Home Screed: The Complete Guide

When designing or renovating a floor, attention almost always falls on the final floor covering: tiles, parquet, resin. Yet the true silent protagonist of any floor is a layer you never see: the screed.

liquid screed machine

Although it’s often an overlooked or under-considered detail, screed raises a surprising number of technical questions: its function and the different types available, the materials it’s made from, how it’s laid, its thickness, drying and curing times, and how it differs from the base beneath it (and from concrete itself). This article is intended as a complete guide to everything you need to know about screed.

Contents

What Is Screed? The Official Definition (BS 8204-1)

BS 8204-1, “Screeds, bases and in situ floorings — Part 1: Concrete bases and cementitious levelling screeds to receive floorings”, together with the related European standard BS EN 13318, distinguishes between a levelling screed (finished to achieve a defined level and to receive the final floor covering) and a wearing screed (one that serves as the floor surface itself). In both cases, the screed makes no contribution to the structural performance of the building.

In practical terms, screed is a layer of mortar (cement-based or based on another binder) applied or poured over the base to create a flat, strong surface at the correct level, ready to receive the final floor covering.

Screed is distinct from the base beneath it (covered in more detail further on) and from the floor covering itself. It is not a structural element: it plays no part in the stability of the building. Yet it is the component that, more than any other, determines whether the finished floor performs well.

 

What Is Screed Used For?

Screed doesn’t have just one job — it performs several at once, all of them critical to a successful floor.

  • Levelling and reaching the design height — every building has a finished floor level to meet, allowing for door thresholds, shower trays, stair landings and lift wells. Screed achieves that level by taking up irregularities in the base. Surface regularity is checked with a straightedge over a set length and classified into regularity classes (SR1, SR2, SR3) under BS 8204-1, according to the flooring that will be laid.
  • Distributing loads — screed spreads foot traffic and furniture loads evenly, avoiding concentrations of stress that could damage the floor covering or the structure below. Its load-bearing capacity is assessed via the in situ crushing resistance categories (A, B or C) set out in BS 8204-1, and depends on thickness and mix strength.
  • Receiving the final floor covering — screed is the surface the flooring is bonded or laid onto. It must be flat, uniform and have the surface cohesion required by the relevant flooring code (BS 5385 for tiling, BS 8201 for timber, BS 8203 for resilient and textile coverings).
  • Housing services — screed is the layer in which underfloor heating pipework (designed to BS EN 1264) and small-diameter cables and pipes are embedded. Correct cover is essential: a minimum of 65 mm is typical over the pipe for a sand:cement screed, or 50 mm for a flowing calcium sulfate screed, and the combined thermal resistance of the floor covering and underlay is generally limited to 0.15 m²·K/W so the system can perform properly.
  • Contributing to sound insulation — combined with an acoustic insulation layer, a floating screed helps the floor meet impact sound insulation requirements under the Building Regulations (Part E in England and Wales). Its thickness and density directly affect both acoustic comfort and the efficiency of underfloor heating.

 

What Is Screed Made Of?

Screed is the result of a carefully designed mix of components that must work together to deliver the required performance. The mix varies by type, but the basic ingredients are always the same.

The binder

  • Cement (CT) — the most common and versatile binder, suitable both indoors and outdoors.
  • Calcium sulfate / anhydrite (CA) — self-smoothing by nature and particularly well suited to underfloor heating, with low shrinkage. It must not be combined with steel reinforcement, which it corrodes in damp conditions, and is generally unsuitable for damp or external areas.
  • Proprietary rapid-hardening binders — special blends formulated for very fast strength gain, often used on fast-track projects.

The aggregates

  • Sharp sand — the traditional aggregate, with a maximum particle size of 4 mm for cement:sand screeds, typically mixed at a ratio of around 1:3 to 1:4.5 (cement:sand) under BS 8204-1.
  • Lightweight aggregates (expanded polystyrene beads, expanded clay, vermiculite) — used to produce lightweight or insulating screeds when the load on the structural floor needs to be reduced.

