Concrete Screed Mix Ratio: UK Guide
Quick summary
Standard UK screed mix is 1:4 cement to sharp sand (by volume). Use semi-dry consistency: damp enough to bind when squeezed, not wet. Apply at 50-75mm thickness. Cures to 20-25 MPa compressive strength. For heavy-duty use, increase to 1:3 mix (25-30 MPa). Always use washed sharp sand, never soft building sand.
Standard screed mix ratios
UK floor screeds are made from Portland cement and sharp sand mixed with water. The ratio determines strength, workability, and durability.
1:4 mix (1 part cement, 4 parts sharp sand)
Most common ratio for domestic and light commercial floors. Compressive strength: 20-25 MPa after 28 days. Suitable for foot traffic, domestic furniture, and light storage. Easier to work than stronger mixes. This is the default mix unless you have a specific reason to use a richer blend.
1:3 mix (1 part cement, 3 parts sharp sand)
Stronger mix for heavy-duty applications. Compressive strength: 25-30 MPa after 28 days. Use for commercial kitchens, warehouses, workshops, or areas with racking and heavy loads. Harder to trowel smooth and more prone to shrinkage cracks if not cured properly. More expensive due to higher cement content.
1:5 mix (weaker, rarely used)
Lower strength (15-18 MPa). Only suitable for very light-duty areas or as a levelling layer over existing screed. Not recommended for new installations. Risk of dusting and low abrasion resistance.
Sharp sand vs building sand
The type of sand is as important as the mix ratio. Always use sharp sand (also called concreting sand or grit sand) for screed. Do not use soft building sand (bricklaying sand).
Sharp sand
Coarse angular grains, typically 0-4mm particle size. Provides strength and reduces shrinkage. Specified in BS 8204 for floor screeds. Washed sharp sand is cleanest and gives best results. Costs £3-5 per 25kg bag or £50-70 per tonne (bulk).
Building sand (soft sand)
Fine rounded grains. Used for bricklaying mortar and rendering. Too fine for screed: it shrinks excessively, cracks, and has low strength. Never use it for floor screeds even if someone suggests it is cheaper.
Grading specification
Sharp sand for screed should conform to BS EN 13139 (aggregates for mortar). Grading zone M (medium) or C (coarse). Check the supplier's datasheet or ask for concreting sand. If buying from a builders merchant, specify "sharp sand for screed" to avoid confusion with other sand types.
Water content and consistency
Water content dramatically affects screed strength and durability. Too much water weakens the screed and causes shrinkage cracks. Too little and the mix will not compact or finish properly.
Semi-dry screed (most common)
Water-cement ratio: 0.4-0.5 (0.4-0.5 litres water per kg cement). The mix should be damp, not wet. When you squeeze a handful, it binds together and releases a small amount of moisture, but does not drip. This is the standard UK method. Easier to level and trowel. Lower shrinkage. Walk-on after 24 hours.
Wet screed (less common)
Water-cement ratio: 0.6-0.7. The mix is fluid and pourable. Self-levels to some extent. Easier to pump and spread over large areas but much higher shrinkage and weaker final strength. Requires more skill to finish without surface defects. Not recommended for DIY.
How to measure water content
For a 1:4 mix using 25kg cement: you need 100kg sharp sand (4 x 25kg) and 10-12.5 litres of water (0.4-0.5 litres per kg cement). Add water gradually and mix thoroughly. Stop when the consistency is right (squeeze test). Do not add extra water to make the mix easier to spread. It will weaken the screed.
Mixing methods
Hand mixing (small jobs)
Measure cement and sand by volume (buckets or shovels). Use a 1:4 ratio: 1 bucket cement to 4 buckets sand. Mix dry first on a clean board or in a wheelbarrow. Add water gradually while mixing with a shovel. Mix thoroughly until the colour is uniform. Labour-intensive and inconsistent. Only viable for small areas (under 10m²).
Cement mixer (most common)
Measure materials by volume or weight. Add half the water and half the sand to the mixer first. Then add the cement, then the remaining sand and water. Mix for 3-5 minutes until uniform. Discharge and use immediately. A standard 150-litre mixer handles one batch at a time (enough for around 0.15m³ of screed).
Forced-action mixer (best quality)
Pan mixer with rotating blades (not a drum mixer). Produces more uniform mix with better water distribution. Used by professionals for large jobs. Hire costs £30-50 per day. Worth it for jobs over 30m².
