Floor Levelling Compounds Guide

Quick Answer: Self-levelling compounds (SLC) flow under gravity to create a flat surface — use for depths of 1–40mm over concrete and screed. Flexible fibre-reinforced smoothing compounds are required over timber subfloors. Always prime the substrate first — unprimed concrete draws water from the compound, causing surface crazing and weak bond. Most compounds are walkable in 2–4 hours and ready for floor coverings in 24 hours, but drying time increases significantly below 15°C.

Summary

Floor levelling compounds are the most-used preparation product in commercial and domestic flooring, yet they are also frequently misused. The common failures — compounds that crack, crumble, delaminate, or peel away from the substrate — almost always trace back to three mistakes: no priming, wrong product for the substrate, or exceeding the maximum application depth in a single pour.

There are four main product families used in UK flooring installation: self-levelling compounds (SLC), smoothing compounds, floor repair mortars, and flexible compounds for timber substrates. These are not interchangeable. Applying a rigid SLC over a timber subfloor will crack as the timber moves. Applying a thin smoothing compound over a heavily uneven concrete base with 30mm hollows will result in a layer too thin to level the deep areas and too thick in others. Getting the specification right before opening any bags is the most important preparation step.

Key Facts

• BS 8204-3 — Screeds, bases and in-situ floorings; Part 3: polymer-modified cementitious levelling screeds (covers SLC products used for subfloor preparation)
• Maximum single pour depth — standard SLC — typically 10–15mm; some products up to 30mm in one pour; check data sheet
• Minimum application depth — SLC — typically 3mm minimum; thinner applications may not flow or may crack
• Primer requirement — mandatory for all substrates; allows SLC to flow without losing water to absorbent base; unprimed = craze, crumble, or delamination
• PVA primer dilution — typically 1:4 or 1:3 (PVA:water); follow product data sheet — some floor-specific PVAs must not be diluted
• SBR bonding agent — alternative primer for difficult substrates (old adhesive residue, concrete with variable absorption); brush on undiluted, allow to become tacky before pouring compound
• Polymer-modified compounds — include plasticisers that improve flow and flexibility; better adhesion to challenging substrates; higher cost than basic SLC
• Fibre-reinforced compounds — required over timber and flexible substrates; glass fibres or polypropylene fibres reduce shrinkage cracking; not intended for use on concrete (unnecessary cost)
• Anhydrite screed primer — anhydrite substrates require specialist anhydrite-specific primer (e.g. BAL Bond SBR or Mapei Primer T) before SLC; standard PVA primer is insufficient
• Epoxy tack coat — for very high moisture conditions (RH 75–97%), an epoxy DPM acts as primer and DPM simultaneously; allows levelling compound to be applied without a separate DPM
• Time before floor finish — 24 hours at 20°C standard; up to 48 hours below 15°C; verify surface hardness before laying
• Thermal performance — levelling compound adds thermal mass and a small insulating effect; for UFH, the thinner the levelling layer above the pipe/mat, the better the thermal response

Quick Reference Table

Detailed Guidance

Choosing the Right Product

Over concrete and cementitious screed: Self-levelling compound is the standard choice. Flow-grade products (high fluidity — they self-level without trowelling) are suitable for areas of more than approximately 10m²; smoothing compounds (applied and trowelled) are used for small areas and repairs. For depths over 10mm in a single pour, check whether a two-pour system is needed or whether a deep-fill product is specified.

Over anhydrite (calcium sulphate) screed: Standard SLC will adhere to anhydrite only with an anhydrite-specific primer. Without correct priming, the sulphates in the anhydrite react with the cementitious SLC and cause delamination. Remove laitance first (sand or grit blast), then apply the specific primer as directed before pouring SLC.

Over timber (ply subfloor): Never use a standard rigid SLC over timber — the timber moves with humidity and the rigid compound will crack. Use a flexible fibre-reinforced compound rated for non-rigid substrates. Maximum depth is typically 10mm; for deeper undulations in a timber floor, the correct solution is additional ply to raise the level, then a thin skim of flexible compound for flatness.

Over old adhesive residue: If there is residual adhesive on the concrete (e.g. from old vinyl tiles), test adhesion by applying compound to a small area and checking that it bonds. SBR bonding agent is generally effective over most adhesive types. Over bitumen adhesive: do not attempt to level over it unless you have confirmed with the compound manufacturer that it is compatible; bitumen can plasticise through the compound and affect the floor finish above.

Over existing ceramic tile: Use a compound rated for non-porous substrates. Prime with a flexible primer (not standard PVA). The SLC must be polymer-modified to adhere to the non-absorbent tile surface. Test the adhesion on a small area if the tile surface is glazed or coated.

