Dave Rowley, training and technical support manager at ARDEX Group, explains how to deal with challenges that arise from flowing screeds for installation projects
FLOWING screeds – whether cement-based or anhydrite (calcium sulphate) have become increasingly popular in modern construction owing to their ease of installation, speed, and compatibility with underfloor heating (UFH).
However, while these benefits are often marketed to screeders and developers, they can present unique challenges for tiler fixers and floor finishing trades.
In this article we explore the critical considerations for correct identification, preparation, and UK Building Regulations, particularly Part L.
With advancements in formulation and appearance, distinguishing between cementitious and anhydrite screeds has become increasingly difficult.
Both can be delivered as pumped, ‘self-smoothing’ screeds and may look similar once cured. However, misidentification can lead to serious compatibility issues with adhesives and primers.
– Anhydrite screeds: Made from calcium sulphate, these screeds offer excellent thermal conductivity and minimal shrinkage, making them ideal for UFH systems. However, they must be dry, mechanically prepared and primed when fixing with cement-based adhesives.
– Cement-based screeds: More moisture-tolerant and generally compatible with a wider range of adhesives, including cementitious types. They’re often preferred in wet areas or where faster drying is required
Always confirm the screed type with the supplier or installer before proceeding with tiling.
Removal of laitance
Regardless of screed type, laitance – a weak, powdery surface layer – must be removed before tiling. This layer forms during curing and can severely compromise adhesive bond strength.
While ‘low laitance’ screeds aids drying times, the top surface should always be ground off before tiling. Mechanical abrasion using diamond grinding or sanding is recommended.
Cementitious screeds may form less laitance, but curing agents are also widely used, which means that in every case, surface preparation is still essential.
It’s worth pointing out, that irrespective of laitance, tile fixers are among the final trades operating on-site, and in most cases, floors are subject to high levels of general dirt and debris. It’s only by mechanical abrasion that laitance and general contaminants such as paint and plaster dropping are removed properly.
Part L changes to building regulations
The 2022 update to Part L of the building regulations introduced stricter requirements for energy efficiency and carbon reduction – this directly impacts floor screeds by requiring improved thermal performance and compatibility with modern, lower-temperature heating systems such as integral heat pump systems.
New heating systems (including those with screeds) must be designed to operate at a maximum flow temperature of 55deg C, making low thermal mass screeds and UFH a preferred solution.
Other clauses include:
– Insulation requirements beneath screeds to minimise heat loss.
– Zoning and control systems for UFH.
– Specific thermal resistance values for insulation layers depending on system type
Tilers must ensure the screed has cured and dried under good ambient conditions before tiling can begin. Moisture readings should be taken to confirm dryness (75% RH max).
Where wet underfloor heating systems are present, it’s critical to ensure the systems have been properly installed, and heat commissioned before final preparation or tile installation can be carried out.
Priming and preparation – essential for adhesion
Proper priming is crucial, for all types of flowing screed, but especially for anhydrite or calcium sulphate screeds.
For cement-based flowing screeds, acrylic or SBR-based primers – such as ARDEX P 51, BAL Bond SBR or BAL Prime APD – help to stabilise friable surfaces while also providing extended open times for cement-based levelling and smoothing compounds and tile adhesives by preventing the substrate from drawing the water from the product.
Without priming on anhydrite screeds, a chemical reaction between the calcium sulphate and cement-based smoothing compound or tile adhesive can lead to the formation of ettringite crystals – these crystals can lead to levelling compounds or tile adhesive debonding. Acrylic, SBR-based or in cases epoxy priming is required and should be confirmed with the screed manufacturer.
Always check with your primer manufacturer the correct dilution rates, number of coats required and drying time.
Flow screed benefits: screeder vs tiler perspective
While flowing screeds offer many benefits to screeders, these benefits don’t always apply to tilers.
Flowing screeds offer fast installation (up to 2,000sq m per day), minimal shrinkage and excellent thermal conductivity – making them perfect for encapsulated water-fed underfloor heating systems as already discussed.
These benefits are not always shared by tilers, who can face issues such as extended drying times (especially with anhydrite), complex preparation requirements and sometime adhesive compatibility issues.
Always check with your tile adhesive or materials manufacturer for guidance on preparation and installation on flowing screeds.
At ARDEX Group, our training and technical support team can provide on-site support, including moisture and screed testing and product recommendations.
Contact our team here: https://ardexacademy.uk/meet-our-team/ g
www.bal-adhesives.com