Ready-mix screed
Order from a ready-mix supplier who delivers in a truck. Only economical for large jobs (50m²+). Consistency is guaranteed. Costs £80-120 per m³ delivered. Must have good site access and enough labour to spread it quickly before it stiffens.
Additives and modifications
Plasticiser
Liquid additive that improves workability without adding extra water. Typical dose: 100-200ml per 25kg cement. Makes the mix easier to trowel and finish. Does not increase strength but reduces the temptation to add too much water. Costs £5-10 per litre. Widely used by professionals.
Fibres
Polypropylene fibres (6-12mm long) added to the mix to reduce shrinkage cracking. Typical dose: 600g per m³ (around 40g per 25kg cement bag). Does not increase strength but improves crack resistance in large screed areas. Costs £10-15 per 900g bag. Useful for screeds over 50m² or in areas with movement joints more than 5m apart.
Accelerators and retarders
Accelerators speed up setting (useful in cold weather). Retarders slow setting (useful in hot weather or for large pours). Not commonly used in domestic work. Follow manufacturer's dosage instructions carefully.
Strength and compressive testing
Screed strength is measured as compressive strength in megapascals (MPa). Higher MPa means stronger screed.
| Mix ratio | Compressive strength (28 days) | Typical use |
|---|---|---|
| 1:3 | 25-30 MPa | Heavy-duty commercial, industrial |
| 1:4 | 20-25 MPa | Domestic, light commercial (standard) |
| 1:5 | 15-18 MPa | Light-duty only (rarely used) |
Strength develops over time. Screed reaches around 70% of final strength after 7 days, 90% after 14 days, full strength after 28 days. Do not subject the screed to heavy loads until it has cured for at least 7 days.
Thickness requirements
Screed thickness depends on the type of screed and the loads it must carry.
Bonded screed (laid on clean, primed concrete)
Minimum 25mm, typical 40-50mm. The screed bonds directly to the substrate. Used for refurbishments or toppings over existing concrete. Thinner than unbonded screed because it derives support from the base.
Unbonded screed (laid on polythene membrane)
Minimum 50mm, typical 65-75mm. The membrane prevents bonding, so the screed must be self-supporting. Used when the substrate is contaminated or thermal/acoustic separation is needed.
Floating screed (laid on insulation)
Minimum 65mm for domestic, 75mm for commercial. The screed floats on compressible insulation, so it needs thickness to resist loads without flexing. Common in new builds and underfloor heating installations.
Curing and drying
Screed must cure (gain strength) and dry (lose moisture) before final flooring is laid.
Curing
Keep the screed damp for the first 7 days after laying. Cover with polythene or spray with water daily. This allows the cement to hydrate fully and develop strength. Do not let it dry out too quickly or it will crack and dust.
Drying
After curing, the screed must dry to below 75% relative humidity before most floor finishes can be laid. Drying time: 1 day per mm thickness up to 50mm. A 50mm screed takes around 50 days to dry in normal conditions (good ventilation, 15-20°C temperature). Thicker screeds or damp conditions extend drying time significantly.
Testing moisture
Use a calibrated hygrometer or carbide bomb test. Do not rely on surface dryness. Laying impermeable flooring (vinyl, resin) on wet screed traps moisture and causes failure.
Cost and material quantities
Materials for a 1:4 screed at 50mm thickness covering 10m² (0.5m³ of screed):
- Cement: 6 x 25kg bags = £30-40
- Sharp sand: 600kg (0.6 tonne) = £35-45 (bulk) or £70-90 (bagged)
- Plasticiser (optional): £5
- Total material cost: £70-135
Labour (if hiring a professional): £150-250 for 10m² including mixing, laying, levelling, and finishing. Total installed cost: £220-385.
Related guides
Sources
- British Standard BS 8204-1:2022, "Screeds, bases and in-situ floorings. Concrete bases and cementitious levelling screeds to receive floorings"
- British Standard BS EN 13139:2002, "Aggregates for mortar"
- The Concrete Society, "Technical Report 70: Guidance on the Specification of Screeds" (2024)
- Cement Admixtures Association, "Guide to Admixtures for Concrete and Mortar" (2024)
- NHBC Standards Chapter 6.6, "Floor screeds and toppings" (2024)