When someone tells me their floor is uneven and they are not sure whether to use self-levelling compound or screed, the first question I ask is: how uneven, and what is there now?
Those two answers almost always tell you which product to use. Not because the products are complicated, but because they solve different problems at different scales.
A floor that is 3mm out of flat across 20m2 needs SLC. A floor being constructed from bare structural concrete needs screed. The confusion usually comes when the two problems exist on the same site - and they often do.
What each product is for
Self-levelling compound (SLC) is a finishing product. It flows under gravity to create a flat surface. It is designed for thin applications - typically 2-20mm depending on the product. It is quick, clean to apply, and relatively expensive per kg compared to screed materials.
Floor screed is a construction product. It builds depth, levels a rough slab, encapsulates underfloor heating, and provides a structural base for floor coverings. It works at 25-75mm depth. It is slower to apply, more labour-intensive, much cheaper per m3, and needs significantly longer to dry.
Head-to-head comparison
| Factor | Self-levelling compound | Floor screed |
|---|---|---|
| Typical depth | 2-20mm | 25-75mm |
| Application | Poured and raked | Mixed and laid manually |
| Labour | Low - pours itself | Higher - manual laying and screeding |
| Drying time | 1-24 hours to walk on | Days to weeks |
| Cost per m2 at typical depth | Higher | Lower |
| Suitable over UFH pipes | No | Yes |
| Can go over existing floor | Yes (with primer) | No - structural application only |
| Minimum depth | 2-3mm | 25mm (bonded) |
When to use self-levelling compound
- Existing concrete or screed floor that is basically sound but uneven
- Renovation project preparing an existing floor for tiling
- New build where the structural slab is flat but not perfectly level
- Thin correction layer (under 15mm) before a floor covering
- Quick turnaround needed - most SLC products allow tiling within 24 hours
When to use floor screed
- New build or extension floor construction from scratch
- Covering underfloor heating pipes
- Raising floor level significantly (over 20mm)
- Floating floor over insulation boards
- When cost per m2 matters - screed is substantially cheaper than SLC at depth
When you need both
This is more common than people realise. A new extension might have a structural concrete slab that is well constructed but has minor irregularities. The contractor lays sand:cement screed to the correct depth and with the correct falls. Once cured, they apply a thin layer of SLC to achieve the perfectly flat, smooth finish that modern large-format tiles require.
The screed does the heavy lifting - building depth, covering the insulation and UFH pipes, providing the structural floor. The SLC does the fine work - creating the flatness tolerance that a rigid tile adhesive and large format tile demand.
Cost comparison
Costs vary by region and supplier, but as a rough guide per m2 of finished floor:
| Method | Material cost per m2 (approx) |
|---|---|
| SLC at 5mm | £3-6 |
| SLC at 10mm | £6-12 |
| Sand:cement screed at 50mm | £2-4 (materials only) |
| Anhydrite liquid screed at 65mm | £8-15 (pumped) |
SLC is approximately 3-5x the material cost of sand:cement screed per unit volume. This matters on large floors - for a 30m2 floor needing 50mm of build-up, SLC would cost around £150-400 in materials where screed would cost £60-120. The economics of using SLC for depth correction quickly become unworkable.
My tips on choosing
Check the existing floor condition first. If the floor has hollow areas, cracks or significant structural problems, no surface product will fix that. The base needs sorting before either screed or SLC is applied.
Do not use SLC as a cheap shortcut. Some people pour SLC thick to avoid a proper screed on a new build. At depth, SLC is more expensive than screed and not designed for the structural role screed performs. It will be thinner in some areas, cracking is possible, and costs are higher.
Test moisture before applying SLC to old screed. An existing floor that appears dry may have high residual moisture that will cause adhesion failure with SLC or the floor covering above it. A basic calcium carbide test or RH probe will tell you if the floor is ready.
For tiling over either product, the floor needs to be flat to within 3mm under a 3m straight edge for standard tiles, and closer to 2mm for large-format tiles. If existing screed is within those tolerances, you may not need SLC at all.
Use the Self-Levelling Compound Calculator or Floor Screed Calculator for your specific project.