Water

Mixing water triggers the binder’s hydration reaction. In semi-dry screeds the target consistency is often described as “damp sand”: too much water weakens the screed and extends drying time, too little makes it hard to compact.

Admixtures

  • Plasticisers/superplasticisers — increase flow without adding water, allowing flowing screeds to be produced without compromising strength.
  • Accelerators and retarders — adjust working time to suit site conditions (cold weather, heat, large pours).
  • Fibres (polypropylene, glass or steel) — control cracking, sometimes as an alternative to a reinforcing mesh.
  • Shrinkage-compensating admixtures — used in some levelling/repair mortars to reduce shrinkage.

 

What Types of Screed Are There? The Classification

BS 8204-1 distinguishes screed types mainly by how they’re laid, while the European standard BS EN 13813 classifies them by binder type.

By construction type

  • Bonded screed — laid onto a mechanically prepared base with the intention of maximising bond. Minimum thickness 25 mm; a nominal design thickness of around 40 mm (tolerance ±15 mm) is typically specified to allow for deviations in the base level.
  • Unbonded screed — intentionally separated from the base by a membrane (commonly 300 micron/1200 gauge polyethylene, lapped and sealed). Thickness generally 50–70 mm.
  • Floating screed — a type of unbonded screed laid on acoustic or thermal insulation. Minimum thickness 75 mm; BS 8204-1 flags a high risk of curling with floating and unbonded screeds unless they are reinforced or laid at 100 mm or more.
  • Heated screed — incorporates underfloor heating pipework or cabling; its design and commissioning are governed by BS EN 1264. Minimum cover over the pipe is typically 65 mm for a sand:cement screed or 50 mm for a flowing calcium sulfate screed. Where floor build-up depth or loading is tightly restricted, low-profile underfloor heating overlay systems are also available that work without a screed at all, at the cost of lower heat output than a screeded system.

By consistency

  • Traditional semi-dry screed — “damp sand” consistency, hand-laid with manual compaction and trowelling. Quality depends heavily on the skill of the screeder.
  • Liquid / flowing / self-smoothing screed — covered by BS 8204-7 (pumpable self-smoothing screeds), cement- or calcium-sulfate-based, pumped into place and self-levelling with no manual intervention. Calcium sulfate flowing screeds can cover very large areas in a single day (up to roughly 2,000 m²) but, as noted above, cannot be reinforced. In the UK, flowing screeds have grown from around 15–16 % to roughly 40 % of all screeded floor area over the past five years, largely thanks to underfloor heating.

Specialist and wearing screeds

  • Granolithic screed — a cement-based wearing screed made with a hard, dense aggregate (often crushed granite, hence the name), power-trowelled to a dense, abrasion-resistant finish. Unlike a standard levelling screed, a granolithic screed is designed to be the final floor surface itself, and is widely specified for warehouses, workshops and other heavy-traffic industrial floors.

By binder type

Screed Type Designation Binder Key Notes
Cementitious screed CT Cement Also commonly sold or searched for as “concrete screed”. Tolerates a higher residual moisture content before laying, but takes longer to dry. Suitable indoors and outdoors.
Calcium sulfate screed (anhydrite) CA Calcium sulfate / anhydrite Also known as gypsum screed. Self-smoothing by nature, excellent flatness, faster drying, but reserved for dry internal areas (must not be used with steel reinforcement, which it corrodes in damp conditions).
Mastic asphalt screed AS Bitumen Also called asphalt screed. Laid hot, with no mixing water: no drying time and no moisture testing required. Trafficable as soon as it has cooled.
Synthetic resin screed SR Synthetic resin (polyurethane, epoxy) Often referred to as epoxy screed. High mechanical performance, mainly used as a thin repair or finishing screed.

Screed for Specific Applications

Beyond construction type and binder, the right screed also depends on what the room is used for.

