What is cementitious screed?
Cementitious screed is a floor screed that uses cement as its binder, mixed with sand or fine aggregate, water and often an admixture. It includes traditional semi dry sand and cement, fine concrete screed, polymer modified screed and cement based flowing screed. It is laid over a slab or insulation to give a level base for the floor finish.
The Mineral Products Association notes that the majority of cement sand levelling screeds consist of a cementitious material, fine aggregate and an admixture. The opposite group is calcium sulphate screed, where the binder is anhydrite or a similar calcium sulphate rather than cement.
Types of cementitious screed
- Semi dry sand and cement. Mixed with just enough water to compact, then laid by hand and ruled off. Flowcrete gives typical thicknesses of 65 to 75mm when unbonded or floating (bonded semi dry screeds can be 25 to 40mm) and hand laying rates of around 125 to 150m² a day. This is our sand and cement screed.
- Fine concrete screed. A cement screed with aggregate up to 10mm, for which BS 8204 recommends proportions between 1:4 and 1:5.
- Polymer modified screed. Covered by BS 8204-3, the part of the standard for polymer modified cementitious levelling and wearing screeds.
- Cement based flowing screed. A pumped, self levelling screed with a cement binder. Cemfloor, for example, is a mix of sand, cement, water, superplasticiser and a proprietary binder.
- Modified and fibre screeds. Sand and cement with rapid drying additives or polypropylene fibres, such as our fibre reinforced screed.
Typical mix and strength
BS 8204-1 recommends nominal proportions by weight of 1 part cement to 4 parts fine aggregate, with a lower limit of 1:3 and an upper limit of 1:4.5. The MPA notes the standard states a screed made in these proportions would meet the expected in situ crushing resistance, as would a screed of compressive strength class C20 or C25. Factory produced screeds must conform to BS EN 13813, which classifies them by compressive and flexural strength.
Typical depths
| Construction | Semi dry sand and cement (MPA) | Cement based liquid (Cemfloor) |
|---|---|---|
| Bonded | 25mm minimum, 40mm normally specified | 25mm minimum |
| Unbonded | 50mm minimum, 70mm specified | 30mm minimum |
| Floating | 75mm minimum, 65mm normally accepted in housing | 35mm domestic, 40mm commercial |
Swipe the table sideways to see every column.
Over underfloor heating, Cemfloor specifies at least 25mm of its screed above the pipes. Our screed thickness guide compares every build-up.
Cementitious or calcium sulphate?
Both are good screeds; they suit different floors.
- Water and wet areas. The Concrete Centre notes that calcium sulphate screeds are generally unsuitable for damp areas. Sand and cement screed works in wet areas where calcium sulphate cannot be used.
- Reinforcement. Calcium sulphate is corrosive to steel in damp conditions and must not be reinforced, whereas cement screeds can take steel mesh or fibres.
- Laitance. Calcium sulphate screeds form a surface laitance that has to be removed before tiling. Cemfloor says cement based tile adhesives can go straight onto its screed without sanding, sealing or priming.
- Curling and bay sizes. The Designing Buildings Wiki notes that unbonded sand and cement screeds are more prone to curling when they dry quickly, while calcium sulphate screeds shrink less and are not prone to it. Cemfloor allows bays of up to 150m² unheated and 100m² heated without control joints.
- Adhesive compatibility. The Tile Association warns that cement in tile adhesive can react with the gypsum in a calcium sulphate screed to form ettringite, which can debond tiles, so the primer and adhesive must suit the screed type.
Drying and curing
The MPA advises covering a cement sand screed with plastic sheeting or similar for at least five days to retain moisture while it cures. Drying is then roughly 1mm a day up to 40mm and slower beyond that in good conditions, a rule of thumb Gyvlon attributes to BS 8204 for both traditional and flowing screeds. Screeds are generally regarded as dry when they reach 75% RH or below, and Cemfloor gives 75% RH as the usual target for moisture sensitive coverings such as timber and vinyl. If underfloor heating is fitted, Cemfloor allows it to be commissioned and used to force dry the screed after 7 days.
Common mistakes
- Laying unbonded or floating screed too thin. Thin unbonded cement screeds are the ones most likely to curl and sound hollow at joints and edges.
- Drying too fast. The MPA says adequate curing is essential, and fast drying makes unbonded sand and cement screeds more prone to curling.
- Poor compaction of semi dry screed. Voids left in the mix weaken it.
- Treating cement and calcium sulphate screeds as interchangeable when choosing primers and adhesives.
How this relates to our services
We lay cementitious screeds: hand laid sand and cement screed, fibre reinforced sand and cement, and a cement based flowing screed where calcium sulphate is not suitable. For most heated floors and large open rooms, calcium sulphate anhydrite flow screed is still the better fit. Our liquid screed page explains how the pumped options compare. Tell us what is going on top, and whether the floor may get wet, and we will recommend the right binder.
Sources
- Mortar data sheet 22: Screeds, Mineral Products Association, issue 3, October 2023
- Floor screeds, The Concrete Centre, accessed 25 October 2026
- Installation of Cemfloor screed, Cemfloor, accessed 25 October 2026
- What's the difference between sand and cement and self levelling screeds?, Flowcrete UK, accessed 25 October 2026
- Screed, Designing Buildings Wiki, accessed 25 October 2026
- The Tiling Guide, The Tile Association, accessed 25 October 2026
- Post installation guidance, Gyvlon (Anhydritec), 2024
Part of our screeding glossary, which defines every term we use on site.
