What is the mix ratio for floor screed?

The standard mix for floor screed is 1:3 to 1:4 (cement to sharp sand) by volume. A 1:4 mix is most common for domestic floors. The consistency should be semi-dry — when squeezed in your hand, it should hold its shape but not ooze water. Over-wet screed shrinks more and is more prone to cracking.

Screed Calculator — Floor Screed Volume & Materials (UK)

Q: How much screed do I need per square metre?

At the standard 75mm depth, you need 0.075 m³ of screed per m². For sand and cement screed (1:4 mix), this means approximately 22kg of cement and 88kg of sand per m². Self-levelling compound at 5mm depth requires approximately 8-10kg per m².

Q: How thick should floor screed be?

Bonded screed (laid on damp concrete): minimum 25mm. Unbonded screed (on a DPM or insulation): minimum 50mm, typically 65-75mm. Floating screed over underfloor heating: minimum 65mm above the pipes (25mm above the top of the pipes). Self-levelling compound: typically 3-10mm.

Q: How long does screed take to dry?

The general rule is 1mm per day for the first 40mm, then 0.5mm per day after that. So a 75mm screed takes approximately 40 + 70 = 110 days to fully dry (to below 75% RH for tiling). Fast-drying screeds can reduce this to 7-14 days. Liquid anhydrite screed dries faster but cannot be tiled until moisture levels are below 0.5%.

Complete Guide to Floor Screed (UK)

Floor screed is a thin layer of cement-based material laid over a concrete sub-floor to create a smooth, level surface for the final floor covering (tiles, wood, carpet, or vinyl). Getting the screed right is essential — an uneven or poorly cured screed leads to cracked tiles, squeaky floorboards, and uneven carpet.

Types of Floor Screed

Screed Type	Typical Depth	Cost per m²	Drying Time	Best For
Sand & cement (1:4)	50-75mm	£8-£15	1mm/day (first 40mm)	General domestic use, DIY
Self-levelling compound	3-10mm	£6-£12	4-24 hours walkable	Levelling, thin skim over existing floor
Liquid anhydrite	25-50mm	£12-£20	24-48 hours walkable	Underfloor heating, large areas
Fast-drying screed	50-75mm	£15-£25	7-14 days to tile	Time-critical projects
Fibre-reinforced	50-75mm	£10-£18	Similar to standard	Reduced cracking risk

Sand and Cement Screed

Traditional sand and cement screed is the most common type in UK domestic construction. The standard mix is 1:3 to 1:4 (cement to sharp sand) by volume. A 1:4 mix is recommended for most domestic floors — it provides adequate strength (25-28 N/mm²) while being economical with cement.

Screed Depth Guidelines

Bonded screed (on clean concrete with bonding agent): minimum 25mm
Unbonded screed (on DPM or polythene): minimum 50mm, ideally 65mm
Floating screed (on insulation): minimum 65mm, ideally 75mm
Over underfloor heating: minimum 25mm above the top of the pipes, typically 65-75mm total

Self-Levelling Compound

Self-levelling compound (SLC) is a pourable cementitious product used to level uneven floors before tiling or laying other finishes. It flows into low spots and levels itself to create a flat surface. Typical products include Mapei Ultraplan, Weber Flow, and Ardex K301.

Coverage: typically 1.5-1.8 kg per m² per mm depth
A 25kg bag covers approximately 2.5-3 m² at 5mm depth
Walkable in 4-24 hours depending on product
Tileable in 12-48 hours
Always prime the floor with the manufacturer's recommended primer before pouring

Drying Times

Screed drying rule of thumb: 1mm per day for the first 40mm, then 0.5mm per day after that. A 75mm screed needs approximately 110 days to dry to the 75% relative humidity required for tiling. Temperature, ventilation, and ambient humidity all affect drying time. Never tile onto wet screed — it causes adhesive failure and tile popping.

Screed for Underfloor Heating

If you are laying screed over underfloor heating pipes, the screed must fully encapsulate the pipes and provide at least 25mm of coverage above the top of the pipe. The total depth is typically 65-75mm. Liquid anhydrite screed is popular for UFH because it flows around the pipes completely, eliminating air pockets that reduce heat transfer.

Important: The UFH system must be pressure-tested before screeding. Do NOT turn on the heating for at least 21 days after laying sand and cement screed. Then gradually increase the temperature over 5-7 days to prevent thermal shock cracking.

Frequently Asked Questions

How much screed do I need per square metre?

At 75mm depth: 0.075 m³ per m². For a 1:4 sand and cement mix, that is approximately 22kg cement and 88kg sand per m².

How thick should floor screed be?

Bonded: min 25mm. Unbonded: min 50mm (ideally 65-75mm). Over UFH: min 65mm above pipes.

How long does screed take to dry?

1mm/day for first 40mm, 0.5mm/day after. A 75mm screed takes ~110 days to fully dry. Fast-drying screeds reduce this to 7-14 days.

What mix ratio for floor screed?

1:3 to 1:4 (cement:sharp sand). 1:4 is standard for domestic floors. Semi-dry consistency — holds shape when squeezed but does not ooze water.

Screed Calculator — Floor Screed Volume & Materials