  • Garage floor screed — needs to withstand vehicle loads and point loads from jacks or stands, so a stronger cement screed (or a granolithic finish for heavier use) is usually preferable to a standard residential mix, with adequate falls for any drainage.
  • Bathroom floor screed — always requires a fall to a drainage point and a waterproofing/tanking system above the screed, in addition to the general waterproofing covered further on for wet areas.
  • External screed — should always be cement-based (never calcium sulfate, which is not moisture- or frost-resistant), with adequate falls and frost resistance built into the mix design.

 

What Thickness Should Screed Be? Minimum and Nominal Thickness

BS 8204 sets out clear thickness thresholds according to how the screed is laid:

  • Bonded screed — minimum 25 mm; a nominal thickness of around 40 mm (±15 mm tolerance) is normally specified.
  • Unbonded screed — typically 50–70 mm.
  • Floating screed — minimum 75 mm, and ideally 100 mm or more (or reinforced) to control the risk of curling.
  • Heated screed — minimum cover over the pipe of 65 mm (sand:cement) or 50 mm (flowing calcium sulfate); overall thickness also depends on pipe diameter.
  • Pumpable self-smoothing screed (BS 8204-7) — bonded applications from 25 mm minimum, with proprietary systems commonly specified up to 80 mm.

Beyond these minimums, a practical margin is always allowed to take up the variations in level found on site.

 

How Long Does Screed Take to Dry? Drying and Curing Times

Curing (the binder’s chemical hardening) and drying (the loss of free water) are two distinct processes. Screed is usually trafficable within 24–48 hours, but that doesn’t mean it’s ready to receive a floor covering.

Before laying any covering, moisture testing is essential. The recognised method in the UK is the hygrometer box test under BS 8203, BS 5325 and BS 8201, leaving a sealed hygrometer in contact with the screed for at least 72 hours. A reading of 75 % RH or below is generally required for resilient and textile coverings, and 65 % RH or below for timber. Some manufacturers also accept the calcium carbide (CM) test as an alternative.

  • Cement screeds — dry at a widely used rule-of-thumb rate of around 1 mm per day up to 40 mm thickness, then about 0.5 mm per day beyond that. On that basis, a 40 mm bonded screed takes roughly 40 days to dry, while a 75 mm floating screed can take around 110 days.
  • Calcium sulfate (anhydrite) screeds — generally dry faster than cement screeds for an equivalent thickness, although the residual moisture limits before laying remain strict and must still be verified by testing.
  • Mastic asphalt screeds — contain no mixing water, so there is no drying time and no moisture test to carry out: the floor is trafficable as soon as it has cooled.

Where underfloor heating is present, BS EN 1264-4 requires cementitious screeds to cure for at least 21 days before the system is commissioned. Commissioning itself should be gradual: starting at around 25 °C and increasing by about 5 °C per day up to the design flow temperature, after which the screed should be kept at temperature, then allowed to cool, before any moisture testing is carried out (at least 48 hours after the heating is switched off).

 

How Is Screed Produced?

On today’s sites, two production methods are used side by side.

Manual production

The binder, aggregates, water and any admixtures are batched and mixed on site using a tumble mixer or forced-action mixer, then hand-laid and trowelled to a finish. This method remains demanding in terms of time and labour, with material often barrowed manually to the point of laying. The consistency of the mix, and the quality of the finished screed, depend heavily on the experience of the screeder, leaving considerable room for variation. An alternative to site-batching is a bagged or ready-mixed screed, supplied pre-blended by major building materials suppliers and mixed with water on site or delivered ready to pour by truck — a convenient option for smaller jobs, though less cost-effective at scale than a mobile mixing plant.

Production via automated mixing plant

This is now the preferred method for modern construction, whether for traditional semi-dry screeds or flowing self-smoothing ones. Mobile mixing units (screed trucks or skid-mounted screed machines, such as those developed by Overmat) make the process continuous and tightly controlled: each component is metered electronically with precision, even compensating for the natural moisture content of the aggregates. The mix is then blended in a screed mixer and delivered with a screed pump directly to the point of lay through hose lines, removing human error, delivering consistently reliable mechanical performance and significantly cutting laying time and labour requirements.

