A customer asked me to look at his conservatory floor before they started a kitchen-diner extension. The conservatory had been built over a sand:cement screed on a ground bearing slab, with underfloor heating pipes running through it. Several tiles had debonded and two had cracked clean through. The screed had been laid at 35mm over the UFH pipes - well under the 65mm minimum for sand:cement over underfloor heating. The pipes had created stress planes in the thin screed and the thermal expansion cycles had done the rest.
The screed type specifies more than just what to order. It determines minimum depth, drying schedule, underfloor heating compatibility, and floor covering requirements.
Sand:cement screed (traditional)
The standard floor screed of UK construction. Mixed on site (or delivered as a dry-mix in bags or bulk) from Portland cement and sharp sand at a 1:3-1:4.5 ratio by weight.
Strengths: Widely understood, easy to repair, relatively cheap materials, structural strength for load-bearing applications, good compressive strength.
Weaknesses: Long drying time (1mm per day rule), minimum depth requirements (65mm floating, 25mm bonded), not ideal over UFH without deep cover, needs careful curing to prevent shrinkage cracking.
Typical depths:
- Bonded to concrete slab: 25-40mm
- Unbonded (over DPM): 50mm minimum
- Floating (over insulation): 65-75mm
- Over UFH pipes: 65mm above top of pipe
Best for: Standard new build and extension floors, renovation projects where programme time is flexible, structural applications.
Mix ratio
1:3 to 1:4.5 cement:sharp sand by weight. The mix should be semi-dry (holds its shape when squeezed but does not exude water) for manual laying. A wetter mix is used for pump applications.
Anhydrite (calcium sulphate) liquid screed
A factory-supplied liquid screed pumped in rather than mixed on site. The binder is calcium sulphate (anhydrite or hemihydrate gypsum) rather than Portland cement.
Strengths: Self-levelling (eliminates manual screeding), excellent thermal conductivity for UFH, shallow depth over UFH pipes (30mm above pipe minimum), very flat finish, large areas covered quickly (1,500-2,500 m2 per day with a pump), reduced cracking risk.
Weaknesses: More expensive than sand:cement materials, laitance must be removed before tiling, incompatible with adhesives that need a cementitious surface without special primer, sensitive to water - will re-soften if regularly wetted, cannot be used in wet areas without appropriate tanking, longer drying time at greater depths.
Typical depths:
- Over UFH pipes: 30mm above top of pipe
- Maximum practical depth: 100mm
- Minimum depth: 25mm
Best for: Underfloor heating systems, large floor areas, new build where a fast programme is needed after pouring.
Polymer-modified screed
Sand:cement screed with polymer additives (typically SBR latex or similar) added to the mixing water. The polymer improves flexibility, adhesion and resistance to cracking.
Strengths: Greater flexibility than plain sand:cement (reduces cracking), improved bond strength, can be used at reduced depths in some applications.
Weaknesses: Higher material cost, mixing protocol is more exacting (polymer must be added correctly), not needed for standard applications where plain screed performs fine.
Best for: Bonded applications over substrates with movement risk, repairs and patches, heated screeds over existing floors.
Rapid-setting screed
Portland cement screed with accelerator additives that achieve structural strength in 2-4 hours rather than days. The floor can be walked on and loaded much sooner.
Strengths: Fast turnaround - can be tiled the same day in some cases, useful for commercial or phased renovation projects where access needs to be maintained.
Weaknesses: Significantly more expensive than standard screed, mixing and laying must be fast (short pot life), moisture content still needs to reach correct level before floor coverings despite fast structural strength.
Best for: Commercial projects, renovations with tight timescales, where the programme cost of waiting justifies the material premium.
Comparison table
| Screed type | Min depth (floating) | Drying rate | UFH compatible | Relative cost |
|---|---|---|---|---|
| Sand:cement | 65mm | 1mm/day | Yes (65mm cover) | Low |
| Anhydrite | 30mm over pipe | 1mm/day to 40mm | Yes (30mm cover) | Medium |
| Polymer-modified | 50mm | 1mm/day | Yes | Medium |
| Rapid-setting | 50mm | Fast strength, moisture still time-dependent | Yes | High |
My tips on choosing floor screed type
Let the UFH specification drive the choice. Most UFH manufacturers specify anhydrite as the preferred screed. If you are installing underfloor heating and have a choice, anhydrite at 30mm over pipe versus sand:cement at 65mm over pipe is a significant depth saving that affects build-up height, window sill heights and external DPC levels.
Consider the programme. Sand:cement is cheaper but takes longer. On a new build where the programme allows 8-10 weeks of screed drying before second fix, this is fine. On a renovation with a tenant moving in, a rapid-set or anhydrite with a heating programme may justify the extra cost.
Anhydrite requires additional steps. The laitance removal and the requirement for a compatible primer before tiling are sometimes forgotten by contractors unfamiliar with the product. Confirm your tile contractor has experience with anhydrite before committing.
Match the repair material to the original. If you are patching an existing screed, use the same type. Sand:cement on anhydrite, or vice versa, creates incompatibility at the join.
Use the Floor Screed Calculator to work out material quantities for your project.