“Concrete” and “screed” are often used interchangeably on site and in technical documentation alike, and it’s easy to see why: in most floor build-ups, one is laid directly on top of the other, and both are cement-based. But they’re two different materials with two different jobs, and getting the distinction right matters when specifying a product, planning a floor build-up, or selecting the right mixing plant for the job. This article answers the questions most commonly asked about the difference between the two.
Contents
Concrete vs Screed: Quick Definitions
Concrete is a structural material made with a coarse aggregate (typically 10–20 mm), used for the load-bearing slab or base of a floor. It’s what carries the weight of the building, and it’s designed and tested for that job first. Surface flatness is secondary.
Screed, covered by BS 8204-1, is a fine-aggregate mortar (sand up to 4 mm) laid over that base to provide a flat, level surface ready to receive the final floor covering. As a standard levelling screed, it is non-structural: its entire purpose is to get the surface flat, level, and ready for tiling, resilient flooring, timber, or carpet — not to hold the building up.
There’s also a second, unrelated meaning worth clearing up before going any further, because it trips a lot of people up. “Screed” as a noun is the material described above. But “to screed” is also a verb, describing the act of levelling off a wet concrete pour with a straightedge (a wooden board, an aluminium screed board, or a mechanical/laser screed for larger pours) while the concrete is still fresh. That’s a finishing technique used during the placing of concrete itself, not the application of a separate screed material afterwards. Confusing the two meanings is common, since the same word covers a material and an action performed on a completely different material. From here on, unless stated otherwise, this article uses “screed” to mean the material.
What Is the Difference Between Concrete and Screed?
Once the basic definitions are out of the way, the difference comes down to a handful of practical points: aggregate size and composition, structural role, thickness, purpose and finish, curing and drying time, and typical use case. Each one follows from the same underlying fact: the two materials are built to do different jobs in the floor build-up, so every other difference is really a consequence of that.
Concrete’s coarse aggregate gives it the bulk and compressive strength needed to carry structural loads, which is why it’s poured thick and doesn’t need to be finished to a fine tolerance. Screed’s fine sand aggregate is what allows it to be laid in a thin, controllable layer and trowelled, pumped, or self-levelled to a tight flatness tolerance, something a coarse aggregate mix physically can’t achieve. That’s also why screed can be laid so much thinner than concrete: without large stones in the mix, there’s nothing to restrict the section down to 25–40 mm without cracking.
The table below summarises how the two materials compare on each of the four points.
| Aspect | Concrete | Screed |
|---|---|---|
| Aggregate | Coarse, typically 10–20 mm gravel or crushed stone | Fine sand-based mortar, max 4 mm (BS 8204-1) |
| Structural role | Structural, load-bearing | Non-structural (standard levelling screed) |
| Thickness | Generally at least 100 mm | Min. 40 mm for traditional sand:cement mixes; from 25 mm for polymer-modified/self-smoothing screeds |
| Purpose and finish | Strength and load-bearing capacity | Flatness and surface quality (regularity classes SR1–SR3) |
| Compressive strength | Typically 25–30 N/mm² or more | Assessed via crushing resistance categories A, B, C (BS 8204-1), not load-bearing capacity |
| Position in the floor build-up | Base/foundation of the floor | Top layer, laid over the concrete |
| Curing/drying time | Walkable in 24–48 hours; reaches full design strength at around 28 days | Under BS 8204-1, traditional screed dries at roughly 1 mm/day, so a 50 mm layer can take around 2 months before flooring can be laid; fast-drying and liquid screeds can cut this to days or a few weeks |
| Typical use case | Foundations, structural slabs, load-bearing floors and walls | Levelling layer beneath tiling, resilient flooring, timber, carpet, or as a base for underfloor heating |
In practice, this is why the two materials almost always appear together rather than as alternatives: concrete gives a floor its strength, screed gives it the flat, level surface a floor covering can actually be laid on. Trying to get both properties from a single material means compromising on one of them, which is exactly the trade-off explored in the next section.
What Is a Concrete Screed?
“Concrete screed” is the name many UK builders’ merchants and retailers give to bagged sand:cement screed mixes, and it’s also how a lot of people search for what is technically a cementitious screed (CT, under BS EN 13813). Despite the name, it isn’t made with the coarse aggregate that defines true structural concrete. It’s a colloquial term, not a different product: a concrete screed and a cement screed are the same thing, just labelled differently depending on the retailer or region. True structural concrete, with its coarse 10–20 mm aggregate, isn’t sold or used as a screed.
Can Concrete Be Used as Screed?
