Summary
"Self-levelling compound" and "screed" get used interchangeably on site, but they're different products doing different jobs, and picking the wrong one costs the customer money and costs you a callback. The confusion is understandable — both are cementitious, both are poured or pumped, and both end up as a flat, level floor. But a self-levelling compound is a thin, self-smoothing finishing layer applied over an existing sound subfloor. A screed is a structural floor build-up, applied at much greater depth, that can carry loads, incorporate insulation and underfloor heating, and form the actual floor structure rather than just dress the surface of one.
The decision usually comes down to how much material you need to move and why. If you're skimming an old concrete slab or plywood subfloor that's slightly out of level before laying vinyl or laminate, you want SLC — quick to mix, quick to walk on, and cheap per square metre for a thin pour. If you're building a new ground floor over insulation, correcting a slab that's out by more than about 10-15mm, or need to bury 15-18mm UFH pipework, you need a proper screed at structural depth. Trying to use SLC at screed depths (or vice versa — trying to use a full screed system where a quick skim would do) is where jobs go wrong: SLC isn't designed to be poured 40mm deep in one go without specialist deep-fill formulations, and full screed systems are overkill, slower, and more expensive for a simple flatness correction.
This article sets out the decision framework — depth, purpose, drying time, and cost — so you can specify the right product at quoting stage rather than discovering the mismatch on site.
Key Facts
- Self-levelling compound (SLC) — a polymer-modified cementitious product, pumped or poured, self-smoothing to a flat, level surface without extensive manual finishing; typically applied from a feather edge up to around 10mm in a standard formulation, with specialist deep-fill/high-build products rated for greater single-pour depths.
- Screed — a structural floor topping, either traditional semi-dry sand:cement (typically 4:1 or 3:1 mix) or a pumped flowing screed (calcium sulfate/anhydrite or polymer-modified sand:cement), applied at 25mm and upwards depending on the build-up.
- Governing standard for both — BS 8204 (Screeds, bases and in-situ floorings — code of practice), split into parts covering different screed types: BS 8204-1 (sand:cement), BS 8204-3 (polymer modified), BS 8204-6 (synthetic resin flooring), BS 8204-7 (pumpable self-smoothing screeds, which covers both deep-fill self-smoothing screeds and thinner self-levelling compounds).
- Minimum screed depths — unbonded/floating sand:cement screed typically needs a minimum of around 65mm; bonded screed direct to a prepared slab can be thinner, from around 25–40mm; always check the specific product's technical data sheet.
- Anhydrite/calcium sulfate flowing screed over UFH — minimum cover over the pipe typically 30mm, per BS EN 1264-4, with total screed depth commonly 35–75mm depending on the system.
- SLC depth limits — most standard SLCs are rated feather-edge to around 10mm in a single pour; going deeper without a manufacturer-approved deep-fill product risks cracking, shrinkage, and slow drying.
- Combined build-ups are normal — a floating or bonded screed is often finished with a thin SLC skim on top to achieve the smoothness tolerance (SR1) required for resilient flooring like LVT.
- Surface regularity tolerances — BS 8204-7 defines SR1 (most stringent, ±3mm under a 2m straightedge) and SR2 (±5mm) tolerance classes; which one you need depends on the final floor covering.
- Drying/curing rule of thumb for screeds — approximately 1 day per mm of depth up to 40–50mm, then roughly 2 days per mm beyond that, before floor covering can go down (product-dependent — always confirm with the manufacturer's data sheet).
- SLC drying — touch-dry in 1–4 hours, walkable within around 24 hours, but the time before you can lay a floor covering on top depends on the product and the pour depth — thin pours can be ready for floor coverings within 24–48 hours; thicker deep-fill pours take longer.
- Moisture testing — both screeds and SLC need moisture checked with a hygrometer before final floor coverings go down; ≤75% relative humidity is the widely used threshold for standard floor coverings, per BS 8203, tightening to around 65% RH for timber flooring or over UFH — always follow the flooring manufacturer's specific limit.
- Priming — porous substrates (concrete, sand:cement screed) need an appropriate primer (often SBR or PVA-based) before SLC is applied, to control absorption and improve bond; anhydrite screeds need their own specific primer system before SLC or adhesive is applied, due to the different chemistry.
- UFH and SLC — self-levelling compound is not designed to encase UFH pipework structurally; that's a job for a proper flowing screed system. SLC can be used as a thin finishing skim over a UFH screed once it's dry, not as the primary UFH encasement.
- Cost driver — SLC is generally cheaper per square metre for thin applications because of the lower material volume and faster labour turnaround; full screed is a bigger material and labour cost because of the depth and (for pumped systems) specialist plant needed.
Quick Reference Table
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Try squote free →| Scenario | Product | Typical depth | Time to walk on | Time before floor covering |
|---|---|---|---|---|
| Minor undulations on existing sound slab before LVT/laminate | Self-levelling compound | Feather edge – 10mm | ~24 hours | 24–48 hours (product dependent) |
| Significant unevenness (10–25mm) on existing subfloor | Deep-fill/high-build self-smoothing compound | 10–40mm (manufacturer-rated) | 24–48 hours | Several days (manufacturer dependent) |
| New floating floor over rigid insulation | Sand:cement or flowing screed | 65mm+ (floating) | 3–7 days | 1mm/day rule up to 50mm, then 2mm/day |
| Bonded screed to existing prepared slab | Sand:cement or flowing screed | 25–40mm (bonded) | 2–3 days | 1mm/day rule |
| Encasing underfloor heating pipework | Flowing calcium sulfate/anhydrite screed | 35–75mm total, 30mm min cover over pipe (BS EN 1264-4) | 2–3 days | Manufacturer commissioning programme, then flooring after RH test |
| Final smoothness pass over a cured screed for LVT/vinyl | Self-levelling compound (skim) | Feather edge – 5mm | ~24 hours | 24–48 hours |
Detailed Guidance
Reading the existing subfloor before you specify anything
Start with a straightedge and a 2m spirit level across the existing floor. If the deviation is within SLC's rated depth range and the substrate itself is sound (no structural cracking, no significant moisture issue, adequate compressive strength), SLC alone will do the job. If deviations exceed what a standard SLC can safely take in one pour, or if the substrate itself needs rebuilding (a new slab, or a floating floor over insulation for a refurbishment), you're into screed territory. Always check for a DPM (damp-proof membrane) requirement before either product goes down — moisture rising through an unprotected slab will fail flooring adhesives and coverings regardless of which levelling product sits above it.
