Calcium Sulphate Liquid Screed – Volume, Drying Time, Cost & Coverage. Updated 2026 Prices.
Enter your floor area and screed depth to calculate the volume, weight, drying time, and installed cost of anhydrite (calcium sulphate) liquid screed. Results include UFH commissioning guidance and minimum cover requirements. Typical installed prices for 2026 UK projects included.
Anhydrite screed (also called calcium sulphate screed, liquid screed, or flowing screed) is a floor screed manufactured from calcium sulphate (anhydrite or hemihydrate) binder, fine aggregate, water, and plasticiser admixtures. It is supplied to site in a specialist pump truck that mixes the screed fresh on arrival and pumps it directly into the building through a hose. The liquid consistency allows it to flow under gravity to produce a self-levelling, flat surface with minimal manual finishing — typically just a dappling (de-aeration) pass with a spiked roller to release any trapped air.
Anhydrite liquid screed has become the dominant floor screed for new build domestic and commercial construction in the UK since the mid-2010s, particularly for floors over underfloor heating (UFH) systems. Its advantages over traditional sand and cement screed are compelling: it can be laid at faster rates (a two-person team can typically install 500–1,500 m² per day with a pump truck), achieves a much flatter surface with less manual effort, has significantly lower shrinkage (typically less than 0.05% vs 0.1–0.3% for sand and cement), and has superior thermal conductivity (approximately 1.8–2.0 W/mK vs 1.0–1.5 W/mK for sand and cement) — making it the optimal choice for embedding UFH pipes.
The main limitations of anhydrite screed are its sensitivity to water (calcium sulphate dissolves in water, so anhydrite screed cannot be used in wet rooms, bathrooms, or external applications without a completely waterproof barrier), the need to sand and prime the surface before floor finishes (the surface laitance must be removed), and the requirement for UFH commissioning before floor finishes can be applied. Drying times are also relatively long — a 50mm anhydrite screed takes approximately 50 days to reach the ≤75% RH threshold required before most floor finishes at standard temperature and humidity conditions.
The three main floor screed types used in UK construction each have distinct properties:
Anhydrite screed resources UK 2026: Gyvlon Liquid Screed | Cemfloor | Checkatrade – Find a Screed Contractor | NHBC Standards | HIA – Heating & Hotwater Industry Council
| Application | UFH Pipe OD | Min. Cover Above Pipe | Min. Total Screed Depth | Recommended Depth | Notes |
|---|---|---|---|---|---|
| No UFH – over insulation only | N/A | N/A | 25mm | 40–50mm | 25mm is technically minimum; thicker is more robust and flatter |
| UFH – 12mm OD pipe | 12mm | 25mm | 37mm (≥40mm recommended) | 40–45mm | Pipe clip height typically 12–18mm; total depth = clip + pipe + cover |
| UFH – 15mm OD pipe | 15mm | 25mm | 40mm (≥40mm) | 45–50mm | — |
| UFH – 16mm OD pipe (most common) | 16mm | 25mm | 41mm (≥45mm recommended) | 45–50mm | Most common domestic UFH pipe size; 50mm standard depth |
| UFH – 20mm OD pipe | 20mm | 25mm | 45mm (≥50mm recommended) | 50–55mm | — |
| UFH – 25mm OD pipe | 25mm | 30mm | 55mm (≥60mm recommended) | 60–65mm | Larger commercial pipe; requires deeper screed |
| Light commercial (no UFH) | N/A | N/A | 30mm | 50–65mm | Deeper depth for better compressive strength and flatness |
Anhydrite screed drying to ≤75% RH (required before most impermeable floor finishes). Based on the 1mm = 1 day rule at standard conditions (20°C, 50–60% RH). Adjusted factors: summer (×0.80), spring/autumn (×1.20), winter (×1.60).
