When people picture a finished floor, they usually think of the tiles, parquet or resin on top. But before that finish goes down, a hidden layer has already done the groundwork: the screed. It’s the screed that gives the floor its flatness, its final level, and its ability to properly support the chosen finish over time. This guide covers what a screed is, the different types available, and how it’s installed on site.
What is a screed and what is it for?
A screed is a layer of mortar, usually based on a hydraulic binder, applied over a substrate (concrete base, thermal insulation or underfloor heating system) to create a flat, even surface strong enough to receive the final floor finish. In the UK, screed specification and installation are covered by BS 8204, the British Standard for screeds, bases and in-situ floorings, published in several parts covering different screed types and finishes (concrete bases and cementitious levelling screeds, polymer-modified screeds, mastic asphalt, synthetic resin floorings and pumpable self-smoothing screeds among them).
Unlike a structural slab, a screed is not a load-bearing element: it doesn’t contribute to the building’s structural stability. Its role is nonetheless essential, since it performs several functions at once:
- reaching the finished floor level required by the project, by compensating for irregularities in the substrate;
- distributing loads from foot traffic and furniture evenly, before they reach the finish or the structure below;
- providing the surface that receives the final finish, whether tiles, bonded parquet or resin;
- accommodating building services, particularly underfloor heating pipes or low-diameter cabling;
- contributing to acoustic performance, especially against impact sound, when combined with a suitable insulating layer.
The different types of screed
Screeds can be classified in two complementary ways: by installation method, largely as set out in BS 8204, and by binder type, as defined in BS EN 13813, the European standard for screed material specifications.
By installation method:
- bonded screed, applied directly onto a prepared substrate, sometimes with a bonding slurry;
- unbonded screed, where a separating membrane is placed between the screed and the substrate;
- floating screed, laid over an insulating layer, typically used where good thermal or acoustic performance is required;
- heated screed, which encases the pipes or cables of an underfloor heating system;
- dry screed, based on dry panels or aggregates, laid without any mixing water and therefore requiring no drying time, at the cost of lower mechanical strength;
- reinforced screed, which incorporates mesh or metal reinforcement to limit cracking on large or industrial floor areas.
By binder type:
- sand-cement screed (CT under BS EN 13813), the most common type, which tolerates slightly higher residual moisture before a floor covering can be laid, but takes longer to dry;
- calcium sulfate/anhydrite screed (CA), self-levelling and offering excellent flatness with faster drying, but limited to dry indoor areas;
- mastic asphalt screed (AS), laid hot without any mixing water;
- synthetic resin screed, with high mechanical performance, mainly used for thin repair or finishing layers. (Note: in a UK context this shouldn’t be confused with the SR designation in BS 8204, which refers to Surface Regularity classes SR1–SR3, an unrelated measure of flatness tolerance.)
This last group of binders can also be applied in two different ways: traditional (semi-dry, hand-laid) screed, and flowing (self-levelling) screed, supplied ready-mixed or produced on site, which self-levels without manual intervention and allows very large areas to be covered without movement joints. Both can be laid using any of the installation methods described above (bonded, unbonded or floating).
The choice depends on the use of the room: sand-cement screed for external or terrace areas, a reinforced solution or a structural slab for garages, additional waterproofing in bathrooms, and a lightweight or dry screed on timber substrates to limit added weight.
How is a screed produced?
Screed can be produced in two quite different ways.
Manual production, where the binder, aggregates, water and any admixtures are batched and mixed on site using a cement mixer or site mixer, remains suitable for small areas but is labour- and time-intensive, with the quality of the result depending heavily on the operator’s experience.
Production via mobile batching plant, now the preferred method on most medium to large sites, whether for traditional or flowing screed. Screed machines carry all the necessary components on board and dose them electronically with precision, before mixing them continuously and pumping the mortar to the point of application (including upper floors) through a flexible hose.
Modern screed machines have moved well beyond simple mixing and pumping.Â
Many are now highly advanced technologically, and can even feature sophisticated tools such as a moisture probe that continuously measures the sand’s water content, automatically adjusting the amount of mixing water added so the mix stays consistent even when the raw materials vary from load to load.
Remote monitoring systems allow the fleet to be tracked in real time — both in terms of geolocation, useful for coordinating multiple machines and crews across different sites, and production data, such as output volumes, mixing times and overall efficiency, which can be checked remotely without a site visit.Â
Dedicated software is also used to store and manage mix recipes, so that a formulation validated on one job can be recalled and reproduced exactly on the next, reducing the risk of human error and ensuring consistent quality across different sites and operators.
Taken together, these features offer clear advantages over manual mixing: consistent dosing accuracy regardless of the quantity produced, uniform mix quality throughout the job, the ability to reach hard-to-access areas thanks to the length of the pumping hose, and a significant reduction in site time and labour required. The use of a screed machine has therefore become a standard solution on modern construction sites, particularly where speed, traceability and consistent quality are priorities.
Drying time
It’s worth distinguishing between hardening, the chemical reaction of the binder, and drying itself, which is the evaporation of free water from the mix. Screed is generally walkable within 24 to 48 hours, but that doesn’t mean it’s ready for the floor finish to be laid.
Residual moisture must be checked before installing any covering. As a rule of thumb widely used in the UK industry, cementitious screeds dry at roughly 1mm per day under normal ventilation conditions — meaning a 50mm screed can take around 50 days to dry sufficiently. Anhydrite screeds generally require lower moisture thresholds before covering but tend to dry faster than sand-cement screeds. Mastic asphalt screed, having no mixing water, needs no particular drying time at all and is walkable as soon as it has cooled.
Where underfloor heating is present, a suitable natural drying period must be allowed before the system is brought into service, followed by a gradual, staged heat-up in line with the manufacturer’s instructions and the relevant part of BS 8204.
Screed thickness
Recommended thicknesses vary according to the installation method:
bonded screed: minimum thickness of around 25–40mm, depending on the specific application;
unbonded screed: generally thicker than a bonded screed, to absorb tolerances in the substrate;
floating screed: typically a minimum of around 65–75mm over the insulation layer, though this depends on the imposed loading and insulation performance;
heated screed: a minimum cover of around 25–30mm above the pipework is generally recommended for flowing screeds, though this should always be confirmed against manufacturer guidance and the applicable part of BS 8204.
Beyond these baseline figures, a practical margin is usually allowed to compensate for any unevenness found on site: too thin a screed weakens its mechanical strength, while too thick a screed unnecessarily extends drying times and adds unwanted load to the structure.
Conclusion
Choosing the right screed isn’t a matter of habit or personal preference: it’s a combination of technical factors — installation method, binder type, thickness and drying time — that need to work together to deliver a durable, reliable floor. A poorly sized or badly installed screed can undermine the entire floor build-up, even when the final finish has been carefully chosen and fitted.
The way the screed is produced matters just as much as the material itself. Modern batching plants, with electronic dosing, moisture sensing and remote monitoring built in, have made it possible to achieve a level of consistency that manual mixing simply can’t match — a factor that’s becoming increasingly important as project timelines get tighter and quality expectations rise. Following BS 8204 and BS EN 13813, and adapting the solution to the actual conditions on site, remains the safest way to avoid costly problems further down the line — whether that’s cracking, delamination, or a finish that simply won’t sit flat.
