves use of a dimensionally stable plywood base.
Similar perimeter gaps should be provisioned for as with tiling, but the exact measurements depend on the type of installation – for instance, a floating engineered wood floor would require in-built gaps of 8-10mm, whereas a comparable glue-down wood floor would require gaps of only 3-5mm due to the strong bond of the adhesive limiting its expansion and contraction. Threshold joints should be installed at doorways, intermediate joints are not required and structural joints should be honoured as always.
Though they may seem like a minor detail when observed in a finished installation, movement joints are part of a wider picture. Through careful attention to the requirements of each layer of the flooring assembly and connected thinking as to how they relate to and support each other, the effects of movement can be contained. Plan thoroughly, respect your materials and environment, and movement joints will serve you well.
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 – 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 – contact us at the details below.
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