Concrete Screed Calculator

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

Multiply the floor area by the screed depth to get the volume in m3. For a 75 mm sand-cement screed over 25 m2: 25 x 0.075 = 1.875 m3. For dry weight, a sand-cement screed weighs approximately 2,000 kg/m3, so 1.875 m3 = 3.75 tonnes of dry material. A standard semi-dry 1:3.5 mix uses approximately 350 kg of cement and 1,200 kg of sharp sand per m3 of screed. Our calculator gives you the exact quantities of cement and sand, plus the number of 25 kg bags if you are mixing by hand.

Q: What is the minimum screed depth?

Minimum screed depth depends on the screed type and how it is bonded to the base. Bonded screed (mechanically keyed or bonded to the structural slab): minimum 25-40 mm. Unbonded screed (laid over a polythene sheet separating layer): minimum 50-65 mm. Floating screed (laid over insulation): minimum 65-75 mm. Liquid or flowing screeds (calcium sulphate): minimum 25-35 mm. When underfloor heating pipes are embedded, the minimum depth above the pipes is typically 25-30 mm for sand-cement screed and 20-25 mm for liquid screed.

Q: What is the difference between sand-cement screed and liquid screed?

Sand-cement screed is a traditional semi-dry or wet mix of cement and sharp sand, laid and trowelled by hand. It has a compressive strength of around 20-25 N/mm2 when cured. It can be walked on after 24-48 hours and is ready for floor coverings after approximately 4 weeks curing time (1 day per mm thickness is a general guide). Liquid screed (calcium sulphate or anhydrite) is a pumpable flowing product that self-levels, requiring minimal labour. It achieves a flatter finish with fewer joints and is better suited to underfloor heating. It typically requires 6-8 weeks drying time before floor coverings are applied.

Q: How long does screed take to dry before tiling or floor coverings?

Sand-cement screed: approximately 1 day drying time per 1 mm of thickness, so a 75 mm screed takes approximately 75 days to be fully dry. In practice, floor coverings can often be applied after 28-42 days for moderate-thickness screeds in well-ventilated conditions. Liquid (calcium sulphate) screed: typically 6-8 weeks for standard depths. Always test with a hygrometer before applying moisture-sensitive coverings - the screed should be below 75% RH (approximately 3% moisture content by weight) before tiling, and below 65% RH before laying wood or carpet.

Q: What Building Regulations apply to floor screeds?

Floor screeds must comply with Part A (Structure) for loadbearing applications and Part L (Energy Efficiency) when used above insulation - the insulation layer must achieve the required floor U-value (maximum 0.25 W/m2K for new ground floors in England). For screeds containing underfloor heating pipes, Part L requires the system to be designed to BS EN 1264 to ensure efficient heat distribution. For wet areas such as shower rooms, Part C requires adequate waterproofing of the screed and any joints before tiling.

Q: Can I screed over underfloor heating pipes?

Yes - screeding over underfloor heating pipes is the standard installation method for in-screed UFH systems. The pipes are secured to the insulation before screeding. Minimum cover above the pipes is 25-30 mm for sand-cement screed and 20-25 mm for liquid screed. Before applying screed, the system should be pressure-tested and the pipes filled with water. After screeding, wait until the screed has achieved full strength (usually 28 days for sand-cement) before commissioning the heating. The system should then be brought up to temperature gradually over several days.

Add each area, choose your screed type and thickness, and get volumes, materials and drying times

Metric (m) Imperial (ft / in)

Areas

Screed Type

Screed Thickness (mm) Unbonded on insulation: 65-75mm • Bonded to concrete: 25-40mm • UFH: min 65mm over pipes

Mix Ratio (cement : sand)

Waste / Overage (%)

Allow 5-10% for uneven sub-floors and mixing losses

Include underfloor heating (UFH) options

Job Costing

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Results

Total Area
-
m²

Screed Volume

m²

Cement Bags

x 25kg bags

Sand

tonnes

UFH Pipe

- loops / ports

Edge Strip Rolls

rolls to order

Estimated Drying / Curing Times

Walk on

Light use / tiling

Full cure / floor covering

Drying times are estimates only and depend on temperature, humidity and ventilation. Always check with your screed supplier.

