# Self-Levelling Underlayment (Compound) for Vinyl, LVT and Carpet Tile Floors

> Self-levelling underlayment is a flowable, usually cementitious compound poured over a primed slab or screed to give a smooth, flat surface for thin floor finishes such as vinyl, LVT and carpet tile. It corrects small deviations and surface defects, not level or falls, and the finish can only go down once both slab and compound pass moisture testing to the flooring manufacturer's limit.

## Why floor flatness decides the finish

**Self-levelling underlayment**, also called self-levelling compound or SLC, is
a flowable material, usually cement-based, poured or pumped over a primed slab or
screed. It spreads under its own weight, with light help from a spiked roller or
gauge rake, and sets to a smooth, flat surface.

In offices it matters because more floors now go straight onto the slab. Where
there is no [raised access floor](/glossary/raised-access-floor), thin finishes
such as [vinyl and LVT](/materials/vinyl-flooring-lvt) sit directly on whatever
the slab gives them. Every trowel ridge, joint and dip shows through as a line or
a shadow under raking light. [Carpet tile](/materials/carpet-tile) is more
forgiving, but tiles still lift and gap on an uneven floor.

The common misconception is that levelling compound "fixes" a bad floor. It
corrects **flatness and surface defects**, not level, falls or a weak, wet slab.
The cost of misunderstanding it is either a vinyl floor that shows every flaw, or
a compound bill that grows because it is being used to fill a floor that needed
re-screeding.

## Self-levelling compound against sand-cement screed

| | Self-levelling underlayment | Sand-cement screed |
| --- | --- | --- |
| **Job** | Smoothness and flatness under thin finishes | Sets floor level, falls, covers services |
| **Thickness** | Typically a few mm to a few tens of mm, per product | Typically much thicker |
| **Laying** | Poured or pumped, flows to a smooth surface | Placed, compacted and trowelled by hand |
| **Speed** | Fast to lay, large areas in a day | Slower, labour-intensive |
| **Drying** | Faster for thin layers; product-dependent | Slow; often the critical path |
| **Falls** | No, it seeks level | Yes |
| **Wearing surface** | Usually no; designed to be covered | Usually covered too |

See the glossary entry on [screed](/glossary/screed) for bonded and unbonded
screeds, drying and flatness. Many floors use both layers: screed to set the
level, compound to give a surface fit for a thin finish.

Cementitious levelling products are often declared against **EN 13813**, which
classifies screed materials by binder type (CT for cementitious) and properties
such as compressive and flexural strength (C and F classes). The class tells you
what the material is; the product data sheet tells you where it may be used.

## How it is laid

1. **Assess the slab.** Check flatness, cracks, contamination and moisture
   before deciding the build-up. **ASTM F710** describes preparing concrete
   floors to receive resilient flooring, including cleanliness, moisture and
   alkalinity checks.
2. **Prepare the surface.** Remove laitance, curing compounds, paint, adhesive
   residue and dust, usually by grinding or shot-blasting. Treat cracks and fill
   deep holes.
3. **Prime.** Use the primer the manufacturer specifies for that substrate, so
   the compound bonds and does not lose its water into the slab.
4. **Form edges and joints.** Seal gaps at walls and penetrations, form dams at
   thresholds, and carry structural movement joints through.
5. **Mix and pour.** Mix to the stated water content, pour continuously in
   strips, and roll out air. Too much water weakens the surface and causes
   cracking and dusting.
6. **Protect while curing.** Keep out direct sun, strong draughts and traffic
   for the manufacturer's stated period.

## Drying and moisture testing

Drying time depends on product, thickness, temperature, humidity and airflow.
The deciding test is not elapsed days. Before adhesive goes down, the slab and
compound are tested against the **flooring and adhesive manufacturer's
moisture limit**. **ASTM F2170** measures relative humidity inside the concrete
using in-situ probes in drilled holes, which gives a better picture of moisture
deeper in the slab than surface readings.

Self-levelling compound does **not** block moisture from below. If the slab is
too wet, the remedy is more drying time or a moisture-mitigation system
specified by the flooring manufacturer, applied before the compound.

