Floor screed is the layer laid over a structural concrete slab or insulation to provide a smooth, level surface for the final floor finish. It plays a critical role in floor flatness, load distribution, acoustic performance, and — increasingly in UK new-build and retrofit — in how well underfloor heating (UFH) transfers heat into the room. Two types dominate: traditional sand-cement (also called cementitious screed) and liquid calcium sulphate (anhydrite) screed. Each suits different situations; choosing the wrong one adds cost and delays.
Bonded, Unbonded and Floating Screeds
Before comparing materials, it helps to understand the three structural approaches — which apply to both types:
- Bonded screed is keyed directly into the concrete substrate using a bonding agent (typically SBR). It can be thinner (25–40 mm sand-cement; 25–30 mm liquid) but demands a scrupulously prepared base. Use where headroom is tight.
- Unbonded screed has a polythene slip membrane between screed and slab. Minimum 50 mm sand-cement; 35 mm liquid.
- Floating screed sits on top of insulation (rigid PIR or EPS boards). Requires at least 65 mm sand-cement to prevent cracking under point loads; liquid anhydrite can go to 35–50 mm because it self-levels and has lower shrinkage stress.
The thickness choice directly affects drying time and cost, so it matters from the outset.
Sand-Cement Screed
Sand-cement screed is a site-batched or ready-mixed blend of sharp sand and Portland cement, typically in a 3:1 or 4:1 ratio by weight, with a water-cement ratio kept as low as practicable to reduce shrinkage cracking.
Strengths: widely available, any builder can lay it, tolerates damp substrate, can be laid outside (frost permitting), fully compatible with tile adhesives and most flooring adhesives without preparation.
Weaknesses: slow to dry — the industry rule of thumb is 1 day per mm up to 40 mm, then slower beyond that (so 75 mm floating screed may need 10–12 weeks before carpet or timber can be fixed). It also requires expansion joints at doorways and changes in direction to control cracking.
Minimum thicknesses (BS 8204-1):
| Application | Minimum thickness |
|---|---|
| Bonded to concrete | 25 mm (monolithic) / 40 mm (separate) |
| Unbonded (on membrane) | 50 mm |
| Floating (on insulation) | 65 mm |
| Floating with UFH pipes | 65–75 mm (pipes 25 mm cover) |
The relevant standard is BS 8204-1:2003+A1:2009 (screeds, bases and in-situ flooring — concrete bases and cement sand levelling screeds). Compressive strength class should be specified: C20 is typical for domestic, C25 for commercial.
Liquid Anhydrite Screed
Calcium sulphate (anhydrite) liquid screeds are pumped onto the floor as a self-levelling fluid. The binder is calcium sulphate (from natural anhydrite or as a by-product of power-station flue-gas desulphurisation). The product arrives in a mixer truck, is pumped through a 50 mm hose, and is spread with a dappling bar — a 300 m² slab can be laid in a single day by a two-person crew.
Strengths: flows around UFH pipes without voids; lower minimum thickness (35–50 mm floating); lower shrinkage means fewer expansion joints; slightly better thermal conductivity (1.0–1.2 W/mK vs 0.8–1.0 W/mK for sand-cement), which matters for UFH efficiency.
Weaknesses: must be primed before tile adhesive or resin floor finish (laitance on the surface must be abraded or primed); cannot be used externally; must be protected from rain for at least 24 hours; not suitable where the substrate is permanently wet or where sulphate attack on adjacent concrete is a risk. Check the screed manufacturer’s data sheet: some require forced ventilation during the first 48–72 hours to accelerate drying.
Minimum thicknesses:
| Application | Minimum thickness |
|---|---|
| Bonded | 25 mm |
| Unbonded | 35 mm |
| Floating (on insulation) | 35 mm |
| Floating with UFH pipes | 30–40 mm (15 mm min cover over pipe) |
Calcium sulphate screeds are covered by BS EN 13813:2002 (screed material and floor screeds — properties and requirements). Specify compressive strength class CA-C20-F4 minimum for domestic floating applications.
Drying Times Compared
Drying time governs the programme on site. Both materials must reach the equilibrium moisture content (EMC) acceptable to the floor finish before anything goes on top — typically below 75% RH (measured with a hygrometer in a sealed hood per BS 8201).
| Screed type | Approx drying rate | 65 mm floating (no heating) |
|---|---|---|
| Sand-cement | 1 mm/day up to 40 mm; 0.5 mm/day beyond | 11–13 weeks |
| Liquid anhydrite | 1 mm/day up to 40 mm; faster with ventilation | 5–7 weeks |
| Liquid anhydrite (with heating protocol) | Conditioning cycle from week 3 | 4–5 weeks |
Anhydrite screeds benefit from a graduated heating protocol: from week 3 (once initial set is complete), UFH is ramped up gradually from 20°C to maximum output, then ramped back, then held at 20°C — this drives off moisture without thermal shock cracking. Most screed manufacturers publish a specific protocol; follow it and get a written sign-off.
Cost Per m²
Costs below are supply and installation, England, 2026 mid-range. They exclude concrete slab, insulation, and floor finish.
| Screed type | Typical cost (£/m²) | Notes |
|---|---|---|
| Sand-cement (site-batched) | £8–£12 | Suitable for small areas |
| Sand-cement (ready-mixed) | £10–£14 | Better consistency |
| Liquid anhydrite | £12–£18 | Efficiency gains on large floors |
| Liquid anhydrite (small pour < 100 m²) | £15–£22 | Pump call-out cost spread over less area |
For areas under 150 m², sand-cement is often cheaper overall once you account for the liquid screed pump mobilisation charge (typically £300–£500). Above 250 m², liquid anhydrite often wins on programme and quality.
UFH Compatibility
Liquid anhydrite is the preferred screed for wet underfloor heating systems in UK housebuilding for three reasons: lower cover requirement (15 mm over pipe vs 25 mm for sand-cement), fewer joints across the heated zone, and better conductivity. However, sand-cement screeds reinforced with polypropylene fibres are widely used and perform adequately where system water temperature is kept at or below 45°C (as is typical for heat-pump-driven systems compliant with Part L of the Building Regulations).
Both screeds require the UFH pipework to be pressurised and tested before the pour. Pipes should be held at 3 bar during the pour to prevent flotation.
[!tip] Specifier checklist Confirm with your screed supplier: minimum thickness for your specific product over your insulation type; EMC target for your intended floor finish; primer requirements before adhesive; and whether a heating protocol certificate is included.
UK Standards Summary
- BS 8204-1:2003+A1:2009 — cementitious screeds and bases
- BS EN 13813:2002 — screed material and floor screed properties
- BS 8201 — Code of practice for installation of flooring
- Building Regs Part C — moisture resistance (subfloor damp-proofing must be continuous with DPC)
- Building Regs Part L — thermal performance (floor U-value ≤ 0.25 W/m²K in new dwellings; the insulation below the screed, not the screed itself, delivers this)
