Screed is the layer of material — typically a cementitious mix — applied to a structural concrete floor slab or existing substrate to create a smooth, level surface ready for finished floor coverings. It is an essential component of most UK ground floors and any floor incorporating underfloor heating, and the choice of screed type significantly affects both cost and programme.
What Is Screed Used For?
Screed serves several functions:
- Levelling an uneven or rough concrete slab to receive tiles, stone, engineered timber, vinyl or carpet
- Encapsulating underfloor heating pipes or electric mat systems
- Achieving the correct finished floor level in new builds and extensions
- Providing a consistent base that allows floor coverings to be bonded or laid evenly
Screed is distinct from concrete: concrete is a structural material; screed is a wearing or receiving surface. Screed should not be relied on for structural load-carrying.
Types of Screed
| Type | Description | Thickness | Drying Rate | Cost (per m², installed) |
|---|---|---|---|---|
| Traditional sand-and-cement (unbonded) | Mixed on site or bagged; laid over a slip membrane | 65–75 mm | 1 day per mm | £15–£22 |
| Traditional sand-and-cement (bonded) | Laid directly onto primed concrete | 25–40 mm | 1 day per mm | £18–£25 |
| Calcium sulphate (anhydrite) liquid screed | Pumpable; self-compacting; contains no cement | 35–50 mm | 1 day per mm + drying aids | £22–£35 |
| Self-levelling compound (SLC) | Thin repair or finishing layer; polymer-modified | 3–10 mm | 24–48 hrs typically | £12–£25 |
| Rapid-drying screed | Modified binders that accelerate moisture release | 40–65 mm | 1 day per 1.5 mm | £28–£45 |
| Heated screed (UFH encapsulant) | Any of the above; must be thermal-cycle commissioning | As above | Extended | As above + UFH cost |
Sand-and-Cement Screed
The traditional and most widely used type in the UK, mixed to approximately 1:3 or 1:4.5 cement:sharp sand by volume. It is placed as a semi-dry mix (sometimes described as a “paving consistency” or “floor screed consistency”) and tamped or floated level. Minimal equipment needed, wide contractor base, and easily repaired. The main drawback is its long natural drying time — approximately one week per 10 mm depth, or roughly 1 day per mm as a practical rule.
Liquid Calcium Sulphate Screed (Anhydrite)
Increasingly common in UK construction, anhydrite screed is pumped as a free-flowing liquid and self-compacts. It is particularly well-suited to underfloor heating installations because it has better thermal conductivity than sand-and-cement and can be installed at lower depths (35–45 mm over pipes, compared to 65–75 mm for traditional screed), reducing floor build-up. Key disadvantages: it requires de-watering ventilation and drying-aid heating before most floor coverings can be applied, and it must not be used in areas of high moisture without specialist surface treatment.
Self-Levelling Compound
Not a structural screed but a thin repair or levelling product poured over an existing sub-floor to fill low spots and create a smooth, flat surface. Useful for renovations where the existing floor is serviceable but uneven. Not suitable as a standalone screed over new construction.
Screed Depths and Building Regulations
In England and Wales, Part C of the Building Regulations (Site preparation and resistance to contaminants and moisture) requires that ground floors are protected from moisture. A damp-proof membrane (DPM) — typically 1200-gauge polythene laid over the slab — should be incorporated. Screed is laid over the DPM and insulation.
Minimum screed depths:
- Bonded screed (directly onto concrete, no membrane): minimum 25 mm, typically 25–40 mm
- Unbonded screed (over DPM or insulation): minimum 50 mm, typically 65–75 mm
- Floating screed over UFH (anhydrite): minimum 25 mm over pipes for heated rooms
- Floating screed over UFH (sand-and-cement): minimum 65 mm over pipes
When underfloor heating is incorporated, the screed must be commissioned (heated progressively from cold to operating temperature and back) before floor coverings are laid. This process typically takes two to three weeks.
Screed Drying Times
This is the most frequently misunderstood aspect of screed. Applying floor coverings — particularly moisture-sensitive materials such as engineered timber, solid wood, or adhesive-fixed tiles — before the screed has dried to the required moisture content causes significant damage: timber swells and cups, adhesives fail, tiles debond.
| Screed Type | Rule of Thumb | Typical Wait (65 mm) |
|---|---|---|
| Sand-and-cement | 1 day per mm depth | 65 days |
| Anhydrite liquid | 1 day per mm + drying aids | 45–75 days (with dehumidification) |
| Rapid-drying | 1 day per 1.5 mm | 40–45 days |
| Heated rapidly (specialised) | Per manufacturer’s specification | 10–28 days |
Required moisture content before laying:
- Most floor coverings: ≤75% relative humidity measured in situ (using a hygrometer probe or Tramex meter)
- Moisture-sensitive wood flooring: ≤65% RH
- Adhesive-critical applications: check adhesive manufacturer specification
Dehumidification, forced ventilation and underfloor heating commissioning all accelerate screed drying but do not eliminate the requirement for moisture testing before flooring.
Underfloor Heating and Screed
UFH is typically embedded in screed rather than below it to maximise heat transfer to the room above. Pipe centres are typically 100–200 mm for water-fed systems. The screed acts as a thermal store and radiator.
Key specification points:
- Anhydrite screed: better thermal conductivity (approximately 1.0–1.5 W/(m·K) depending on mix)
- Sand-and-cement screed: approximately 0.9–1.1 W/(m·K)
- Maximum aggregate size for screed over UFH pipes: 10 mm to ensure pipes are fully encased
- Always commission the UFH system before laying finished floor covering
Cost Summary (2026)
| Scope | Typical Cost |
|---|---|
| Traditional screed, average room (25 m²) | £375–£550 |
| Liquid screed, average room (25 m²) | £550–£875 |
| Rapid-drying screed, average room | £700–£1,125 |
| Self-levelling compound, average room | £300–£625 |
| Full ground floor, 4-bed house (80 m²) | £1,200–£3,500 |
| UFH installation before screeding | £60–£120/m² additional |
Materials represent roughly 40–60% of the installed cost; specialist pumped liquid screeds carry a higher proportion of materials cost.
Choosing a Screeding Contractor
Not all general builders have experience with liquid anhydrite screeds. For pumped screeds, look for contractors using equipment supplied by manufacturers such as Gypsol, Cemfloor or Lafarge. Ask for evidence of moisture testing on previous projects before floor coverings were laid — this is the clearest indicator of a contractor who understands the critical interface between screeding and flooring.
