Anhydrite screed is one of the most common flooring solutions in modern construction, especially wherever underfloor heating is involved. Yet many questions remain: is it the same as cement screed, does it need sanding before tiling, and how strong actually is it? This guide answers all of that with concrete data rather than vague claims.
Table of Contents
- What is anhydrite screed?
- Anhydrite screed vs. cement screed: what’s the difference?
- What does anhydrite screed look like? How to identify it
- What is anhydrite screed used for? Underfloor heating and beyond
- Anhydrite screed thickness: bonded, unbonded and floating applications
- How strong is anhydrite screed?
- Does anhydrite screed crack? Common issues and how to prevent them
- Does anhydrite screed need sanding? Primer and surface preparation
- Can you tile straight onto anhydrite screed?
- How long does anhydrite screed take to dry?
- Advantages and disadvantages of anhydrite screed
- Overmat solutions for anhydrite screed
What is anhydrite screed?
Anhydrite screed, also called calcium sulfate screed, is a self-levelling flooring screed made from a calcium sulfate binder mixed with sand and water. It is classified under BS EN 13813 as a CA (calcium sulfate) screed, in the same way cement screeds are classified as CT. The binder itself can come from two sources: natural anhydrite, mined directly as a mineral, or synthetic anhydrite, produced as a by-product of industrial processes such as flue-gas desulphurisation (FGD) in power generation. Both are chemically equivalent (calcium sulfate) and perform similarly once formulated into a screed mix, though the exact additive package used by each manufacturer varies.
Unlike traditional semi-dry cement screed, anhydrite screed is naturally free-flowing and self-levelling once pumped: it spreads and settles on its own, without manual tamping or floating. Fluidity is typically checked on site with a flow test similar in principle to the flow table test described in BS EN 13454, where the mix is expected to spread to a specific diameter once released from a cone, confirming it has the correct water content before pumping continues. The water-to-binder ratio is carefully controlled by the mixing plant, since too much water weakens the cured screed and extends drying time, while too little compromises the self-levelling flow. Typical mixes also include additives such as plasticisers (to improve flow without adding excess water) and, in some products, retarders to extend working time on larger pours.
📌 Note
Anhydrite screed is not suitable for wet areas or external use, since calcium sulfate is sensitive to sustained moisture and can dissolve or lose strength if repeatedly saturated. Cement screed remains the standard choice for bathrooms, exterior applications and any area exposed to standing water.
What is anhydrite screed used for? Underfloor heating and beyond
Anhydrite screed is most widely used as the standard choice for underfloor heating systems, since its self-levelling, liquid consistency flows around heating pipes without leaving air pockets, ensuring full contact and efficient heat transfer to the floor surface, in line with the general performance requirements set out in BS EN 1264 for radiant heating and cooling systems. Because it fully encases the pipework without voids, it also reduces the risk of localised hot spots on the finished floor that can occur when air gaps form around pipes in a less fluid mix.
Beyond underfloor heating, it is frequently chosen for large open-plan areas such as apartment developments and open-plan new builds, since its lower shrinkage allows bigger pours with fewer movement joints than an equivalent cement screed, reducing both material handling and the risk of joint-related defects later. It is also a common choice on beam-and-block floor structures, where its lighter application method and self-levelling nature help achieve a flat, consistent surface over a structural deck without extensive manual labour. Because it can typically be laid thinner than an equivalent cement screed for the same performance, it is also useful in renovation projects where floor build-up height is constrained.
Anhydrite screed vs. cement screed: what’s the difference?
Anhydrite and cement screed are often treated as interchangeable, but they differ in binder, behaviour, density and typical use case. In short: anhydrite screed is based on calcium sulfate rather than cement, is naturally self-levelling rather than hand-tamped, shrinks less and can therefore be poured over much larger areas without joints, and conducts heat more efficiently, which is why it is the preferred choice under underfloor heating. In exchange, it cannot be used in wet or external areas, and it requires sanding and priming before most floor coverings can be applied, two steps that cement screed generally does not need. The table below summarises these differences point by point.
| Criterion | Anhydrite (calcium sulfate) screed | Cement screed |
|---|---|---|
| Binder | Calcium sulfate (CA) | Cement (CT) |
| Consistency | Naturally self-levelling, pumped as a liquid | Semi-dry, hand-laid and tamped, or pumped as a fluid mix |
| Typical density | Around 1,900 to 2,100 kg/m³ | Around 1,900 to 2,200 kg/m³ |
| Thermal conductivity | Higher, typically around 1.2 to 2.0 W/(m·K) | Typically around 1.0 to 1.4 W/(m·K) |
| Shrinkage | Low, fewer movement joints needed | Higher, more frequent joints required |
| Typical unjointed pour area | Can extend to several hundred square metres in some products | Often limited to around 40 m² per bay as a general rule of thumb |
| Underfloor heating | Excellent pipe encasement, efficient heat transfer thanks to higher conductivity | Suitable, but generally less efficient than anhydrite |
| Moisture tolerance | Not suitable for wet areas or exterior use | Suitable for wet areas and exterior use |
| Surface preparation before covering | Requires sanding and priming | Usually does not require sanding |
📌 Note
The higher thermal conductivity of anhydrite screed is one of the main technical reasons it is preferred for underfloor heating: heat transfers more efficiently from the pipes to the floor surface, which can translate into faster response times and, in some cases, lower running temperatures for the same room comfort.
