Liquid Screed: UK Guide
Quick summary
Liquid screed (flowing anhydrite screed) is a pumpable floor screed that self-levels. Faster to lay than traditional sand-cement screed, thinner (25-35mm vs 50-75mm), and ideal for underfloor heating. Dries in 1mm per day up to 40mm. Costs £20-30 per m² installed. Requires specialist contractor and equipment.
What is liquid screed?
Liquid screed (also called flowing screed, pumpable screed, or anhydrite screed) is a floor levelling material made from calcium sulphate (anhydrite) binder, fine sand, and water. Unlike traditional sand-cement screed which is mixed stiff and spread by hand, liquid screed is mixed to a fluid consistency and pumped onto the floor where it flows to find its own level.
The flowing nature means it achieves a perfectly flat surface with minimal labour. A two-person crew can screed 200-300m² in a day, compared to 30-50m² for traditional hand-laid screed. The most common UK product is Gypsol (British Gypsum), though other brands like Isocrete and Supaflo are also widely used.
Liquid screed is particularly suited to underfloor heating (UFH) installations because it flows around the pipes and encapsulates them fully, improving heat transfer efficiency.
Benefits vs traditional screed
Speed of installation
Liquid screed is pumped and flows to level itself. A typical house (100m²) can be screeded in 2-3 hours. Traditional screed takes 2-3 days for the same area because it is hand-laid and trowelled.
Thinner application
Minimum thickness for bonded liquid screed is 25mm, compared to 50mm for traditional screed. This saves floor height, reduces dead load on the structure, and is ideal for renovations where ceiling height is limited.
Better surface flatness
Self-levelling produces a flatter surface (SR2 classification, deviation under 5mm per 2m) without skilled trowelling. Traditional screed relies on the operative's skill and may need additional levelling compound before laying sensitive floor finishes.
Excellent for underfloor heating
Flows completely around UFH pipes, eliminating air pockets. This improves thermal conductivity and system efficiency. Traditional screed can leave voids under pipes, reducing heat transfer.
Lower shrinkage and cracking
Anhydrite screed has lower drying shrinkage than sand-cement screed. Fewer cracks and joints needed (movement joints every 100m² vs every 40m² for cement screed).
Liquid screed vs traditional screed
| Aspect | Liquid Screed | Traditional Screed |
|---|---|---|
| Binder | Anhydrite (calcium sulphate) | Cement |
| Application | Pumped and self-levelling | Hand-laid and trowelled |
| Min thickness (bonded) | 25mm | 50mm |
| Drying time (50mm) | 60 days (natural), 7-14 days (forced) | 7 days walk-on, 28 days full cure |
| Labour rate (100m²) | 2-3 hours | 2-3 days |
| Cost (installed) | £20-30/m² | £15-25/m² |
Thickness and application depth
Bonded screed (over concrete slab)
Minimum 25mm, typical 30-35mm. The screed is bonded to the substrate with a primer. Suitable for refurbishments or areas without underfloor heating.
Unbonded screed (over membrane or insulation)
Minimum 35mm, typical 40-50mm. A separating layer (polythene membrane or insulation board) sits between the substrate and screed. Used in new builds or where thermal/acoustic separation is needed.
Floating screed (over underfloor heating)
Minimum 25mm above the top of the UFH pipes. Typical total thickness is 50-75mm (insulation + pipes + screed). The screed encapsulates the pipes and distributes the heat.
Do not apply liquid screed thicker than 100mm in one pour. For greater depths, use traditional concrete or screed in layers.
Underfloor heating compatibility
Liquid screed is the preferred choice for underfloor heating systems. The fluid mix flows around pipes completely, eliminating air voids. This improves thermal conductivity (heat transfer from pipe to floor surface) by 20-30% compared to traditional screed.
Installation over UFH pipes
Lay insulation boards, then UFH pipes clipped to the insulation. Pressure-test the system and leave pressurised during screeding. Pour liquid screed to minimum 25mm above the top of the pipes. Typical total depth is 50-65mm (insulation + 16mm pipes + 30-50mm screed).