| Screed Depth | Standard (20°C) | Summer (warm) | Spring / Autumn | Winter (cold) | Notes |
|---|---|---|---|---|---|
| 25mm | 25 days | 20 days | 30 days | 40 days | — |
| 35mm | 35 days | 28 days | 42 days | 56 days | Minimum typical UFH depth |
| 40mm | 40 days | 32 days | 48 days | 64 days | — |
| 50mm | 50 days | 40 days | 60 days | 80 days | Standard new build depth – most common |
| 60mm | 60 days | 48 days | 72 days | 96 days | — |
| 75mm | 75 days | 60 days | 90 days | 120 days | Maximum standard depth; long drying time |
After UFH commissioning (which must be done before floor finish is laid), residual moisture is significantly reduced and the drying time effectively restarts from a lower baseline moisture level. Forced drying using dehumidifiers and ventilation can reduce drying times by up to 30–40%, but must not begin before the screed has achieved initial hardness (typically 7 days). Always test with a calibrated hygrometer before laying any floor finish.
| Depth | SE England | Midlands / SW England | North England / Wales | Scotland / N. Ireland | Greater London | Notes |
|---|---|---|---|---|---|---|
| 35mm | £18–£26/m² | £17–£24/m² | £16–£23/m² | £15–£22/m² | £22–£32/m² | Minimum over 16mm UFH |
| 40mm | £19–£28/m² | £18–£26/m² | £17–£25/m² | £16–£24/m² | £23–£34/m² | — |
| 50mm | £22–£32/m² | £21–£30/m² | £20–£28/m² | £18–£26/m² | £27–£40/m² | Standard new build – most common |
| 60mm | £25–£37/m² | £24–£35/m² | £23–£33/m² | £21–£31/m² | £31–£46/m² | — |
| 75mm | £30–£44/m² | £28–£42/m² | £27–£40/m² | £25–£37/m² | £37–£55/m² | Commercial / heavy loading |
All prices are installed costs (materials + pump truck + labour) excluding edge trim, UFH commissioning, primers, and floor finishes. VAT at 20% is additional. Most anhydrite screed contractors have a minimum charge equivalent to approximately 50–80 m² of screed at the quoted rate (approximately £1,200–£2,400 minimum). For areas below this threshold, an additional mobilisation charge is typically applied. Always obtain at least two quotes from local liquid screed contractors.
| Property | Anhydrite Liquid Screed | Sand & Cement Screed | Cementitious Liquid Screed | Notes |
|---|---|---|---|---|
| Thermal conductivity λ | 1.8–2.0 W/mK | 1.0–1.5 W/mK | 1.4–1.8 W/mK | Higher = better for UFH efficiency |
| Compressive strength | C20–C40 (grade dependent) | C20–C35 | C20–C40 | — |
| Drying shrinkage | <0.05% | 0.1–0.3% | 0.05–0.10% | Lower = fewer cracks |
| Wet density | ~2,050 kg/m³ | ~2,000 kg/m³ | ~2,000 kg/m³ | — |
| Foot traffic | 24–48 hours | 24–72 hours | 24–48 hours | — |
| Suitable for wet rooms | No | Yes | Yes | Critical limitation of anhydrite |
| Minimum depth | 25mm (35mm over UFH) | 25mm (bonded), 50mm (unbonded) | 30mm | — |
| Installation rate (m²/day) | 500–1,500 | 80–200 | 400–1,200 | Per 2-person team with pump |
| Movement joints required | Rarely (perimeter only for <300m²) | Every 6–8m | Every 8–12m | Major advantage of anhydrite |
| Surface prep before finish | Sand + prime (essential) | Prime only | Sand + prime | — |
Anhydrite screed uses calcium sulphate (CaSO₄) binder rather than Portland cement. The key practical consequence is that calcium sulphate is not water resistant — it is soluble in water over time. This is why anhydrite screed absolutely must not be used in wet rooms, bathrooms, or anywhere subject to prolonged water contact. A completely waterproof barrier (such as tanking membrane or epoxy coating) must be provided before any wet area use.
Anhydrite screed has a thermal conductivity of approximately 1.8–2.0 W/mK — significantly higher than sand and cement screed (1.0–1.5 W/mK). This means heat from UFH pipes transfers to the floor surface more efficiently, resulting in lower flow temperatures, faster heat-up times, and reduced running costs. In a well-designed UFH system, anhydrite screed can reduce the required UFH flow temperature by 3–5°C compared with sand and cement screed.