What does anhydrite screed look like? How to identify it
Freshly poured anhydrite screed has a smooth, self-levelled surface with a pale grey to beige tone, noticeably flatter and more uniform than a hand-tamped cement screed, since it settles under its own weight rather than being manually floated. The finished surface often has a subtle sheen while still curing, which dulls as it dries. Once cured, the surface typically develops a thin, slightly powdery surface layer known as laitance, made up of fine binder particles that rise during self-levelling and settle at the top: this is a useful visual and tactile clue, since running a hand or a dark cloth across the surface will usually pick up a fine pale residue. Cement screed, by contrast, does not typically produce this same powdery film in the same way, and its surface tends to show more visible tamping or trowel marks rather than a perfectly self-levelled sheen.
Anhydrite screed thickness: bonded, unbonded and floating applications
Minimum screed thickness in the UK is generally referenced against BS 8204-7, the standard covering pumpable self-smoothing screeds based on calcium sulfate binders. As a general guide:
| Application | Typical minimum thickness | Approximate weight per m² at minimum thickness* |
|---|---|---|
| Unbonded screed (over a separating membrane) | Around 30 mm | Around 60 kg/m² |
| Floating screed (over insulation) | Around 35 mm, increasing to around 40 mm over more compressible insulation | Around 70 to 80 kg/m² |
| Screed over underfloor heating pipes/cables | Typically a minimum cover of around 30 mm above the pipe or cable | Around 60 kg/m² plus the depth of the pipe itself |
*Based on an approximate density of 2,000 kg/m³; actual figures vary by product formulation.
📌 Note
These figures are commonly cited industry guide values associated with BS 8204-7 and should be verified against the current edition of the standard and the specific manufacturer’s technical data sheet before being used on an actual project, particularly for heavily loaded or commercial floors. On a floating application, the total floor build-up (insulation plus screed plus floor covering) should always be checked against available headroom, especially in renovation projects with fixed door thresholds or staircases.
How strong is anhydrite screed?
Anhydrite screed strength is classified under BS EN 13813 using compressive strength classes (C, in N/mm²) and flexural strength classes (F), the same system used for cement screed.
| Strength class | Approximate compressive strength | Typical use case |
|---|---|---|
| CA-C20-F4 | 20 N/mm² | Standard residential floors, light domestic loading |
| CA-C25-F5 | 25 N/mm² | Higher-traffic residential or light commercial floors |
| CA-C30-F6 | 30 N/mm² | Commercial floors with moderate loading |
| CA-C40-F10 | 40 N/mm² | Heavier-duty commercial applications |
Selecting the right class depends on the anticipated floor loading, the type of covering to be applied, and whether the screed is bonded, unbonded or floating, since floating applications on a compressible insulation layer generally call for a higher strength class than a bonded application on a rigid substrate.
Does anhydrite screed crack? Common issues and how to prevent them
Anhydrite screed generally shrinks less than cement screed, so cracking tends to be less frequent, but it is not immune to it. The most common causes are:
- Drying too quickly: strong draughts, direct sun exposure or forced heating during the early curing period can dry the surface faster than the core, leading to differential shrinkage and surface cracking or curling at the edges.
- Insufficient thickness relative to the load or the compressibility of the insulation layer beneath it, which allows excessive flexing under point loads.
- Inadequate edge isolation: missing or damaged perimeter isolation strips prevent the screed from moving independently of the surrounding walls and structure, transferring restraint stresses into the slab.
- Restraint at columns, door openings or changes in room shape, where movement is locally restricted and stress concentrates if no isolation or joint is provided.
While anhydrite screed can generally be poured over much larger areas without movement joints than cement screed, very large or irregularly shaped areas, or those with significant changes in direction, may still require movement joints as specified by the screed manufacturer for that particular product and area.
Does anhydrite screed need sanding? Primer and surface preparation
Yes. Anhydrite screed develops a thin surface laitance during curing that must be mechanically sanded off, typically using a floor sander with dust extraction, before any adhesive, primer or floor covering is applied, otherwise adhesion will be compromised. After sanding, the surface must be thoroughly vacuumed to remove residual dust, since even fine dust left on the surface can interfere with primer adhesion.
Once sanded and cleaned, a suitable primer is applied to seal the surface and improve adhesion of the subsequent adhesive or covering. The primer type depends on the covering to be installed: an SBR-based or manufacturer-specific primer is common ahead of tile adhesive, while some engineered timber flooring systems call for a different, often moisture-resistant primer formulation. Primer must generally be allowed to dry fully, following the manufacturer’s specified time, before proceeding with adhesive or covering installation.