 

The Difference Between Screed and Base

The base is the supporting layer beneath the screed: its job is to take up large differences in level, house bulky services and reduce the load on the structural floor below. It cannot directly receive a floor covering. Screed, by contrast, is purpose-designed to receive the floor covering (tiles, timber, resin, stone), with the mechanical strength and the much tighter flatness tolerances that this requires. Some proprietary screed types can themselves serve as the wearing surface, without a separate covering on top.

In a typical residential build-up, the layers from bottom to top are: structural floor/slab → lightweight base or backfill (housing services) → separating layer → screed → final floor covering.

 

Screed vs Concrete: What’s the Difference?

The two terms are often used loosely — “concrete screed” is even sold as a retail product name for bagged sand:cement mixes — but screed and concrete are technically different materials with different jobs to do. Concrete is a structural material made with a coarse aggregate (typically 10–20 mm), used for the load-bearing slab or base beneath the floor build-up. Screed, by contrast, is a fine-aggregate mortar (sand up to 4 mm under BS 8204-1), and as a levelling screed it is non-structural: its job is to provide the level, regularity and surface needed to receive the floor covering, not to carry the building’s loads.

There is one recognised exception: a structural screed (also called a structural topping), bonded directly onto a precast concrete slab and designed to act compositely with it, does contribute to the floor’s structural capacity. This is a specific engineered application, not the default for a levelling screed, and should always be specified by a structural engineer rather than assumed.

For heavy-duty floors that need to perform almost like a wearing concrete surface, the screed of choice is the granolithic screed described above: cement-based, with a hard aggregate, trowelled to a dense, abrasion-resistant finish that can take the place of a separate floor covering altogether.

 

What Goes Between the Screed and the Floor Covering?

Once the screed has finished curing and reached the correct drying level, the floor covering isn’t always laid straight away. Depending on the flooring chosen (timber, tiles, resin) and the demands of the site, certain intermediate layers are essential to ensure the long-term stability of the finished floor:

  • Primers and consolidants — liquid products applied to the surface to regulate the screed’s water absorption, improve adhesive bond, or consolidate a dusty surface.
  • Adhesives — specific products (cementitious, polyurethane or epoxy) for securely fixing the floor covering.
  • Decoupling and anti-fracture membranes — strongly recommended over underfloor heating or with large-format tiles; they absorb differential movement between the screed and the covering, reducing the risk of cracking in the finished floor.
  • Liquid or sheet waterproofing (tanking) — essential in wet areas such as bathrooms, utility rooms, balconies and terraces, to protect the screed and the structure below from water ingress.

 

Common Screed Defects and How to Avoid Them

A poorly executed screed doesn’t always show its flaws straight away — they tend to surface over time. The most common defects are cracking, caused by excess water in the mix, the absence of movement joints, or drying that’s too rapid; debonding, recognisable by the hollow sound the floor makes when tapped, caused by a poorly prepared base or insufficient bond; curling, particularly in unbonded or floating screeds laid thinner than 100 mm without reinforcement; and surface dusting, a sign of a screed that was too wet or cured under poor conditions. Most of these problems are prevented with three essential precautions: respecting the correct mix proportions, allowing the prescribed curing and drying times, and always carrying out a hygrometer moisture test (BS 8203) before laying any covering. A reading outside the required threshold is the most reliable sign that a screed isn’t ready yet, even when it looks dry on the surface.

 

Overmat Solutions for Every Type of Screed

Whatever type of screed or levelling layer needs to be produced, Overmat has a dedicated mixing plant for it. The T Series, available in electric and hybrid versions, is the solution for traditional sand:cement screeds, dry pre-blends and heated screeds. The liquid screed machine from the ALS Series delivers extreme precision in the production of flowing, self-smoothing screeds based on anhydrite and cement. 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.

If you’re looking for a screed machine for sale, Overmat’s range covers every need: from compact screed mixers and liquid screed machines to a complete concrete screed machine for sale, available as fixed plants or fully equipped screed trucks.