This is a programme decision as much as a technical one. On a fast fit-out, the
slab often arrives late and wet, and the flooring date does not move. Deciding
early whether to allow drying time or to specify a moisture-tolerant adhesive or
barrier costs little; discovering the conflict a week before handover leaves only
expensive options, or a floor that lifts in its first monsoon.

## Common mistakes

- **Using compound to correct level.** Volumes, cost and drying time all
  balloon; the floor needed a screed.
- **Skipping or mixing the wrong primer.** Pinholes, weak surfaces and poor bond
  follow.
- **Adding extra water** to help it flow. The surface becomes weak and dusty and
  the adhesive fails on it.
- **Pouring over unrecorded movement joints.** The compound cracks along them and
  the crack shows through the vinyl.
- **Laying vinyl by the calendar** rather than after a moisture test.
- **Leaving compound exposed to site traffic** before the finish, so it is
  scratched, stained and dusty when the flooring team arrives.

## What to ask your contractor or flooring specialist

- What flatness does the chosen finish require, and how will it be checked?
- Which compound, primer and thickness are proposed, and why?
- How will slab and compound moisture be tested, and against what limit?
- How are movement joints and thresholds handled?
- What drying time is allowed in the programme before flooring starts?

## Standards referenced

Screed material properties and classification under **EN 13813**; preparation of
concrete floors for resilient flooring under **ASTM F710**; in-situ relative
humidity testing of concrete slabs under **ASTM F2170**. Flatness tolerance,
compound thickness and acceptable moisture levels for a specific floor must be
confirmed by the flooring and adhesive manufacturers and the project's
consultant.

## Frequently asked questions

### How thick can self-levelling compound be laid?

It depends on the product. Some are designed for thin smoothing layers of a few millimetres, others for thicker build-ups in one pour, and some need aggregate added beyond a certain depth. The manufacturer's data sheet sets the minimum and maximum thickness for each product, and a floor that needs more than that is a screed problem, not a levelling one.

### How long before vinyl or carpet tile can go on self-levelling compound?

Drying time depends on the product, the thickness, temperature, humidity and ventilation. Rapid-drying products allow flooring sooner; standard ones take longer, and thicker layers take longer still. The manufacturer states a guide time, but the deciding check is a moisture test on the compound and slab against the flooring adhesive manufacturer's limit, not the calendar.

### Do I need a primer before self-levelling compound?

Almost always. The primer controls how fast the slab draws water out of the compound, which otherwise causes pinholes, weak surfaces and poor bond, and it improves adhesion. Different primers suit absorbent concrete, dense or power-floated slabs, and non-absorbent surfaces such as old tiles or adhesive residues. The compound manufacturer specifies which.

### Is self-levelling compound the same as screed?

No. A sand-cement screed is a thicker layer that sets the floor level, can be laid to falls and can carry conduits. Self-levelling compound is a thin smoothing layer that improves flatness and surface finish. A floor often has both: screed for level, compound for smoothness under a thin finish. Using compound to correct a badly out-of-level floor is slow and expensive.

### Does self-levelling compound stop moisture from the slab?

No. A cementitious compound is permeable and adds its own water to the floor. If the slab is too wet for the flooring adhesive, the compound will not fix it. The options are more drying time or a moisture-mitigation system specified by the flooring and adhesive manufacturers, applied before the compound.

### Can self-levelling compound be left as the finished floor?

Usually not. Most underlayments are designed to be covered and are too soft or dusty to be a wearing surface. Some products are made as wearing toppings for light-duty areas, but they are a different product class. If a floor is to be left exposed, choose a product made for that purpose and confirm its use with the manufacturer.

## Sources

- [EN 13813](https://standards.cencenelec.eu/ords/f?p=CEN:105) — Screed material and floor screeds — screed material — properties and requirements
- [ASTM F2170](https://www.astm.org/products-services/standards-and-publications.html) — Determining relative humidity in concrete floor slabs using in situ probes
- [ASTM F710](https://www.astm.org/products-services/standards-and-publications.html) — Preparing concrete floors to receive resilient flooring

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Source: https://hagerstone.com/materials/self-levelling-underlayment