Home Ready-mix screed vs site-mixed screed: which to choose?
Concrete screed pump

Choosing between ready-mix screed and site-mixed (bulk) screed is not just a question of convenience. It affects programme, waste, consistency, and how much control you retain over the mix once work is underway. Both routes can achieve the same technical classification under BS EN 13813, and both are subject to the same UK codes of practice on thickness, jointing and drying (BS 8204), but the way that classification is reached, and the operational trade-offs involved, are very different. For a broader introduction to screed types and terminology, see our complete guide to screed.

 

Table of contents

 

What is ready-mix screed?

“Ready-mix screed” covers material that is batched under controlled, factory conditions rather than mixed by hand on site. In practice it reaches the contractor in one of three formats:

  • bagged or big-bag premixed screed, mixed with water on site using a forced-action or pan mixer;
  • volumetric or truck-mixed screed, batched at a plant and delivered ready to pour or pump directly from the vehicle;
  • pumpable self-smoothing (liquid) screed, delivered by specialist tanker and pumped straight into place, covered specifically by BS 8204-7.

Factory-produced traditional screed typically achieves classifications in the C16/F1 to C30/F3 range depending on the mix design (a 1:5 sand:cement ratio corresponds broadly to C16/F1, 1:4 to C20/F2, and 1:3 to C30/F3 under BS EN 13813, see our guide to sand and cement screed for more detail on traditional mix design). These correspondences are indicative and commonly used in UK practice, but the actual class achieved also depends on the sand grading curve, the water/cement ratio and the level of compaction on site, not on the sand:cement ratio alone; classification should always be confirmed by testing to BS EN 13892-2. Retarding admixtures are commonly used to extend the working window, generally to somewhere in the region of half a working day, so the load remains usable until it is fully placed. Once that window closes, or once a bag is opened and part-used, the remaining material is effectively lost.

 

What is site-mixed (bulk) screed?

Site-mixed screed, also referred to as bulk screed or in-situ mixed screed, is produced directly on site using an automated batching and pumping plant. Raw materials, sand, cementitious or anhydrite binder, additives and fibres, are stored loose on board the machine and dosed by an on-board computer against a fixed recipe, then mixed and pumped straight to the point of application.

The distinction from ready-mix is not the final classification the screed can achieve (both routes can be formulated to the same BS EN 13813 classes), but where the batching control sits: in a fixed plant off site, or in a mobile unit that travels with the job.

📌 Note

Ready-mix and site-mixed are delivery methods, not screed types. Both can be used to produce traditional sand-cement screed or self-levelling (liquid) screed, cementitious or anhydrite-based. Deciding how the screed is produced, and deciding which screed type is right for the floor build-up, are two separate specification questions.

 

Classification: BS EN 13813 applies to both

Regardless of how it is produced, screed placed in the UK is classified under BS EN 13813, which sets compressive strength classes from C5 to C80 and flexural strength classes from F1 to F50 (values expressed in N/mm²). A delivery note for ready-mix screed and a batch record from an on-site plant should reference the same classification system, which is what makes the two routes directly comparable on a technical level, and why neither can be assumed to be “stronger” simply because of how it was produced. What differs is how consistently that classification is achieved batch after batch, which depends on dosing accuracy, water/cement ratio and aggregate grading, rather than on delivery method as such.

 

Thickness requirements under BS 8204

Minimum and specified thicknesses are set out in BS 8204-1 (traditional cementitious screeds) and BS 8204-7 (pumpable self-smoothing screeds), and apply equally whether the material is ready-mix or site-mixed:

Screed type / construction Minimum thickness Typical specified thickness
Bonded traditional screed 25 mm 40 mm
Unbonded traditional screed 50 mm 70 mm
Floating traditional screed 65 mm (light loads) 75 mm (heavier loads)
Bonded pumpable self-smoothing screed 25 mm 40 mm (±15 mm)
Unbonded calcium sulfate self-smoothing screed 30 mm 45 mm (±15 mm)

 

Movement joints and bay sizes

Both ready-mix and site-mixed traditional screed need to be laid in bays, with joints positioned to control shrinkage cracking as the screed dries. Under BS 8204 guidance:

  • Bonded screed bays should not exceed 40 m², with no dimension greater than 6 m in any direction.
  • Unbonded and floating screed bays are more restrictive, generally limited to around 16 m², with no dimension greater than 4.5 m.
  • The aspect ratio of any bay should stay within roughly 1.5:1 (length to width), since elongated, narrow bays are more prone to mid-bay cracking.
  • Stress-relief joints within a strip are typically spaced at around 5 to 6 m for traditional cementitious screeds, with bay widths commonly kept to 3 to 4 m.

Existing structural movement or expansion joints in the base should never be screeded over: they need to be carried through the full depth of the screed. This applies identically whichever production method is used, since joint layout is a design decision made before the pour, not a function of how the material reaches site.

