What is screed, and how is it different from concrete?
A levelling layer, not a structural one — what screed is made of, the three types you will meet in a house, and why laying it like concrete makes it fail.
- Difficulty
- beginner
- Time
- 10 min
- Read
- 3 min
Short answer
Screed is cement and sharp sand with no coarse aggregate, laid in a thin layer over a slab to give a flat, true floor surface. It is not structural: the slab underneath carries the load. Concrete has stone in it and is the thing that carries load. Using one where the other belongs gives either a floor that cracks or a floor you cannot get flat.
The word turns up in flooring quotes, underfloor heating specifications and damp-proofing jobs, usually without explanation. The distinction that matters is structural: concrete does a job, screed provides a surface. Screed laid too thin, on a dusty slab, or without curing is the commonest cause of a hollow, drummy, cracking floor.
Step by step
- Understand what a screed is for.Levelling. It takes an uneven structural slab and gives a flat, smooth, true plane for a floor finish to go onto, and it buries pipes and underfloor heating. It is not designed to carry load across a span.
- Recognise a traditional sand-and-cement screed.A stiff, almost damp mix of sharp sand and cement, laid to level, compacted and floated. It is cheap, it takes a long time to dry before a floor covering can go down, and its flatness depends entirely on the person laying it.
- Recognise a flowing or liquid screed.Anhydrite or calcium sulphate based, or a flowing cement one, pumped in and self-levelling. It goes down fast, achieves excellent flatness, and works very well with underfloor heating. Anhydrite screeds need their surface laitance removing before tiling and are not suitable in permanently wet areas.
- Recognise a levelling compound.Not really a screed — a thin polymer-modified self-smoothing layer, a few millimetres thick, used to take out minor unevenness before vinyl, LVT or tiles. It is a finishing operation on a floor that is already close.
- Note that screed is bonded, unbonded or floating.Bonded directly to a prepared slab, so it can be laid relatively thin. Unbonded over a membrane, so it must be thicker. Floating over insulation, thicker still and usually reinforced with mesh or fibres. The thickness required follows from which of the three it is, so this is decided first.
- Get the thickness from the specification, not a guess.Minimum thicknesses differ for bonded, unbonded and floating construction, and for whether it will carry a heavy floor or underfloor heating pipes. Screed laid thinner than its minimum cracks and debonds, and that is not repairable by filling.
- Prepare the slab properly.For a bonded screed the slab must be clean, sound, mechanically prepared, free of dust and laitance, and primed with a cement-and-SBR slurry brushed in immediately before laying. Screed laid on a dusty slab rings hollow.
- Cure it and let it dry.Two different things. Cure it damp under polythene for several days so it gains strength, then allow it to dry before a floor covering goes on, which takes far longer — a moisture test on the floor is the only reliable way to know it is ready.
Tips
- Screed drying time is the item most commonly missed from a renovation programme. A traditional screed of any thickness takes weeks to months to dry enough to take an impermeable floor covering.
- If underfloor heating is in the screed, it has a commissioning heating cycle that must be run before flooring goes down, and it is not the same thing as drying.
- A hollow-sounding screed has debonded. Small areas can be resin-injected; a whole floor that has lifted needs taking up.
Questions people ask
Can I use concrete instead of screed to level a floor?
Not in a thin layer. Concrete contains 20mm stone, which cannot be floated flat in a thin depth and will not compact properly. Use a screed mix, or a levelling compound if the depth is small.
How long before I can tile onto a new screed?
It depends on the screed type, its thickness and the drying conditions, and the honest answer is that you test it rather than count days. Manufacturers give a drying guide, and a hygrometer or moisture test on the slab is what the flooring supplier will want to see.
Is a levelling compound the same as a screed?
No. It is a thin polymer-modified smoothing layer for taking out minor unevenness, typically a few millimetres deep. It cannot make up a significant level difference and it is not a structural or load-spreading layer.