📌 Note
This sanding and priming step is one of the main practical differences from cement screed, which typically does not require surface sanding before covering, and is frequently underestimated by contractors more familiar with traditional cement screed workflows.
Can you tile straight onto anhydrite screed?
Not directly onto the raw surface. Tiling is possible once the screed has been sanded to remove the laitance, primed appropriately, and confirmed to have reached the moisture level required by the tile adhesive manufacturer. This is typically verified with an in-situ relative humidity (RH) probe test in line with BS 8203, rather than by surface appearance or elapsed time alone.
As a general guide, calcium sulfate screeds are commonly required to reach around 75% RH or below before tiling with standard adhesives, though this threshold can vary by adhesive manufacturer and product, and some floor coverings (particularly timber) may require an even lower moisture level, sometimes around 65% RH or below, depending on the specific flooring system. Skipping any of these steps, especially moisture verification, is one of the most common causes of tile adhesion failure or bond breakdown over calcium sulfate screeds.
How long does anhydrite screed take to dry?
Anhydrite screed is generally faster drying than an equivalent thickness of cement screed, but the exact time still depends on thickness, ventilation, ambient temperature and relative humidity.
| Screed thickness | Approximate time to reach suitable RH for covering (good drying conditions) |
|---|---|
| Up to 40 mm | Roughly 2 to 4 weeks |
| Around 50 to 60 mm | Roughly 4 to 6 weeks |
| Above 75 mm | Considerably longer, often 8 weeks or more |
📌 Note
These timeframes are general indications only, based on good ventilation and moderate ambient temperature. Cold, poorly ventilated or humid site conditions can extend drying times significantly. The only reliable way to confirm a screed is ready for covering is an in-situ relative humidity probe test to BS 8203, not an estimate based on elapsed time or surface appearance. Where underfloor heating is present, a controlled commissioning heat-up procedure is typically required before final moisture testing and covering, following the manufacturer’s protocol.
Advantages and disadvantages of anhydrite screed
Advantages:
- Self-levelling application, producing a very flat surface with minimal manual finishing.
- Lower shrinkage than cement screed, allowing larger pours with fewer movement joints.
- Excellent encasement of underfloor heating pipes, improving heat transfer efficiency thanks to its higher thermal conductivity.
- Can generally be laid thinner than an equivalent cement screed for the same application.
- Faster drying than a comparable thickness of cement screed under good conditions.
- Well suited to large open-plan floors and beam-and-block structural decks.
Disadvantages:
- Not suitable for wet areas or external applications due to moisture sensitivity.
- Requires sanding and priming before tiling or applying most floor coverings, adding a labour step not needed with cement screed.
- Needs protection from rain and standing water throughout the construction period, before it is covered.
- Direct contact with unprotected steel elements should be avoided, as calcium sulfate can promote corrosion without proper isolation.
- Generally more sensitive to incorrect water dosing during mixing than a semi-dry cement screed, making consistent, controlled production important.
In short, the disadvantages of anhydrite screed mostly come down to moisture management and surface preparation: it cannot be used in wet areas or externally, it must be protected from rain and standing water throughout construction, it requires sanding and priming before most floor coverings can be applied, and unprotected steel elements should not be in direct contact with it. None of these are dealbreakers on the right project, but they do mean anhydrite screed needs to be planned for, not treated as a drop-in replacement for cement screed.
Overmat solutions for anhydrite screed
Overmat, an Italian company and leading manufacturer of automated mixing and pumping plants, produces screed machines dedicated to the production of liquid screed based on both cement and calcium sulfate (anhydrite).
The ALS Series screed trucks are purpose-built for the production of liquid screed, delivering the level of dosing accuracy that professional applicators demand. Every batch is managed by an automated system that precisely controls the ratio of binder, water and additives, the three variables that ultimately determine the flowability, workability time and long-term stability of the finished screed.
Consistency is one of the biggest challenges in on-site liquid screed production, since sand moisture can vary significantly depending on storage conditions, season and weather. To address this, the ALS Series liquid screed machine integrates a moisture probe that continuously measures the humidity of the sand and automatically adjusts the water dosage for each batch in real time. This means the mix design stays stable whether the truck is working on a dry summer site or after days of rain, removing the guesswork that traditional manual dosing methods rely on and reducing the risk of non-conforming batches.
For companies looking for a screed machine for sale, the ALS Series can be configured on request with two separate binder silos. This dual-silo setup allows the same truck to produce two different types of screed — for example, cement-based and calcium sulfate-based mixes,, over the course of a single working day, without the need for cleaning cycles between changeovers or for dispatching a second vehicle. This is particularly valuable for contractors managing multiple sites or projects with mixed technical specifications.
Every stage of production is logged automatically by the truck’s on-board electronics, creating a full digital record of each pour. This data can be accessed remotely through MixerApp, Overmat’s proprietary monitoring platform, which lets fleet managers and site supervisors track machine operating status, GPS location and quantities produced in real time from any device. Beyond day-to-day monitoring, this traceability also supports quality control documentation and helps optimize fleet logistics across multiple job sites.