 

Fibre reinforcement and crack control

Both traditional and self-levelling screed can be reinforced with synthetic micro-fibres, typically polypropylene, instead of (or alongside) welded steel mesh, to control early thermal and shrinkage cracking. Typical dosage rates for crack-control micro-fibres in cementitious screed sit in the region of 0.6 to 1.2 kg per m³, with around 0.9 kg/m³ commonly cited as sufficient for general crack resistance. Micro-fibres reduce the risk of plastic and shrinkage cracking in the first hours and days after placing, but they are not a substitute for structural steel reinforcement where the specification calls for it.

Where fibre reinforcement is intended to fully replace welded mesh, rather than simply control early shrinkage cracking, structural macro-synthetic fibres are used instead of micro-fibres, at significantly higher dosage rates: typically in the region of 4 to 6 kg/m³ for temperature and shrinkage reinforcement, rising to 6 to 10 kg/m³ on heavy-duty industrial floors. Micro-fibres and macro-fibres serve different structural purposes and are not interchangeable at the same dosage.

This is one area where the production method makes a practical difference. With bagged or delivered ready-mix screed, the fibre type and dosage are fixed before the load leaves the plant. With an on-site batching plant, fibre content, micro or macro, can be dosed automatically and adjusted batch by batch, alongside liquid, gel or powder additives, without needing to specify and order a different product for each part of the job.

 

Drying time and moisture testing before flooring

Drying time depends on thickness and screed type, not on whether the material was ready-mix or site-mixed. Under BS 8204-1 guidance, unheated traditional cementitious screed dries at roughly 1 mm per day for the first 50 mm, with drying slowing further beyond that thickness. In practice, a 75 mm screed can take around 100 days or more to reach the moisture level generally accepted before floor coverings are laid, unless a force-drying process or a fast-drying additive package is used.

That moisture level needs to be measured, not assumed, before flooring goes down. Two methods are used in the UK:

  • Hygrometer box test (BS 8203, Annex B): a small insulated box with a humidity probe is sealed to the screed surface until the reading stabilises, typically at least 72 hours. A reading of 75% RH or lower is generally required before resilient floor coverings, and 65% RH or lower before timber flooring.
  • Calcium carbide (CM) test: a crushed sample reacts with calcium carbide to produce gas, measured on a pressure gauge; commonly used in the UK on calcium sulfate (anhydrite) and polymer-modified screeds. A reading of around 0.5% on a 50 g sample corresponds broadly to 75% RH.

📌 Note

Switching from ready-mix to site-mixed production, or the reverse, will not by itself change how long a floor takes to dry. What an on-site plant can influence is how consistently the water content is controlled from batch to batch, which is one of the variables that affects drying behaviour in practice.

 

Underfloor heating: curing and commissioning

Where screed covers underfloor heating pipework, an additional commissioning step applies before any floor covering can go down, governed by BS EN 1264. Heat-up must not begin until at least 21 days after laying a cement and sand screed, or 7 days for a calcium sulfate (anhydrite) screed, and the heating loops should be pressure tested before the screed is even poured.

The heat-up protocol itself follows a fixed sequence:

  • Initial phase: flow temperature no higher than 25°C (or no more than 15°C above the screed surface temperature, whichever is lower), held for 3 days.
  • Ramp-up phase: flow temperature increased by a maximum of 5°C per day, up to the design maximum, not exceeding 55°C.
  • Hold phase: maximum design temperature maintained for 4 days.
  • Cool-down: the system is switched off and the screed left to cool naturally, generally around 24 hours, before flooring is installed.

This protocol applies to the screed itself, again independent of whether it was ready-mix or site-mixed. For a full walkthrough of underfloor heating screed choices and thicknesses, see our dedicated guide on screed for underfloor heating.

 

Ready-mix screed: advantages and limitations

Advantages:

  • Mix design fixed and quality-controlled at the point of manufacture, before it ever reaches site.
  • No raw materials (sand, cement, additives) to store or manage on site.
  • Lower labour requirement on pour day, since the material arrives ready to place or pump.

Limitations:

  • Delivery has to be booked in advance, and vehicle access confirmed, which constrains programme flexibility.
  • Load size is fixed by the vehicle or bag format, so unusually large or irregular pours can require multiple coordinated deliveries.
  • Material left over once the retardation window has passed, or once a bag has been opened, is generally wasted.
  • The recipe, including fibre type and dosage, cannot be adjusted once the load has left the plant, which limits flexibility if conditions on site change, for example a different additive package needed for a radiant floor zone versus a standard area.

 

Site-mixed screed: advantages and limitations

Advantages:

  • Materials are dosed and mixed on demand, so waste is limited to what is actually batched and poured.
  • Production can continue for as long as raw materials are loaded on the plant, without waiting on external deliveries or access slots.
  • The recipe, binder type, additive dosage, fibre content, water ratio, can be adjusted batch by batch as conditions or requirements change across the same job.
  • Not dependent on transport logistics or weather affecting delivery vehicles, since mixing happens directly at the point of use.

