Short answer: for sand and cement screeds, typical guidance is 25–40 mm when bonded to a concrete slab, at least 50 mm when unbonded on a membrane, and at least 65 mm when floating over insulation in a home (75 mm for heavier commercial loads). Over underfloor heating, at least 25 mm should cover the pipes. Liquid screeds can usually be thinner, as set by the manufacturer.
Why depth depends on the build-up
A bonded screed shares the load with the slab beneath it, so it can be thin. An unbonded screed is separated by a membrane and must carry itself. A floating screed sits on compressible insulation and needs the most depth so it does not crack as it bends under load.
Sand and cement depths at a glance
| Type | Typical minimum |
|---|---|
| Bonded to slab | 25–40 mm |
| Unbonded on membrane | 50 mm |
| Floating, domestic | 65 mm |
| Floating, heavier commercial | 75 mm |
| Cover over UFH pipes | 25 mm above pipes |
Too thin, too thick
Too thin and a screed may crack or curl. Too thick and it takes much longer to dry, costs more and can add unnecessary weight. Get the build-up drawn out before work starts: slab, membrane, insulation, pipes, screed and finish.
How to work out the depth you need
- Start from the finished floor level: usually the level of the existing house floor you need to match.
- Subtract the thickness of the floor finish (tiles and adhesive, LVT, engineered wood and underlay).
- Subtract the insulation, membranes and any underfloor heating panels below.
- What remains is the space for the screed. Check it meets the minimum for the screed type and build-up.
If the space is too shallow, the options are thinner insulation of a higher performance, a liquid screed that can be laid thinner, or lowering the slab. If it is too deep, extra insulation is usually cheaper and drier than extra screed.
Common questions
Is 50 mm enough over insulation? Not for a typical sand and cement floating screed in a home; 65 mm is the usual minimum. Some liquid screeds can be laid thinner, as set by the manufacturer.
What depth over underfloor heating pipes? At least 25 mm of sand and cement above the pipes. See our underfloor heating screed page.
Written by Ben Phillips, director of Chelmsford Floor Screeding Ltd, floor screeding contractors based in Billericay with 20+ years of experience across Essex and London. More about us.