What Is Floor Screeding?
A floor screed is a cementitious layer applied over a structural floor slab or existing floor surface to create a smooth, level surface ready to receive the final floor covering. Screed provides a clean working surface at the correct finished floor level, and it can be used to encapsulate underfloor heating pipes or cables, to level an uneven slab, or to build up the floor to meet a required height.
The screed is separate from the structural floor beneath it. It is laid on top of the slab — either bonded directly to it, laid on a separating membrane (unbonded), or laid over insulation (floating). The choice of method depends on the specific requirements of the project.
Types of Floor Screed
Traditional Sand and Cement Screed
Traditional screed is a mix of sharp sand and cement gauged with water to a semi-dry consistency. It is laid by hand and compacted, then levelled using screeding rules and a straight edge. Once laid, it is trowelled or floated to the required surface finish.
Sand and cement screed is a well-proven system that has been used for decades. It is suitable for the majority of domestic and commercial applications and can be laid bonded, unbonded or floating depending on the substrate and requirements. It is particularly well suited to areas where the screed needs to be built up to a significant thickness.
Minimum depths depend on the application. As a general guide, bonded screed is typically laid at 25–40 mm, unbonded screed at 50 mm or above, and floating screed (over insulation or underfloor heating) at 65–75 mm for traditional screed. These are indicative figures — the specification for any project should be agreed based on the specific substrate and loading conditions.
Liquid (Flowing) Screed
Liquid screed — also known as self-levelling or self-compacting screed — is a free-flowing, calcium sulphate-based or cementitious product that is pumped into place and levels itself. It is well suited to large-area projects where speed of installation and a very flat finish are important.
Liquid screed is particularly popular for use with underfloor heating systems. Because it is poured rather than compacted, it flows around heating pipes without the risk of voids or bridging that can occur with traditional screed, giving better thermal conductivity and more even heat distribution. It also allows thinner cover depths over heating pipes than traditional screed — typically 30–45 mm over the pipe with a calcium sulphate liquid screed, compared to the deeper depths often required for traditional sand and cement.
After laying, liquid screed needs to be spiked (de-aerated) to remove air bubbles trapped during pumping, and the surface is lightly sanded once cured to remove any laitance before the floor covering is installed.
Applications
- Underfloor heating (UFH): Screed encapsulates heating pipes or cables and acts as the heat emitter. Liquid screed is particularly well suited to UFH installations.
- Ceramic and porcelain tiling: Tiles require a flat, stable substrate. A well-laid screed provides this, particularly important for large-format tiles where any unevenness will show through.
- Vinyl and LVT: Resilient floor coverings such as sheet vinyl and luxury vinyl tile (LVT) are thin and will telegraph any unevenness in the substrate beneath. Screed provides the flat, smooth base these materials require.
- Engineered and solid timber: Timber floor coverings need a stable, dry substrate. Screed provides this, though moisture content must be adequately reduced before installation of the floor covering.
- Levelling uneven slabs: Where an existing slab is uneven or at the wrong level, screed can be used to build up and level the floor to the required finished level.
Levelness and Tolerances
The finished surface of a screed should meet the levelness tolerances required by the final floor covering. For most domestic floor coverings, a tolerance of ±3 mm under a 2 m straightedge is a commonly referenced standard, though more demanding finishes such as large-format tiles or polished concrete may require tighter tolerances.
We aim to achieve a consistently flat surface within the agreed tolerances. Where the substrate below is significantly uneven, achieving a very flat screed surface may require additional care and may have implications for the depth and specification of the screed — we discuss this at the survey stage.
Drying Times
Screed must be sufficiently dry before the floor covering is installed. Laying floor coverings over screed that contains excessive moisture is one of the most common causes of floor covering failures — adhesives can fail, wood can swell, and resilient coverings can blister.
As a rough guide, traditional sand and cement screed dries at approximately 1 mm per day for the first 40 mm, and then approximately 0.5 mm per day beyond that. A 75 mm floating screed therefore takes in the region of 90 days to reach the typical 75% relative humidity threshold for most floor coverings. Liquid calcium sulphate screed dries faster — the exact rate depends on the product and conditions, but 40–50 mm of liquid screed may reach the required moisture level within 4–6 weeks under good drying conditions.
These are indicative figures only — actual drying times depend on site conditions, ventilation, temperature and whether any accelerating measures are taken. We recommend testing screed moisture content with a calibrated hygrometer before installing any floor covering.
Preparation and Substrate Requirements
The structural slab beneath the screed must be sound and free of contaminants such as oil, dust or loose material. For bonded screed, the substrate is primed with a bonding agent. For floating screed applications, insulation boards and damp-proof membranes must be correctly laid and sealed before screeding begins.
For underfloor heating installations, the heating system should be pressure-tested before screeding, and the screed specification should be agreed with the UFH installer to ensure the pipe spacing and cover depth are compatible.
Our Process
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Survey and Specification
We visit the project to assess the substrate, the required finished floor level and the floor covering to be installed. We confirm the screed type, thickness and method (bonded, unbonded or floating) and provide a written quote.
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Substrate Preparation
The substrate is cleaned of all dust, debris and contaminants. Any insulation, membranes or underfloor heating pipework are checked and confirmed as correctly installed. Perimeter edge insulation strips are fitted to allow for expansion of the screed.
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Setting Out and Screeding Pins
We set out the finished floor level using a laser level, establishing the correct height across the area to be screeded. Depth pins or battens are fixed to guide the screed to the correct thickness.
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Screed Application
The screed is mixed and laid — either compacted and levelled by hand for sand and cement screed, or pumped in and spiked for liquid screed. The surface is checked for level throughout the pour.
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Curing and Protection
The screed is protected from foot traffic for the appropriate period — typically 24 hours for initial walking strength, though this varies by product. The floor should be protected from direct sunlight and draughts during initial curing to prevent rapid drying and cracking.
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Handover
We advise on drying times, any sanding requirements (for liquid screed) and the moisture testing procedure before installation of the floor covering. We clean up and leave the site ready for the next stage of works.
Areas Served
We provide floor screeding services across South Wales from our base in Llanbradach, Caerphilly. We regularly work in Cardiff, Newport, Pontypridd, Barry, Bridgend and Merthyr Tydfil. If you are unsure whether we cover your area, contact us.