The widely used rule of thumb for anhydrite screed drying is 1mm depth = 1 day to reach ≤75% RH at standard conditions (20°C, 50–60% RH). A 50mm screed takes approximately 50 days; a 75mm screed takes approximately 75 days. Cold and damp conditions significantly extend drying. Forced drying (dehumidifiers + heating) can reduce times by 30–40% but must not start for at least 7 days. Always test with a calibrated hygrometer before laying floor finishes — never rely on elapsed time alone.
Anhydrite liquid screed requires a specialist pump truck and trained operator for installation. The fixed mobilisation cost (typically £300–£600) makes small areas (<50 m²) significantly more expensive per m² than larger pours. Most contractors quote a minimum charge equivalent to 50–80 m² of screed. For areas below this threshold, the extra cost of the minimum charge should be factored into the project budget, or alternative screed types considered for very small areas.
One of the most significant practical advantages of anhydrite screed over sand and cement is that no intermediate movement joints are required for bays up to approximately 300 m² in area. Sand and cement screed requires movement joints at approximately 6–8m centres to control shrinkage cracking. Eliminating these joints simplifies the floor construction, removes potential weak points in the floor covering, and reduces programme time.
As anhydrite screed dries, it forms a weak surface laitance layer (a thin layer of calcium sulphate crystals at the top surface). This layer must be removed by light sanding (mechanical floor sander or vacuum sander) before any primer or floor finish is applied. Failure to remove the laitance is the most common cause of tile adhesive bond failure and floor finish delamination over anhydrite screed. After sanding, apply the primer specified by the floor finish manufacturer.
Before the floor finish is laid over an anhydrite screed with UFH, the UFH system must be commissioned. The standard commissioning protocol for anhydrite is: wait 7 days after screed installation; then raise UFH flow temperature to 25°C for 3 days; then raise to the maximum design temperature for 4 days; then reduce back to ambient over 2 days. This removes residual moisture, confirms there are no leaks, and checks for air locks. Only after commissioning and subsequent hygrometer confirmation can floor finishes be installed.
All anhydrite screeds must be isolated from walls and fixed structural elements with a perimeter edge strip (typically 5–8mm closed-cell foam) before installation. This prevents the screed from bonding to walls and allows free thermal expansion and contraction, preventing edge cracking. The edge strip is also required around any fixed penetrations (pipework, columns). Without edge strip, perimeter cracking within the first year is virtually certain, particularly for UFH screeds that undergo regular thermal cycling.
Anhydrite screed has a wet density of approximately 2,050 kg/m³. A 50mm deep anhydrite screed adds approximately 102.5 kg/m² (1.0 kN/m²) of dead load to the floor structure. This must be included in the structural engineer's dead load calculations for any suspended floor — particularly relevant for timber suspended floors where screed is being added over a retrofit UFH system. Always confirm the structural adequacy of a suspended floor to carry liquid screed before installation.
Anhydrite screed is fully compatible with ceramic and porcelain floor tiles, but requires careful preparation: (1) sand off the surface laitance with a floor sander; (2) vacuum thoroughly; (3) apply a compatible anhydrite primer (e.g. Ardex P 51, Mapei Primer G, Mapei Eco Prim T+ or equivalent); (4) allow to dry; (5) apply tile adhesive from a manufacturer with specific anhydrite-compatible products. Standard grey cementitious tile adhesives used without correct priming will fail over anhydrite — use only flexible S1 or S2 adhesives from manufacturers who test their products over anhydrite screed.
The term "liquid screed" or "flowing screed" is sometimes used for both anhydrite (calcium sulphate) and cementitious liquid screeds. These are fundamentally different materials with different properties — anhydrite is not water resistant and requires specific primers, while cementitious is water resistant. Always confirm with the screed contractor which binder type is being used. For wet areas, always specify cementitious. For UFH in dry areas, anhydrite is preferred. Check the manufacturer's technical data sheet before specifying or accepting any liquid screed product.
Engineered wood flooring can be installed over anhydrite screed, but moisture content is critical. Most engineered wood manufacturers specify the screed must be at or below 0.5% CM (carbide moisture) or ≤75% RH before installation. Some manufacturers specify even lower values (65% RH). After UFH commissioning, residual moisture levels drop significantly, but the screed must still be tested with a calibrated hygrometer — not an electronic moisture meter, which is unreliable on calcium sulphate substrates. Use only a hygrometer (sleeves method) for moisture testing of anhydrite screed.