Limitations:

  • Requires either owning the plant or engaging a screeding contractor equipped with one.
  • Bulk raw materials still need to be sourced and supplied to site, even if not pre-batched.
  • Best justified by volume and programme: a single small pour is unlikely to make the case for a dedicated plant on site.

 

Ready-mix vs site-mixed: comparison table

Criteria Ready-mix screed Site-mixed (bulk) screed
Where batching control sits Fixed plant, off site Mobile plant, on site
BS EN 13813 classification Set at the factory, fixed recipe Achieved on site, adjustable batch by batch
Fibre/additive dosage Fixed before dispatch Adjustable at every batch
Material waste Possible on part-bags or unused load Limited to what is batched
Dependence on external delivery High, requires booked access windows Low, production happens on site
Best suited for Smaller, single-visit pours with straightforward access Larger, multi-day or multi-zone projects, screeding companies, fleet operators

 

Which option suits your project?

For a small, single-day residential job with straightforward vehicle access, ready-mix screed remains a practical route: the material arrives at the required classification, with no equipment to manage on site.

For larger commercial projects, multi-storey developments, or programmes spread across several days or several zones, an automated on-site plant tends to make more operational sense. It removes dependency on repeated bookings and access windows, keeps production running for as long as raw materials are available, and allows the recipe, including fibre type and dosage, to be adjusted between pours, for example when part of a floor includes underfloor heating and requires a different additive package from the rest of the site.

 

How Overmat plants combine factory-level consistency with on-site flexibility

Overmat, Italian manufacturer of automated mixers for screed and pumping plants, builds machines designed to close the gap between the two approaches described above: factory-level dosing accuracy, but produced directly on site, batch after batch, for as long as the job requires.

The Serie T range includes screed machines and screed mixers that dose sand, binders, admixtures and fibres automatically on board for traditional sand-cement screed. A dedicated model, the 20 TP PTO, is built specifically to handle dry premixed screed loaded from a single silo, for contractors who want to work with a premixed material but still pump directly from a machine on site rather than coordinating separate deliveries batch by batch.

For self-levelling screed, cementitious or anhydrite-based, the Serie ALS includes liquid screed machines that doses binder, water and additives with precision. A moisture probe reads the sand’s water content at every load and the on-board computer corrects the water dosage automatically, keeping the mix, and therefore the classification it achieves, consistent from the first batch to the last, closer to the repeatability normally associated with factory-produced material.

For contractors working with liquid screed additives supplied in bags, Overmat has also developed, in partnership with Gyvlon, an automatic powder-bag dispenser that loads additive bags directly into the mix without manual handling. The system has already been installed in the UK, on an ALS plant operated by Exact Screed, removing the mess and dosing inconsistency that comes with manually tipping bags into the mixer.

Every batch produced on an Overmat screed truck is logged automatically by the on-board computer and can be tracked through MIXER-APP, the company’s remote monitoring platform, giving contractors and screeding companies a certifiable record of what was mixed, when, and in what quantity, across the whole job, similar in principle to a delivery note, but generated for material produced directly on site.

Considering a move from delivered screed to on-site automated production? Get in touch with Overmat to find the right plant for your fleet and project size.

 

FAQ

Is ready-mix screed the same as self-levelling screed?

No. Ready-mix describes how the screed is supplied, factory-batched and delivered, while self-levelling (or liquid) screed describes the material itself, cementitious or anhydrite-based, covered specifically by BS 8204-7. Both traditional and self-levelling screed can be supplied ready-mix or produced on site.

Does site-mixed screed dry faster or slower than ready-mix screed?

Neither, in principle. Drying time under BS 8204-1 depends on thickness and screed type, not on how the material was batched. What an automated on-site plant can affect is the consistency of the water dosage between batches, which is one of the variables that influences drying behaviour in practice.

Can site-mixed screed achieve the same BS EN 13813 classification as ready-mix screed?

Yes, provided the raw materials and the recipe are the same. The classification (for example C20/F2 or C25/F4) depends on the mix design, water/cement ratio, aggregate grading and dosing accuracy, not on whether batching happens in a factory or on a mobile plant on site.

Is site-mixed screed only justified on large projects?

It tends to make the most operational sense where volume and programme justify having a dedicated plant on site, larger jobs, multi-day pours, or screeding companies moving between sites. For a single small pour, ready-mix screed is often the simpler route.

Does ready-mix screed still need to comply with UK standards once it’s on site?

Yes. Regardless of supply method, screed placed in the UK should meet the relevant BS EN 13813 classification and the thickness, jointing and drying guidance in BS 8204, alongside any project-specific requirements such as moisture content before covering, which should always be checked against the manufacturer’s technical data sheet.