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5.3 Laying a floor screed

A level sand-cement layer on top of a concrete slab, ready for tiles, wood or vinyl.

Cross-section of an unbonded floor screed: concrete slab, plastic separating sheet, 50 mm sand-cement screed. 1 Concrete slab cured and clean (see 4.1) 2 Separating sheet plastic sheet, lapped 3 Screed 50 mm · 0–4 mm sand · 1 : 4
Cross-section of an unbonded screed. A bonded screed leaves out the sheet and sticks straight to the slab, and can then be thinner.

At a glance

Difficulty
2 of 4
Drying
Several weeks, longer for thick screeds. Measure before laying a finish.
Tools
Forced-action or drum mixer, wheelbarrow, screed rails, straight edge, hand tamper, steel float, carpenter's level
Parts
  1. 1  Concrete slab
  2. 2  Separating sheet
  3. 3  Screed

What you need per m², 50 mm unbonded screed

MaterialSizeTypeAmount
Sand 0–4 mm Washed, sharp 83 kg
or 0–8 mm Fine concrete, for thicker bonded screeds
Cement – CEM I or CEM II 21 kg
Separating sheet – Plastic sheet, lapped, turned up at walls 1 m² + laps

Why 0–4 mm sharp sand?

0–4 mm

Screeds are made with sharp sand, washed and graded to EN 13139. In EN 12620, sand (fine aggregate) is anything up to 4 mm, so screed sand is 0–4 mm.

0–8 mm

A fine concrete with small stone. Use it for thicker bonded screeds, where plain sand screed would shrink too much.

Step by step

  1. 1

    Prepare the slab.

    For an unbonded screed, sweep the slab and lay a plastic sheet, lapped at joins and turned up at the walls. For a bonded screed, clean the slab and roughen it so the screed can grip, then use a bonding agent.

    Separating sheet laid
  2. 2

    Set screed rails to the finished level.

    Lay level rails or strips of screed about 2–3 m apart, checked with a carpenter's level or laser. The straight edge rides on them.

    Screed rails set level
  3. 3

    Mix 1 cement to 4 sand by weight, semi-dry.

    That is about 1 bucket of cement to 3½ of sand. Use a forced-action mixer if you can, and only as much water as it takes to compact the screed fully.

    The semi-dry mix
  4. 4

    Spread, tamp hard, then rule off.

    Lay the mix a little above the rails, tamp it down firmly over the whole area, then pull the straight edge across the rails. Fill hollows and rule again. Lay screeds over 50 mm in two layers.

    Ruling off between rails
  5. 5

    Lift out the rails, fill the gaps, float the surface.

    Fill where the rails were with the same mix and tamp it. Close the surface with a float. Work in bays and finish each one in one go.

    Floating the screed
  6. 6

    Cover it, then let it dry.

    As soon as it has set, cover it with plastic so it cures slowly. Then let it dry gradually before laying a floor finish.

    Screed covered with plastic
Specifications
Screed typeSieve sizeThicknessHow
Bonded0–4 mmThinnestSlab roughened to the stone, bonding agent.
Unbonded0–4 mmThicker than bondedOn a plastic separating sheet.
Floating0–4 mmThickestOn insulation.

Mix

This page uses 1 part cement to 4 parts sand by weight. Loose cement is a little lighter than dry sand (about 1.4 against 1.6 t/m³), so by bucket that is about 1 : 3½.

Thicker bonded screeds

Bonded sand-cement screeds work best thin, because thick ones tend to come loose. For a thicker bonded screed, use a fine concrete with small stone.

Curing and drying

Cover with plastic as soon as the screed has set. Then let it dry gradually. Thick screeds take much longer to dry than thin ones.

Sources

  1. EN 12620:2002+A1:2008 clause 3.8: fine aggregate is the sizes with D up to 4 mm
  2. Public-sector works specification for screeds and plaster (UK Contracts Finder): sand to EN 13139; forced-action mixer; minimum water for full compaction; compact thoroughly over the whole area; over 50 mm in two layers; cover with polyethylene, then dry gradually
  3. University of Babylon, Methods of construction and estimation, lecture 14: dry materials shrink by about a quarter when mixed; bulk weight of cement 1,400 kg/m³
  4. NPTEL (Indian Institutes of Technology), soil mechanics worked examples: dry sand about 1,600 kg/m³
  5. EN 13813 Screed material and floor screeds; BS 8204-1 UK code of practice for cementitious levelling screeds
  6. EN 13139 Aggregates for mortar (includes screed sand); EN 12620 Aggregates for concrete (fine concrete screed)
  7. EN 197-1 Cement

Amounts: 50 mm of screed is 0.05 m³ per m². Dry materials take about a third more volume than the mixed screed, so about 0.067 m³, split 1 : 4 by weight with loose bulk densities of 1,400 kg/m³ for cement and 1,600 kg/m³ for sand: 21 kg cement and 83 kg sand. A 40 mm bonded screed needs about 17 kg cement and 66 kg sand. They include no waste. Order a little extra, more if the slab is uneven.

Section 5.3 Find these materials near you →

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