How Much Screed for a Floor?
Quick Answer
You need 0.78m³ of screed (~1.25 tonnes sand + 7 bags cement)
Based on: 4m x 3m floor, 65mm screed depth, 1:4 cement:sand mix (semi-dry)
How We Calculated This
Floor screed volume is a simple length x width x depth calculation:
- Area: 4m x 3m = 12m²
- Depth: 65mm = 0.065m
- Volume: 12 x 0.065 = 0.78m³
Materials for a 1:4 Cement:Sand Screed
The standard UK floor screed mix is 1 part cement to 3–4 parts sharp sand by volume (BS 8204 recommends 1:3 to 1:4.5). Using a 1:4 mix for 0.78m³ of screed:
- Sharp sand: 0.78 x (4/5) = 0.624m³ dry volume. With 25% bulking allowance: ~0.78m³ = approximately 1.25 tonnes
- Cement: 0.78 x (1/5) = 0.156m³ = approximately 6.5 bags of 25kg cement (at ~1,500kg/m³)
In practice, you should allow for compaction and slight over-thickness. A safer estimate is:
- Sharp sand: ~1.25 tonnes (2 bulk bags or ~50 x 25kg bags)
- Cement: ~7 bags of 25kg (allowing for compaction)
Screed thickness minima (BS 8204-1)
Bonded screed on a concrete slab: minimum 25–40mm(polymer-modified 25mm, traditional sand/cement 40mm). Unbonded screed (on DPM): minimum 50mm. Floating screed (over insulation, UFH, or acoustic layer): minimum 65mm (75mm over compressible insulation).
Common Screed Depths
- Bonded screed: 25–40mm minimum (directly bonded to slab with PVA/SBR)
- Unbonded screed: 50mm minimum (over DPM)
- Floating screed: 65–75mm (over insulation)
- Screed over UFH pipes: 65–75mm above the pipe crown
Semi-Dry vs Wet Screed
Traditional semi-dry screed is mixed to a “snowball” consistency — it should hold together when squeezed but not drip water. This is the most common method for domestic floors. It can be walked on after 24–48 hours and is ready for tiling after approximately 1 day per mm of thickness (so 65mm = about 65 days drying time before tiling).
Liquid/flowing screeds (anhydrite or gypsum-based) are pumped onto the floor. They self-level and are faster to lay, but require specialist equipment. Coverage rates and materials differ from traditional screed.
Tips
Set level screeds using timber battens or proprietary screed rails. Work in bays of manageable size. Compact the screed firmly with a tamping board and finish with a float. Cure the screed by keeping it damp for at least 7 days (cover with polythene if necessary). Do not force-dry with heating.