Substrate Preparation Before Levelling

1. Remove all loose material — flaking screed, old adhesive blobs, dust, laitance
2. Grind down high spots with a diamond cup wheel (angle grinder) or floor grinder
3. Fill cracks with a floor repair mortar or flexible gap filler; allow to cure
4. Vacuum thoroughly — levelling compound does not bond to dusty surfaces regardless of primer
5. Seal any gaps at the perimeter where compound could run under skirting or doors
6. Apply primer — brush on to ensure full coverage; do not leave puddles; allow to dry until tacky (typically 20–30 minutes for PVA, 60 minutes for SBR)

Mixing and Pouring Self-Levelling Compound

Self-levelling compounds are water-gauged — they require a precise water:powder ratio for correct flow and strength:

1. Measure the water into a clean bucket (weight gauging is more accurate than volume); use water at 10–20°C
2. Add the powder to the water (not water to powder) and mix with a paddle mixer for 2–3 minutes until lump-free
3. Pour into the work area immediately — do not allow the slump to start before pouring; most compounds have a working time of only 15–20 minutes
4. Spread with a gauge rake or dapple roller to the required depth; the compound will self-level to a flat plane (assuming it flows freely — if it is hesitating, the mix water content may be too low or the primer was too dry)
5. Spike-roll with a spiked roller to release air bubbles; do not trowel flow-grade SLC
6. Do not disturb for the walkable cure time; avoid draughts (they accelerate surface drying and cause curling)

For larger areas: work in sections, with a wet edge maintained. Butt each new pour against the previous wet pour — do not allow the previous pour to skin over before the next is applied or the join will be visible.

Application Depth Constraints

Never apply levelling compound in a single pour deeper than the product maximum. Thick single pours do not cure evenly — the surface dries while the bottom remains wet, causing cracking and a weak plane through the depth. Where depths of 30–50mm are required:

• Pour to the product's single-pour maximum
• Allow to cure to walkable (2–4 hours)
• Apply a second pour to the remaining depth
• Allow 24 hours after the final pour before laying the floor

Checking for Flatness Before Laying

Before laying any floor finish, confirm the achieved flatness using a long straightedge:

• For LVT (glue-down): 2mm in 1m maximum; check at multiple points and directions
• For tile: 3mm in 1.8m maximum
• For engineered wood (floating): 3mm in 1.8m maximum
• For carpet: 5mm in 1.8m acceptable (carpet bridges minor undulations)

Identify any remaining high spots (grind) or low spots (spot fill) before proceeding.

Frequently Asked Questions

Can I pour self-levelling compound over existing ceramic tiles?

Yes, if the tiles are: (a) all fully bonded — tap-test every tile; (b) not raised (proud grout joints ground down or filled); (c) primed with a non-porous substrate primer. Use a polymer-modified SLC and apply SBR bonding agent rather than standard PVA. Test adhesion on a small area first. The additional height from compound + floor finish above must be accounted for at door thresholds.

How thick does the levelling compound need to be to be effective?

The minimum useful application depth is 3mm for flow-grade SLC; 1–2mm for smoothing/skim compounds applied by trowel. Below 3mm, flow-grade compounds do not flow reliably and may crack as they cure. For very minor undulations (less than 1–2mm), a smoothing compound is more appropriate than a flow-grade SLC.

Why does my levelling compound crack after drying?

The most common causes in order of frequency: (a) no primer or inadequate primer — the substrate absorbed water too fast and the compound dried unevenly; (b) too cold — below 10°C, curing is impeded and the compound remains soft and friable; (c) mixed too wet — over-watered compound is weaker and more prone to cracking; (d) applied too thin — below the product minimum depth; (e) poured over a previously cratered or poorly prepared surface that did not give consistent bond. Always prime, mix to the correct water ratio, and respect minimum depth limits.

Can levelling compound be applied in cold weather?

Most cementitious levelling compounds require a minimum ambient and substrate temperature of 5–10°C. At lower temperatures, hydration is slowed or stopped and the compound remains uncured indefinitely. At 5–10°C, drying time doubles or triples — budget an additional 48 hours before laying. Do not attempt to apply SLC in a building with no heating if temperatures are below 5°C; protect from frost until the compound has fully hardened.

Regulations & Standards

• BS 8204-3 — Screeds, bases and in-situ floorings; Part 3: polymer-modified cementitious levelling screeds

• BS 8203 — Code of practice for installation of resilient floor coverings; substrate preparation requirements

• BS 5385-1 — Wall and floor tiling; substrate preparation and flatness requirements for ceramic tile

• HSE COSHH Regulations 2002 — cementitious products contain crystalline silica (Class III carcinogen); use RPE (P3 dust mask) when mixing dry powder compounds

• BAL Technical Guidance: Levelling Compounds — UK flooring adhesive and levelling product technical data

• Mapei UK: Floor Preparation Guide — SLC selection and application guidance for UK substrates

• Contract Flooring Association (CFA) — substrate preparation guidance for UK floor installers

• subfloor preparation: assessment and preparation before levelling — the stage before levelling compound

• screed types: when to use sand/cement, anhydrite, or fibre-reinforced — when full screeds are needed rather than levelling compound

• LVT installation: flatness requirements and preparation sequence — using levelling compound to prepare for LVT