Area Breakdown

Area	Floor Area	Volume	Cement Bags	Sand (t)

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Frequently Asked Questions

How much screed do I need per square metre?

Multiply floor area by screed depth. A 75 mm screed over 25 m²: 25 x 0.075 = 1.875 m³. Sand-cement screed weighs approximately 2,000 kg/m³. A standard 1:3.5 mix uses approximately 350 kg cement + 1,200 kg sharp sand per m³. The calculator above gives exact quantities and bag counts.

What is the minimum screed depth?

Bonded screed (keyed to slab): minimum 25-40 mm. Unbonded screed (over polythene sheet): minimum 50-65 mm. Floating screed (over insulation): minimum 65-75 mm. Liquid screed (calcium sulphate): minimum 25-35 mm. Over UFH pipes: minimum 25-30 mm cover for sand-cement, 20-25 mm for liquid screed.

What is the difference between sand-cement screed and liquid screed?

Sand-cement screed: traditional semi-dry or wet mix, trowelled by hand, walkable in 24-48 hours, ready for floor coverings after ~4 weeks (1 day per mm). Liquid screed (calcium sulphate): pumpable, self-levelling, flatter finish, better for UFH, but requires 6-8 weeks drying time before floor coverings.

How long does screed take to dry before tiling or floor coverings?

Sand-cement screed: approximately 1 day per mm thickness (75 mm = ~75 days fully dry). Liquid screed: typically 6-8 weeks. Before tiling: screed must be below 75% RH. Before wood or carpet: below 65% RH. Always test with a hygrometer - do not rely on time estimates alone.

What Building Regulations apply to floor screeds?

Part A for loadbearing applications. Part L when used above insulation - ground floor maximum U-value 0.25 W/m²K in England. UFH systems must be designed to BS EN 1264 for efficient heat distribution. Part C requires waterproofing of screed in wet areas before tiling.

Can I screed over underfloor heating pipes?

Yes - standard method for in-screed UFH. Pipes are secured to insulation before screeding. Minimum cover above pipes: 25-30 mm (sand-cement) or 20-25 mm (liquid screed). Pressure-test pipes before screeding. Wait 28 days (sand-cement) for full strength before commissioning heating, then bring up to temperature gradually over several days.

UK Building Regulations BS 8204 / Part C / Part M

Screed Type Guide

Sand & cement (site mixed) - Traditional 1:3 to 1:4.5 mix. Most common in UK domestic. Needs 24-48h before walking on, weeks to fully dry.
Sand & cement (bagged) - Pre-mixed for convenience. Same drying characteristics as site-mixed. Good for small areas or repairs.
Liquid anhydrite - Calcium sulphate-based. Pump-applied, self-levelling. Thinner minimum depths. Cannot be used in wet rooms. Needs priming before tiling.
Liquid cement - Cementitious flowing screed. Self-levelling, pump-applied. Can be used in wet rooms. Faster than traditional but slower than anhydrite.
Fast-track - Walk on in 4-6 hours. Tile in 24-48h. Premium cost but critical path savings on fast build programmes.

Thickness Guide

Bonded to concrete slab - 25-40mm. Must be mechanically keyed and primed.
Unbonded (on DPM) - 50-75mm minimum. Laid on a polythene slip membrane.
Floating (on insulation) - 65-75mm minimum for sand & cement. 35-45mm for liquid screed.
Underfloor heating - Min 65mm over pipe centres for sand & cement. Min 30-45mm for liquid screed. Check UFH supplier spec.
Liquid screed minimum - 35mm unbonded, 25mm bonded. Much thinner than traditional screed.

Drying Tips

Sand & cement screed needs adequate ventilation to dry. Open windows and use dehumidifiers - do not use direct heat in the first week.

Anhydrite screed must be forced-dried with heating and ventilation before tiling. A priming coat (diluted PVA or specialist primer) is essential before applying adhesive.

Test moisture content with a hygrometer before laying any floor covering - target below 75% RH for most adhesives and floor coverings.

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Concrete Screed Calculator

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