Question 1

How thick should floor screed be?

Accepted Answer

Per BS 8204-1, a traditional sand and cement levelling screed should be a minimum of 25mm thick when bonded directly to a concrete slab, 50mm when laid unbonded (65mm if it contains heating pipes), or 65mm when floating over insulation (75mm with heating pipes), always with at least 25mm cover above any pipes. Self-levelling/liquid screed can be thinner at 25-50mm. Use our Underfloor Heating Coverage Calculator to size a UFH system. The thickness rules above come from BS 8204-1; Approved Document C covers floor moisture protection, not screed depth.

Question 2

What is the correct mix ratio for floor screed?

Accepted Answer

The standard mix for traditional floor screed is 1 part cement to 3-4 parts sharp sand (by volume). A 1:4 mix is most common for general domestic use. For heavier-duty areas like garages, a stronger 1:3 mix is recommended. The sand must be sharp (concreting) sand, not building sand, to achieve the correct strength. This calculator covers traditional, self-levelling and fast-set floor screed mixes used across UK domestic and commercial projects.

Question 3

How long does screed take to dry before tiling?

Accepted Answer

As a rule of thumb, traditional sand and cement screed dries at about 1 day per mm for the first 50mm, then roughly 2 days per mm beyond that, at good drying conditions (20°C and below 65% relative humidity). So a 65mm screed needs approximately 80 days (50 + 15×2), not 65. These are indicative only — always confirm with a moisture (RH) test reading 75% or below per BS 8203 before laying tiles or floor coverings. Self-levelling/liquid screed dries faster, and fast-set screed can be walked on in hours and tiled within days.

Question 4

How much sand do I need per m³ of screed?

Accepted Answer

For a traditional sand-cement screed at any cement:sand ratio between 1:3 and 1:5, you need approximately 1.6 tonnes of sharp sand per m³ of finished screed (using the yield assumption that the cement fills voids in the sand, so mortar volume ≈ sand volume). This gives a set density of about 1,960 kg/m³ at 1:4, which sits inside the typical sand-cement screed set-density range of 1,800-2,000 kg/m³. The mix ratio determines the cement quantity, not the sand. A UK 'bulk bag' (often called a tonne bag) of sharp sand actually holds about 0.8-0.85 tonne and occupies roughly 0.5 m³ at the 1.6 t/m³ bulk density used here, so it is not a full tonne — order one bulk bag per ~0.8 tonne of sand needed. For a typical room (4m × 5m at 65mm depth = 1.30 m³ + 10% wastage = 1.43 m³), you need approximately 2.3 tonnes of sharp sand, which is about 3 bulk bags.

Question 5

Can I screed over underfloor heating pipes?

Accepted Answer

Yes, screeding over underfloor heating (UFH) is standard practice. Per BS 8204-1 you need at least 25mm of screed cover above the top of the pipes, and the total screed thickness must meet the minimum for the construction: 65mm for an unbonded screed containing pipes, or 75mm for a floating screed over insulation containing pipes. Self-levelling/liquid screed can run thinner with around 30mm cover over the pipes. The underfloor heating must be pressure tested before screeding, and do not commission (heat up) traditional screed for at least 21 days after laying. Size your UFH system first using our Underfloor Heating Coverage Calculator.

Screed Calculator — Sand, Cement & Volume for Floor Screeding

How We Calculate This

The formula

UK screed depth guidelines (BS 8204-1)

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