Anhydrite screed (also known as calcium sulphate screed, liquid screed, or flowing screed) is a floor screed made with a calcium sulphate binder rather than Portland cement. It is supplied as a pumpable liquid from a specialist mixing truck and flows under gravity to self-level on the floor. The calcium sulphate binder gives anhydrite screed its key advantages over sand and cement: very low shrinkage (less than 0.05%), excellent thermal conductivity (1.8–2.0 W/mK — ideal for UFH systems), fast installation (500–1,500 m² per day), and the ability to be used without intermediate movement joints in bays up to approximately 300 m². Its critical limitation is that calcium sulphate is not water resistant — anhydrite screed must not be used in wet rooms, bathrooms, or areas subject to regular water contact.
The standard rule of thumb for anhydrite screed drying is 1mm of depth = approximately 1 day to reach ≤75% RH at standard conditions (20°C, 50–60% RH and good ventilation). Therefore: 35mm screed ≈ 35 days; 50mm (standard new build depth) ≈ 50 days; 75mm ≈ 75 days. In cold, damp winter conditions, multiply by approximately 1.6 (50mm screed ≈ 80 days). After UFH commissioning, drying accelerates. Forced drying with dehumidifiers and heating can reduce times by 30–40% but must not begin until at least 7 days after laying. Always confirm drying with a calibrated hygrometer (sleeves method) — never rely solely on elapsed time, as site conditions vary enormously.
Anhydrite screed installed prices in the UK in 2026 (supply, pump truck, and installation labour) vary by depth and region. For the most common specification (50mm over UFH in South East England): approximately £22–£32 per m² excluding VAT. For 40mm: approximately £19–£28/m². For 60mm: approximately £25–£37/m². Greater London prices are typically 20–30% higher; Scotland and Northern Ireland 10–20% lower. Most contractors apply a minimum charge equivalent to approximately 50–80 m² (approximately £1,200–£2,400 minimum call-out). Additional costs include edge strip (approximately £0.50–£1.00/m run), surface sanding and priming (approximately £3–£8/m² additional if not included), and floor finish. Always obtain at least two quotes from specialist liquid screed contractors.
The minimum depth of anhydrite screed over underfloor heating pipes is determined by the pipe diameter plus the minimum cover above the pipe. For the most common domestic UFH pipe (16mm OD): the minimum cover above the top of the pipe is 25mm, giving a minimum total screed depth of approximately 16 + 25 = 41mm (often rounded up to 45mm for practical purposes). Most screed contractors and NHBC Standards recommend a minimum of 45–50mm total depth for 16mm pipe to ensure the screed is robust enough for normal domestic loading and provides adequate cover to prevent pipe surface lines appearing on the finished floor under thermal cycling. For 20mm pipe, minimum 50mm is recommended. Always follow the UFH system manufacturer's installation guidance for minimum screed depths.
No — anhydrite (calcium sulphate) screed must not be used in wet rooms, bathrooms, en-suites, showers, or any other area subject to regular water contact. Calcium sulphate is soluble in water — prolonged or repeated wetting will cause the screed to soften, lose strength, and eventually delaminate. Even with a tanking membrane or epoxy coating above the screed, the risk of water ingress through joints or breaches in the waterproofing makes anhydrite unsuitable for wet room applications. In these areas, use sand and cement screed or cementitious liquid screed, which are inherently water resistant. For a new build or extension that has both wet rooms and dry rooms, a common approach is to use cementitious liquid screed throughout (even in dry areas) to simplify the specification and avoid the risk of anhydrite screed being accidentally installed in wet locations.
Preparing an anhydrite screed surface before tiling requires four essential steps: (1) Check moisture content — use a calibrated hygrometer (sleeves method, not an electronic pin meter) to confirm the screed is at or below 75% RH (or the specific maximum stated by the tile adhesive manufacturer). (2) Remove surface laitance — sand the entire screed surface with a floor sander or vacuum sander until the laitance is fully removed and the screed is uniformly coarse-textured. Vacuum thoroughly. (3) Apply primer — apply a primer specifically approved by the adhesive manufacturer for use over calcium sulphate screeds (e.g. Ardex P 51, Mapei Primer G, Bostik UB One). Allow to dry until just tacky. (4) Apply tile adhesive — use only flexible S1 or S2 adhesive from a manufacturer who specifically lists compatibility with calcium sulphate screed. Failure to follow all four steps is the primary cause of tile delamination over anhydrite.
