Floor Screed for Extensions and Renovations: Types, Costs & Drying Times
Floor screed is a layer of sand-and-cement or specialist material laid over the structural floor slab to create a smooth, level surface ready for the final floor finish. In extensions and ground floor renovations, the screed specification is important — particularly where underfloor heating is installed.
Types of Floor Screed
Traditional sand-and-cement screed: The standard screed used in UK construction for decades — a mix of sharp sand and Portland cement laid at typically 75mm thick (65mm over underfloor heating pipes where those pipes are bonded to the screed). Widely available, relatively low cost.
- Drying time: Approximately 1mm per day, so 75mm = 75 days minimum before normal floor finish installation. This is the critical programme consideration — a traditional screed in an extension poured in Week 6 may not be ready for floor finish until Week 18+.
- Suitable for: All floor finishes, all situations, underfloor heating (with appropriate design)
Anhydrite (calcium sulphate) flowing screed: A liquid screed pumped and poured over the floor rather than trowelled. Typically laid at 50mm (40mm over UFH pipes). Self-levelling and self-compacting, giving a flatter surface than hand-laid sand-and-cement.
- Drying time: Faster than sand-and-cement — approximately 1mm per day up to 40mm, then slower. But fast-drying additives and forced drying with UFH can reduce this significantly.
- Suitable for: Underfloor heating (excellent thermal contact with pipes), large open areas, where a flatter base is required
- Note: Anhydrite screed requires laitance removal (light grinding or acid-etching) before adhesive-fixed floor finishes — not always communicated clearly
Fast-drying screed: Modified sand-and-cement or anhydrite screed with rapid-hardening additives. Can be walked on within hours and ready for floor finish in days rather than weeks. Higher material cost but significant programme benefit.
- Cost premium: 30–60% over standard screed
- Suitable for: Where programme time is critical
Underfloor Heating and Screed
Screed is the most efficient way to embed underfloor heating pipes. The pipes are laid in the insulation layer, the screed is poured over to encase them, and the thermal mass of the screed stores and releases heat. Key UFH + screed considerations:
- The screed must be commissioned (brought up to temperature gradually) before the floor finish is installed — this is the final drying stage
- The floor finish must be compatible with UFH — stone and porcelain tiles are ideal; engineered timber requires a manufacturer-specified maximum temperature; solid timber is not recommended over UFH
Typical Screed Costs in London
| Screed type | Approximate cost per m² (supply and lay) |
|---|---|
| Sand-and-cement screed (75mm) | £20–£35 |
| Anhydrite liquid screed (50mm) | £25–£40 |
| Fast-drying screed | £35–£60 |
RCB Design & Build
RCB Design & Build specifies and coordinates floor screed installation as part of extension and renovation projects — ensuring the screed type, thickness, and programme align with the UFH system and floor finish specification.