Floor Screed Calculator
Calculate screed volume, sharp sand and cement for bonded, unbonded and floating screeds. Covers standard 4:1 and 3:1 mixes with sand delivery options. 2026 UK pricing.
Bonded screed: applied directly to roughened concrete with SBR bonding slurry. Minimum 40mm depth. Most common for renovations where headroom is limited.
4:1 (sharp sand:cement) is standard for most domestic floor screeds. Use 3:1 for commercial, industrial or high-traffic areas.
Screed Guide
Screed Types & Specification
Bonded, unbonded and floating screeds
Bonded screed (min 40mm): Applied directly to a prepared concrete slab using an SBR bonding slurry. The slab must be mechanically roughened (scabbled or shot-blasted), free of dust and contamination, and the SBR slurry applied wet-on-wet immediately before screeding. Minimum 40mm depth.
Unbonded screed (min 50mm): Laid over a polythene DPM (damp-proof membrane) with no adhesion to the substrate. The screed floats on the membrane and must be thick enough to resist cracking from differential thermal movement. Minimum 50mm.
Floating screed (min 65mm): Laid over rigid thermal insulation boards. This is the most common type for new buildings and underfloor heating. Minimum 65mm — deeper over UFH pipework to provide adequate cover. Polypropylene fibre reinforcement is recommended to control cracking over the compressible layer.
Curing and drying times
Sand and cement screed takes approximately 1 day per mm depth to dry sufficiently for foot traffic — but this is just initial set. Full drying (to moisture content suitable for flooring) takes much longer: typically 1mm per day from around day 7 onwards. A 65mm screed may take 6–8 weeks to dry fully in normal conditions.
Never rush screed drying with heaters in the early stages — rapid drying causes surface cracking. Keep the room at even temperature and humidity. Test with a hygrometer before installing sensitive flooring (tiles, wood, vinyl) — moisture content should be below 75% RH (or 65% RH for wood floors).
Liquid (anhydrite) screeds
Liquid flowing screeds (calcium sulphate / anhydrite) are an alternative to traditional sand-and-cement screeds. They are pumped and flow self-levelling into position, covering UFH pipework fully. Minimum depth is 35mm over UFH pipes — significantly thinner than semi-dry screeds.
They cure faster than sand-cement screeds and have better thermal conductivity for underfloor heating. However: they require specialist pumping equipment; they cannot be used in wet areas without sealing (anhydrite is water-soluble); and they must be laitance-ground before tiling. Our calculator covers sand-and-cement screeds only — contact a specialist supplier for liquid screed volumes.
FAQ
Common Questions
Bonded screed: minimum 40mm. Unbonded (over DPM): minimum 50mm. Floating (over insulation): minimum 65mm. UFH floating screed: 65–75mm to cover pipework adequately. Liquid anhydrite screed can be thinner — consult your supplier. Never go below minimum depths as the screed will be weak and prone to cracking.
4:1 sharp sand to cement (by volume) is standard for domestic floor screeds — approximately 20–25 N/mm² compressive strength. Use 3:1 for commercial or heavy duty applications requiring higher strength (~30 N/mm²). Always use sharp sand (BS EN 12620) — building sand will produce a weak screed.
Approximately 1 day per mm for foot traffic (basic set), but full drying to flooring-ready moisture content takes much longer — typically 1mm per day after the first week. A 65mm screed may take 6–8 weeks. Always test with a hygrometer before installing sensitive flooring. Below 75% RH for tiles; below 65% RH for wood flooring.
Polypropylene fibre reinforcement is strongly recommended for floating screeds over insulation and for screeds over UFH pipework. It significantly reduces surface cracking caused by differential movement and temperature cycling. Add approximately 600–900g per m³ of screed. It does not replace the need for correct depth and mix ratio.
| Material | Quantity | Notes | Est. Cost |
|---|