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.
By Dhruv Agarwal · · 4 min read
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, thin finishes such as vinyl and 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 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 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
- 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.
- Prepare the surface. Remove laitance, curing compounds, paint, adhesive residue and dust, usually by grinding or shot-blasting. Treat cracks and fill deep holes.
- Prime. Use the primer the manufacturer specifies for that substrate, so the compound bonds and does not lose its water into the slab.
- Form edges and joints. Seal gaps at walls and penetrations, form dams at thresholds, and carry structural movement joints through.
- 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.
- 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.
Standards referenced
- EN 13813 — Screed material and floor screeds — screed material — properties and requirements (CEN-CENELEC)
- ASTM F2170 — Determining relative humidity in concrete floor slabs using in situ probes (ASTM International)
- ASTM F710 — Preparing concrete floors to receive resilient flooring (ASTM International)
Frequently asked
Related
- Screed: The Layer That Decides Whether the Floor Finish Works
- Vinyl Flooring — LVT, Homogeneous and Heterogeneous
- Carpet Tile — Pile, Backing and Acoustic Value
- Raised Access Floor: The Void That Runs the Office
- Office Flooring — Carpet Tile, Vinyl, Stone and Raised Access
- Carpet Tile vs Vinyl Flooring in an Office: Acoustics, Wear and Upkeep