Screed is the layer of material laid over a concrete structural slab to create a smooth, level surface for final flooring. It's one of the most common sources of programme delays on extension and renovation projects — largely because homeowners and sometimes builders underestimate how long it takes to dry sufficiently for floor coverings to be laid. Getting it right avoids expensive callbacks when tiles crack or wood floors cup months after completion.
Screed Types
Sand and cement (traditional screed) is the most widely used. Mixed at approximately 3–4:1 sand to cement by volume, it's laid at 65–75mm depth over insulation (or 40–50mm bonded directly to a concrete slab). It's reliable, well-understood by trades, and relatively cheap. The downside is drying time: approximately 1 day per mm of depth under normal conditions, meaning a 75mm screed needs 75 days before floor coverings can go down.
Liquid anhydrite (calcium sulphate) screed is poured rather than laid. It self-levels, is ideal over underfloor heating pipes (wrapping around them completely), can be laid thinner (35–40mm), and dries faster — typically 50mm takes 28–35 days. It cannot be used in wet areas without a DPM, and it must be primed before tiling. It's also more expensive per square metre.
Fast-drying screed (brands: Mapei Topcem, Tarmac Topflow) uses modified cement chemistry to reduce drying to 24–72 hours for floor coverings. It costs significantly more but allows faster programme progression in time-critical projects.
Drying vs Curing
Screed cures (gains structural strength) within a few days. Drying — losing moisture to a level where floor coverings can safely be laid — is a different and longer process. Laying tiles over wet screed traps moisture, causing adhesive failure. Laying wood flooring over wet screed causes cupping and buckling as the timber absorbs moisture from below.
The test is a moisture meter reading of <75% relative humidity (RH) in the screed for most floor coverings; <65% RH for solid wood or parquet. Have the screed tested with a calibrated hygrometer, not just a surface probe, before laying any floor covering.
Screed and Underfloor Heating
Where underfloor heating pipes are embedded, the screed must encapsulate them fully with a minimum 25mm cover above the pipe. After initial drying, the UFH system must be commissioned at low temperature (25°C flow) and slowly raised over 7 days before full operation — a commissioning process that also drives out residual moisture. This adds further time to the programme before floor coverings can be laid.
Costs
| Screed type | Cost per m² | Drying time |
|---|---|---|
| Sand and cement | £15–£25 | 1 day per mm |
| Liquid anhydrite | £20–£35 | 28–35 days (50mm) |
| Fast-drying | £45–£80 | 24–72 hours |