One of the most significant practical advantages of anhydrite screed is that no intermediate movement joints are required for bay areas up to approximately 300 m² (some manufacturers specify larger bays of up to 1,500 m²). This is because anhydrite has very low shrinkage (less than 0.05%), so the forces driving cracking in traditional sand and cement screed are largely absent. What is essential is that the screed is completely isolated from walls and fixed elements by a perimeter edge strip (typically 5–8mm closed-cell foam polyethylene). Without edge strip, restraint at the perimeter will cause cracking near the walls. For very large areas or areas with significant temperature variation (e.g. unheated warehouses), intermediate movement joints may be required — consult the screed manufacturer's technical guidelines.
UFH commissioning is a controlled process of gradually heating and then cooling the screed to remove residual moisture and verify the UFH system performance before floor finishes are installed. For anhydrite screed, the standard commissioning protocol is: wait 7 days after screed installation (the screed must have achieved initial hardness); raise UFH flow temperature to 25°C for 3 days; increase to maximum design temperature (typically 45–55°C) for 4 days; reduce back to ambient over 2 days. Commissioning serves three purposes: it removes a significant proportion of residual moisture from the screed (accelerating drying); it confirms that no leaks or air locks exist in the UFH pipework; and it verifies that the screed has not cracked along pipe lines. After commissioning, the screed must be retested with a hygrometer before floor finishes are installed — do not assume commissioning alone is sufficient to achieve the required moisture content.
Anhydrite liquid screed installation requires a specialist contractor with a pumped screed vehicle and trained operatives — it cannot be done by a general builder or tiler without specialist equipment. To find a liquid screed contractor: search specifically for "liquid screed contractor" or "anhydrite screed" combined with your county or region online; the major product manufacturers (Gyvlon, Cemfloor, Bona Flow, Knauf) maintain approved contractor networks accessible via their websites; check reviews on Checkatrade or TrustATrader specifically for screed-related reviews; and ask for references from recent projects of similar size and specification. When requesting quotes, provide the floor area, depth, whether UFH is involved and the pipe diameter, the floor finish intended, and access considerations. Most contractors have a minimum area requirement and will not travel more than approximately 1–2 hours from their base — confirm coverage of your area before inviting quotes.
No — anhydrite screed should not be laid directly over timber floorboards, chipboard, or OSB. Calcium sulphate reacts with some timber treatments and adhesives, and the movement of a timber subfloor (even minor deflection under load) will crack the relatively rigid anhydrite screed. Additionally, the significant additional dead load of anhydrite screed (approximately 100 kg/m² at 50mm depth) may exceed the structural capacity of an existing timber floor, particularly in older properties. If a liquid screed is required over a timber floor (e.g. for retrofit UFH), a cementitious liquid screed (not anhydrite) with specialist flexible bond coat primer and confirmed structural adequacy of the timber floor is the correct approach. Always engage a structural engineer to assess the adequacy of any existing suspended timber floor before adding any liquid screed.
Most floor finishes are compatible with anhydrite screed provided the screed has been correctly prepared (laitance removed, primed, moisture content confirmed at or below threshold). Compatible finishes include: ceramic and porcelain floor tiles (with S1/S2 flexible adhesive); luxury vinyl tile (LVT) and sheet vinyl (with adhesive rated for calcium sulphate — test bond before full installation); engineered wood flooring (floating installation preferred over direct glue; moisture content must be confirmed with hygrometer sleeves method); carpet (standard underlay and grippers; no special requirements); resin flooring (two-component epoxy or polyurethane; specialist primer required). NOT compatible without special preparation: solid hardwood (too sensitive to moisture movement); standard cementitious tile adhesives without anhydrite-specific primer; and any impermeable finish applied before the screed has reached the required moisture threshold.
Additional anhydrite screed resources UK 2026: Gyvlon Liquid Screed | Cemfloor | Mapei UK – Anhydrite Compatible Products | Ardex UK | NHBC Standards | Checkatrade – Screed Contractors