Screed is a layer of cementitious or calcium sulphate (anhydrite) material applied to a concrete sub-floor to produce a smooth, flat, and level surface ready to receive tiles, timber, vinyl, carpet, or other floor finishes. Traditional sand-and-cement screed is a 1:3 to 1:4.5 mix applied and hand-floated to a tolerance of +/-3mm under a 3m straightedge. Liquid (flowing) screed is pumped in and self-levels, giving a flatter and faster-dried result than hand-applied screed.

What is the minimum depth of screed?

Minimum depths vary by type and application. Bonded sand-and-cement screed (mechanically keyed to a clean concrete slab) can be as thin as 25-40mm. Unbonded screed (on a polythene slip membrane) should be a minimum of 50mm. Floating screed over insulation (for underfloor heating or thermal compliance) should be a minimum of 65mm for sand-and-cement or 50mm for liquid anhydrite screed. Below these depths, screed becomes prone to cracking and delamination.

How long does screed take to dry?

Traditional sand-and-cement screed dries at approximately 1mm per day up to 40mm, then 0.5mm per day for greater depths - so a 75mm screed takes around 70 days to dry sufficiently for most floor coverings. Liquid anhydrite screed dries faster and is often 28 days for 40mm depth. Moisture meters should always be used to confirm readiness before laying moisture-sensitive floor finishes such as timber, vinyl, or resin. Drying times are significantly extended in cold or humid weather.

What is the difference between screed and concrete?

Concrete is a structural material containing large aggregate (gravel or crushed stone) mixed with cement and water - it forms the structural floor slab. Screed is a finishing layer applied on top of the concrete slab using fine sand and cement (or gypsum anhydrite in liquid screeds) with no coarse aggregate - its purpose is to provide a level, smooth surface for floor finishes. Screed is not structural and should not be used as a replacement for a concrete slab.

Finishes noun

Screed

/ skriːd /

Also known as: floor screed, sand-and-cement screed, liquid screed, anhydrite screed, self-levelling screed

Definition

Screed is a thin layer of cementitious or calcium sulphate (anhydrite) material applied over a concrete sub-floor to produce a smooth, flat, and level surface ready to receive the floor finish. Traditional sand-and-cement screed is hand-applied in a 1:3-4.5 cement:sand mix at 50-75mm depth and floated level. Liquid (flowing) screed is pumped in and self-levels. Screed may be bonded (keyed to the slab), unbonded (on a slip membrane), or floating (over insulation).

In practice

The three installation methods each have different minimum depths. Bonded screed is mechanically keyed to the concrete slab with a bonding agent (SBR slurry) and can be as thin as 25-40mm, but requires a very clean, prepared surface. Unbonded screed lies over a polythene membrane (allowing differential movement) and should be a minimum 50mm. Floating screed, laid over rigid insulation for underfloor heating or thermal compliance, must be a minimum 65mm for sand-and-cement or 50mm for liquid anhydrite to achieve adequate strength and crack resistance.

Liquid anhydrite screed is increasingly popular for underfloor heating because it encapsulates heating pipes fully with no air pockets, dries flatter than hand-applied screed, and allows thinner depths. It must be laitance-sanded (the surface skin removed) before laying moisture-sensitive finishes. All screeds require expansion joints at slab joints, doorways, and changes in shape, and must be tested with a moisture meter before installing timber, vinyl, or resin finishes - damp screed causes floor failure.

Building Regulations

Relevant PartsPart C (moisture), Part L (energy - floating screed over insulation)

Flatness toleranceSR2: +/-3mm under 3m straightedge (most domestic applications)

Moisture contentBelow 75% RH (CM meter) or 0.5% moisture content before floor finish

Liquid screedBS EN 13813 - Screed material and floor screeds

Approved Document C requires floors to resist moisture rising from the ground - usually achieved by a DPM below the concrete slab and by ensuring screed is dry before moisture-sensitive floor coverings are laid. Approved Document L requires floors to meet minimum U-value targets, which for ground floors typically requires rigid insulation below the screed, making correct screed depth over insulation critical. Screed surface regularity classes are defined in BS 8204.

Full Building Regulations guidance

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Screed

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