Why depth is the deciding factor, not "which is better"
SLC and screed aren't competing products for the same job — they're different tools for different depths and purposes. A self-levelling compound is engineered to flow, self-smooth, and set relatively quickly at thin section depths; push it beyond its rated depth and you risk excessive shrinkage cracking, slow internal drying (the surface looks dry while the body is still wet), and debonding. A screed is engineered to be a structural floor layer in its own right, at a depth that gives it the strength and thermal mass (particularly relevant for UFH) that a thin SLC skim simply can't provide.
When to combine both
The most common professional build-up for a high-spec resilient floor finish (LVT, sheet vinyl, or anything demanding SR1 tolerance) is: substrate preparation → structural screed to the required depth → SLC skim over the top for final smoothness. This gives you the strength and (where relevant) UFH performance of a proper screed, plus the fine surface tolerance that only a self-levelling compound reliably achieves. Skipping the screed and trying to get SLC alone to do both jobs on a poor or uneven substrate is a common cause of failed floor coverings later.
Drying time and programme impact
This is where the wrong choice costs the job money. A thin SLC pour can be ready for floor covering within a day or two. A full screed, particularly a deep sand:cement floating screed, can take weeks to dry sufficiently for moisture-sensitive coverings — the widely used rule of thumb is roughly 1 day per millimetre up to 40–50mm depth, then roughly 2 days per millimetre beyond that. If a customer's programme doesn't allow for that, specifying a fast-drying proprietary screed or limiting depth (using more insulation to reduce screed thickness, for example) needs to be discussed at quoting stage, not discovered once the screed pump has already been and gone.
Underfloor heating specifics
Never rely on self-levelling compound to encase UFH pipework. It isn't designed to give the thermal mass or structural cover a heating screed provides, and manufacturers' warranties for UFH systems will specify a proper flowing screed at the correct minimum cover. Once the heating screed is laid and has gone through its commissioning heating cycle and moisture testing, SLC can then be used as a thin finishing skim if the final smoothness tolerance demands it.
Frequently Asked Questions
Can I use self-levelling compound instead of a screed for a new floor?
Not for a structural new floor build-up — SLC isn't designed to replace a screed at structural depths, and standard formulations aren't rated for the section thickness a new floor typically needs. Use SLC to finish and smooth a floor that already has the right structural depth and level, not to build that depth up from scratch.
How thick can self-levelling compound be poured in one go?
Most standard formulations are rated from a feather edge up to around 10mm in a single pour — always check the specific product's technical data sheet, as this varies by manufacturer and product line. Specialist deep-fill or high-build self-smoothing compounds are rated for greater depths and are the correct choice if you need more than a standard SLC can safely take, rather than over-pouring a standard product.
Does self-levelling compound need a primer?
Yes, on most substrates. Porous substrates like concrete or sand:cement screed need priming (commonly SBR or PVA-based) to control suction and improve bond before SLC is applied. Anhydrite/calcium sulfate screeds need their own specific primer system due to their different chemistry — using the wrong primer, or skipping it, is a common cause of SLC debonding.
Can I put underfloor heating pipework in self-levelling compound?
No — use a proper flowing screed (calcium sulfate/anhydrite or a purpose-designed UFH screed system) with the correct minimum cover over the pipework, typically 30mm per BS EN 1264-4. SLC can be used as a thin finishing skim on top of a cured, commissioned UFH screed, not as the primary encasement.
Regulations & Standards
BS 8204 — Screeds, bases and in-situ floorings, code of practice, covering the material and application standards for both screeds and self-levelling/self-smoothing compounds.
BS 8204-1 — Concrete bases and cementitious levelling screeds to receive floor coverings.
BS 8204-3 — Polymer modified cementitious levelling screeds and wearing screeds.
BS 8204-6 — Synthetic resin floorings.
BS 8204-7 — Pumpable self-smoothing screeds, including surface regularity classes SR1 and SR2.
BS 8203 — Code of practice for installation of resilient floor coverings, relevant to moisture testing and hygrometer thresholds.
BS EN 1264-4 — Water-based surface embedded heating and cooling systems, installation requirements including minimum screed cover over UFH pipework.
BS 5325 — Code of practice for installation of textile floor coverings, relevant to subfloor preparation standards where carpet is the final finish.
BSI Shop — source for current editions of the BS 8204 series and related standards.
Contract Flooring Association — Technical Guidance — UK trade body technical guidance on screeds, levelling compounds, and moisture testing.
Concrete Society — Technical Report on Screeds — background technical guidance on concrete floor bases and screed specification.
floor levelling compounds — detail on levelling compound types, priming, and SR1/SR2 tolerance classes.
floor screed types and drying times — full comparison of sand:cement vs calcium sulfate screed and the drying rule in detail.
underfloor heating screed — screed selection and minimum cover specifically for UFH installations.
subfloor preparation guide — moisture testing, DPM detailing, and substrate preparation ahead of either product.