Not in the normal sense, and not for a standard levelling job. Concrete’s coarse aggregate makes it impossible to lay at screed thicknesses (25–40 mm) without cracking, and it can’t be finished to the tight flatness tolerances (SR1–SR3) that a screed is designed to achieve. Using concrete where a screed is called for would mean sacrificing exactly the properties (flatness, thin-section reliability, surface finish) that make a screed the right material for the job in the first place.
There’s one genuine exception: a structural screed, also called a structural topping. This is a cementitious layer bonded directly onto a precast concrete slab and designed to act compositely with it, so unlike a standard levelling screed, it does contribute to the floor’s structural capacity. It’s a specific engineered solution, specified by a structural engineer for particular precast floor systems, not something to assume applies to an ordinary domestic or commercial screed job.
Can You Screed Straight Onto Concrete? Do You Need To?
Yes, screed is laid directly onto a concrete base or slab in the vast majority of floor build-ups – that’s its normal use. Whether you actually need it depends on the condition and finish of the concrete underneath.
- Surface regularity: a structural concrete slab is cast to structural tolerances, not the tighter flatness classes (SR1–SR3 under BS 8204-1) needed for tiling, resilient flooring or timber. In most cases it simply isn’t flat or smooth enough to take a floor covering directly.
- Surface cohesion and bond: even a well-finished concrete surface may not have the surface strength or bond quality that flooring adhesives require, as set out in the relevant flooring codes (BS 5385 for tiling, BS 8201 for timber, BS 8203 for resilient and textile coverings).
- Falls, services and level: screed is also what allows a floor to reach its exact design level, take up minor irregularities, and house any underfloor heating pipework or cabling, none of which the structural slab is designed to do on its own.
A concrete surface that has been specifically power-floated to a tight tolerance can sometimes receive certain floor coverings directly, but this needs to be confirmed against the flooring manufacturer’s requirements. It isn’t the default, and most concrete bases still need a screed, or at minimum a self-smoothing levelling compound, before flooring is laid.
Overmat Solutions for Every Type of Screed
Whatever type of screed needs to be produced, Overmat has a dedicated mixing plant for it. The range covers everything from automatic mixers for screed handling traditional dry blends to advanced concrete screed machines built for flowing, self-smoothing formulations — a complete line of screed machines and screed mixers for every production need.The T Series, available in electric and hybrid versions, is the ideal solution for traditional sand:cement screeds, dry pre-blends and heated screeds. A liquid screed machine from the ALS Series delivers extreme precision in the production of flowing, self-smoothing screeds based on anhydrite and cement. A screed truck from the A Series produces lightweight levelling layers and pre-blended self-smoothing mixes with great flexibility. The EPS Series is dedicated to lightweight, polystyrene-based levelling layers. The range is completed by the Twin Series, the only plant with dual mixing capability, able to produce both traditional and self-smoothing screeds on the same machine.
Every plant is integrated with the Mixer App, which allows machines to be monitored remotely, recipes to be managed, site reports to be reviewed and technical support to be accessed.
FAQ – Frequently Asked Questions
Is screed stronger than concrete?
No. Structural concrete is designed with a much higher load-bearing capacity than screed needs, easily reaching compressive strengths of 25–30 N/mm² or more. Screed is assessed differently, via the in-situ crushing resistance categories (A, B or C) set out in BS 8204-1, which measure surface hardness rather than structural load-bearing capacity. The exception is a heavy-duty wearing screed, such as a granolithic screed made with a hard aggregate, which can rival a concrete wearing surface in abrasion resistance for industrial floors — without being a structural material in the way concrete is.
Is liquid screed better than concrete?
The comparison mixes up two different jobs. Liquid screed is an excellent levelling and finishing layer (it self-levels, dries reasonably quickly for its type, and gives excellent results over underfloor heating) but it isn’t a substitute for a structural concrete base, and it was never meant to be. A liquid screed still needs a properly designed concrete or block-and-beam structural floor underneath it to bear the building’s loads. Compared to a traditional sand:cement screed, though, liquid screed generally offers better flatness, easier coverage of large areas without joints, and better encasement of underfloor heating pipework.
Can you screed over existing concrete?
Yes, provided the existing concrete is properly prepared first. That means checking for cracks, contamination, and residual moisture, and making sure the surface has enough mechanical key or is treated with an appropriate primer or bonding agent so the new screed can bond correctly. Existing concrete that’s damaged, heavily contaminated, or still curing needs to be addressed before screeding. Skipping preparation is one of the most common causes of screed failure, debonding, and cracking.
How thick should screed be over concrete?
It depends on the type of screed. A traditional sand:cement mix typically requires a minimum of 40 mm under BS 8204-1 to prevent cracking and debonding. Advanced polymer-modified or proprietary self-smoothing screeds can be safely laid as thin as 25 mm. The right thickness also depends on whether the screed needs to accommodate underfloor heating pipework, falls, or other services, which can require additional depth.