Commissioning the heating
Wait until the screed has dried to below 75% relative humidity (test with a hygrometer). Then commission the UFH system gradually: start at 25°C flow temperature, increase by 5°C per day until design temperature is reached. Run for 7 days, then turn off and allow to cool before laying final flooring.
Drying times and moisture testing
Liquid screed takes longer to dry than cement screed because anhydrite is less permeable. Drying time depends on thickness, temperature, and ventilation.
Natural drying
Rule of thumb: 1 day per mm thickness up to 40mm, then 2 days per mm above that. Examples:
- 30mm screed: 30 days
- 40mm screed: 40 days
- 50mm screed: 40 + (2 x 10) = 60 days
- 65mm screed: 40 + (2 x 25) = 90 days
This assumes good ventilation (windows open or dehumidifier running) and ambient temperature above 10°C. Cold or damp conditions extend drying time significantly.
Forced drying
Use industrial dehumidifiers or the UFH system to accelerate drying. Can reduce drying time to 7-14 days for screeds up to 50mm. Specialist screed contractors offer forced drying as a service (costs £3-5/m²).
Moisture testing
Before laying impermeable flooring (vinyl, resin, sealed wood), test moisture content. Use a calibrated hygrometer or carbide bomb test. Maximum allowable RH: 75% for most floor finishes, 65% for moisture-sensitive finishes (engineered wood). Do not skip testing or the floor finish may fail.
Surface preparation before flooring
Liquid screed forms a weak surface layer (laitance) as it dries. This must be removed before laying flooring, otherwise adhesives will not bond properly.
Laitance removal
After the screed has fully dried and been moisture-tested, lightly sand or mechanically abrade the surface to remove laitance. Then vacuum thoroughly. Some contractors use a light acid wash (dilute phosphoric acid) but this requires neutralising and thorough rinsing.
Priming
Apply a primer suitable for anhydrite screed before laying tiles, vinyl, or resin flooring. This seals the surface and provides a key for adhesives. Check the flooring manufacturer's recommendations for compatible primers.
Cost and budget
Liquid screed is supplied and installed by specialist contractors. Material and labour are usually quoted as a package price per m².
- Bonded screed (30mm): £18-24/m²
- Unbonded screed (40mm): £22-28/m²
- Floating screed over UFH (50mm): £25-32/m²
- Forced drying service: £3-5/m²
Minimum order quantities
Most suppliers have a minimum order (typically 5-10m³, covering 100-200m² at 50mm depth). Small jobs (under 20m²) may incur a surcharge or be rejected. Liquid screed is economical for whole-house or commercial projects, less so for single-room refurbishments.
Example project: 100m² new build ground floor with UFH
- Liquid screed (50mm over UFH): 100m² x £28 = £2,800
- Forced drying: 100m² x £4 = £400
- Total: £3,200
Equivalent traditional screed (hand-laid 65mm): 100m² x £22 = £2,200 (cheaper but slower and less suitable for UFH).
Limitations and considerations
Long drying time
Natural drying takes 2-3 months for typical thicknesses. This can delay project completion unless forced drying is used. Traditional cement screed dries faster (4 weeks for 50mm).
Moisture sensitivity
Anhydrite screed is damaged by prolonged exposure to water. Protect it from rain during construction. Not suitable for wet rooms, commercial kitchens, or outdoor use. Use cement screed in those locations.
Specialist installation required
Liquid screed cannot be mixed or laid by hand. You need a specialist contractor with pumping equipment. This limits flexibility and means you cannot DIY.
Surface preparation essential
Laitance removal and priming are non-negotiable. Skipping these steps causes flooring failures. Budget time and cost for surface prep.
Related guides
Sources
- British Gypsum, "Gypsol Flowing Floor Screed Installation Guide" (2025)
- British Standard BS 8204-7:2020, "Screeds, bases and in-situ floorings. Pumpable self-smoothing screeds"
- Isocrete, "Flowing Screed Technical Manual and Application Guide" (2024)
- The Concrete Society, "Technical Report 70: Guidance on the Specification of Screeds" (2024)
- NHBC Standards Chapter 6.6, "Floor screeds and toppings" (2024)