What is screed?
Screed is a levelling layer of sand and cement laid over a structural floor to give a flat, smooth surface for the finish that goes on top. It is not structural, and it is never the surface you walk on — it always gets covered. Two things about it catch people out: the minimum thickness depends entirely on what it is sitting on, and the drying time is measured inmonths, not days.
Screed is not concrete
They look similar in a mixer and behave completely differently in a floor. The difference is aggregate size and purpose:
- Concrete contains coarse stone, typically up to 20 mm. It is structural — the slab carries the load.
- Screed uses fine aggregate (sharp sand) only, is laid far thinner, and exists to level the surface above that slab.
Compacted, set and air-dried, screed weighs2,000–2,200 kg per cubic metre, and one tonne of material lays about0.45 m³ — figures published onTarmac's screed data sheet, which is what our screed calculator computes with.
Bonded, unbonded or floating — and why it sets the minimum
More screeds fail from being too thin for their construction than from any mix problem. From Tarmac's data sheet, applying BS 8204-1:
| Construction | Sits on | Minimum | With heating |
|---|---|---|---|
| Bonded | prepared slab + bonding agent | 40 mm | — |
| Unbonded | damp-proof membrane | 50 mm | 65 mm |
| Floating | insulation | 65 mm | 75 mm |
Our screed calculator applies the right row automatically and warns you if the thickness you entered falls short.
How to screed a floor
- Prepare the base. It must be clean and free of dust, oil, plaster and debris. For a bonded screed, wet the slab and brush in a bonding slurry immediately before laying.
- Set your levels. Screed rails or mortar dabs at the finished height, checked with a level, working around the room.
- Place and compact. Spread the semi-dry mix with a surcharge and compact it thoroughly and evenly — by tamping or mechanically.Poor compaction is the most common cause of screed failure, in Tarmac's own words. Over 50 mm thick, lay in two equal layers, with the second within two hours of the first.
- Level and finish. Rule off the rails with a straight edge, then trowel to the finish your floor covering needs — but avoid over-trowelling, which brings laitance to the surface where it crazes and dusts.
- Cure. Cover with plastic sheeting for at least seven days to hold moisture in.
Drying: the bit that breaks build programmes
Allow roughly one month per 25 mm of thickness — and above 50 mm, two months per 25 mm for the part above 50 mm. That compounds fast:
| Thickness | Indicative drying | Coverage per tonne |
|---|---|---|
| 50 mm | 2 months | 9 m² |
| 65 mm | 3.2 months | 6.9 m² |
| 75 mm | 4 months | 6 m² |
| 100 mm | 6 months | 4.5 m² |
Coverage is 0.45 m³ per tonne divided by the thickness. Tarmac print 7.0 m² at 65 mm where the exact arithmetic gives 6.9 — four rows of their table are rounded to convenient figures, and we show the computed value rather than quietly adopting theirs.
Curing and drying are different. Curing keeps moisturein for the first seven days so the cement hydrates properly. Drying is the months afterwards while moisture leaves so a floor finish can be laid. Skip the curing and the screed is permanently weaker; rush the drying and you trap moisture under vinyl or wood, which lifts it later. And do not force it: hot air blowers and switching the underfloor heating on early will crack and curl a young screed.
Worked example: a 30 m² floating floor
- Construction: floating over insulation with underfloor heating → minimum 75 mm
- Volume at 75 mm plus 5% waste: 30 × 0.075 × 1.05 =2.36 m³
- Material: ÷ 0.45 m³ per tonne =5.3 tonnes of ready-to-use screed
- Drying before a moisture-sensitive finish: about4 months — plan the programme around it
Run your own figures, including the sand-and-cement split if you are site mixing, with the screed calculator.
Frequently asked questions
What is screed?
Screed is a thin, levelling layer of sand and cement laid over a structural floor to give a flat, smooth surface for the final floor finish. It is not the structure and it is not a wearing surface — it always gets covered by tiles, vinyl, carpet or boards. Compacted, set and air-dried it weighs 2,000–2,200 kg per cubic metre.
What is the difference between screed and concrete?
Aggregate size and job. Concrete contains coarse stone up to 20 mm and is structural — it carries load. Screed uses fine aggregate (sand) only, is laid much thinner, and exists purely to level the surface above the structural slab. Using concrete where screed is specified gives a rough surface; using screed where concrete is needed gives a floor with no strength.
How do you screed a floor?
Clean the base thoroughly, set your levels and lay screed rails or dabs to the finished height, then place the semi-dry mix in bays and compact it hard before levelling off the rails with a straight edge. Compaction is the critical step — Tarmac name poor compaction as the most common cause of screed failure. Trowel to the finish the floor covering needs, then cover with plastic sheeting for at least seven days to cure.
How long does screed take to dry?
Allow roughly one month per 25 mm of thickness, and for anything above 50 mm allow two months per 25 mm for the part above 50 mm. So 50 mm is about 2 months and 75 mm about 4 months before a moisture-sensitive floor finish goes down. Damp bases, high humidity and low temperatures all extend it — and the flooring contractor must test moisture content regardless of the calendar.
Curing and drying are not the same thing
Curing means keeping moisture in for at least seven days under plastic sheeting so the cement hydrates and gains strength. Drying is the months afterwards while excess water leaves so a floor covering can be laid. Skipping the curing stage weakens the screed permanently; rushing the drying stage traps moisture under the floor finish and lifts it later.
What is the minimum thickness for screed?
It depends on how the screed sits on the base: bonded (straight onto a prepared slab) at least 40 mm, unbonded (over a damp-proof membrane) at least 50 mm or 65 mm with heating pipes, and floating (over insulation) at least 65 mm or 75 mm with heating pipes. Those are Tarmac's figures applying BS 8204-1, and going thinner is a common cause of cracking.
Can you speed up screed drying?
Not safely with heat. Tarmac are explicit that hot air blowers, underfloor heating and other forced drying must not be used in the early life of a screed — drying it too fast cracks and curls it. What does help is ventilation and a dehumidifier once curing is complete, and choosing a fast-drying proprietary screed at the specification stage if the programme is tight.
About these numbers
The yield, density, minimum thicknesses, drying times and the compaction warning all come from Tarmac's published screed data sheet, which applies BS 8204-1. Every figure on this page renders from the same tested module thescreed calculator computes with — the tests reproduce Tarmac's own coverage table before any change ships — so the two cannot disagree; read how CubicMath works. Your supplier's data sheet wins for your job, and the flooring contractor's moisture meter outranks any drying estimate here. Sharp sand for a site-mixed screed? The sand calculator covers that.