Structural and base materials · Concrete & Cement
Self-Levelling Compound (Floor Levelling)
Sets out the thin correction layer poured over a base that is already broadly right, and why it neither builds up like a screed nor creates a fall.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What self-levelling compound is
A self-levelling compound is a factory-blended powder, usually cement or calcium sulfate based with polymers included, mixed with water or a latex liquid to a pourable consistency. Poured onto a prepared floor it flows out under its own weight, assisted with a spiked roller or trowel, and sets to a smooth plane ready to receive a floor covering.
It corrects rather than constructs. Where a sand-cement screed is laid to a thickness and worked by hand, and can therefore bury pipes, form falls and make up substantial level differences, a levelling compound is applied as a thin layer over a base that is already broadly correct. It cannot be relied on to create a fall, because it seeks level by nature, and it is not a structural layer.
Products differ considerably from one another. Some are formulated for timber floors and tolerate movement in the deck, some are for concrete only, some are intended to be used with a primer specific to the substrate, and some are meant to be covered rather than left exposed. The manufacturer's data sheet governs which substrates a given compound accepts, and combining two systems rarely works.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- Self-leveling compound (US spelling)
- Levelling compound
- Floor leveller
- Smoothing compound
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Smoothing an existing concrete floor before vinyl, carpet or engineered timber is laid.
- Correcting a screed that has cured slightly out of tolerance for a thin finish.
- Taking up small steps and undulations where old floor coverings have been removed.
- Preparing a plywood-overlaid timber floor to receive a resilient covering.
- Evening out a floor after adhesive residues, patch repairs or service chases have been made good.
- Providing a smooth base over a heated floor surface where the heating layer is already in place.
Consideration
Where it is commonly considered
- Where the base is essentially sound and only surface flatness needs improving.
- Where a thin, unforgiving finish such as sheet vinyl would telegraph every imperfection.
- Where the floor level cannot be raised much because of thresholds and door clearances.
- Where a fast-setting preparation layer suits a short working window in an occupied space.
- Where the flooring manufacturer calls for a smooth base of a particular quality.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- Where the level difference is large enough that a screed would be the proper discussion.
- Where the base is moving, deflecting, damp or debonded and the cause has not been addressed.
- Where a fall towards a drain is needed, since the material seeks level rather than slope.
- Where the compound would be used as the visible wearing surface without the manufacturer allowing it.
- Where the substrate is not one that the chosen product's data sheet accepts.
- Where a ground floor has no functioning membrane beneath it and moisture has not been assessed.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Substrate dependency
- Compatibility with the base is the central issue. Concrete, screed, timber sheet, old adhesive residue and tiled surfaces each need different preparation and often a different primer, and the product data sheet is the authority on which are accepted.
- Installation dependency
- Working time is short once the liquid is added, and the pour has to be continuous across an area so that successive batches merge. Mixing consistency, room temperature and how quickly the material is spread all show up in the finished plane.
- Moisture behaviour
- Moisture in the base can affect both the bond and the covering above. Where a ground floor lacks a functioning membrane, applying a smooth layer over it does not deal with the underlying moisture, and the assessment belongs to a specialist.
- Surface wear
- Most compounds are formulated to be covered rather than walked on as a final surface. Where a product is described as tolerating light exposure, that description and its limits come from the manufacturer rather than from general practice.
- Repairability
- Local defects can be ground back and re-poured, but feathered edges and patches often telegraph through thin coverings. A debonded area usually has a cause that a patch on its own will not resolve.
- Documentation
- The data sheet sets out the accepted substrates, the matching primer, the mixing proportions in the manufacturer's own terms, the working time and the permitted layer range, together with what may be laid over the compound and when.
- Appearance
- The surface is usually hidden, but trowel ridges, batch lines where pours met and pinholes can telegraph through thin sheet coverings, so the standard of finish still matters even though it will not be seen.
Exposure
Exposure and environmental conditions
- Is this a ground floor where moisture rising from below has been considered?
- What temperature will the room be at during the pour and while the compound sets?
- Will the floor be in a wet area where water may reach the layer in normal use?
- Is there underfloor heating below, and what does its supplier require before and after pouring?
- Is the area occupied, and can it be kept clear while the material sets?
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- The base has to be sound and firmly attached, because the compound follows whatever is beneath it.
- Priming is normally required, and the correct primer differs by substrate and by product.
- Timber decks need to be rigid and are often overlaid before a compound is considered at all.
- Old adhesive residues, paint, curing agents and dust all interfere with bond unless removed.
- Perimeter and movement joints in the base need respecting rather than bridging over.
- A levelling compound is not a moisture control measure, so the presence, condition and position of any damp-proof membrane or moisture control layer in the floor build-up is a matter to establish with the relevant professional before pouring.
Appearance
Appearance and finish considerations
- The quality of finish shows through thin vinyl and rubber coverings far more than through carpet.
- Batch lines can appear where one mix started to set before the next one arrived.
- Feathered edges around patches can be visible or can break away under traffic.
- Colour is not usually a consideration because the layer is covered by the final finish.
- Where a manufacturer permits an exposed finish, the appearance still varies with pour conditions.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- Does the chosen product accept this substrate according to its data sheet?
- Has the correct primer been used, and was the base properly prepared first?
- What layer range does the manufacturer allow for the product being used?
- How will the bond be checked before the floor covering goes down?
- What does the covering manufacturer require of the surface it is laid on?
Upkeep
Maintenance and repair considerations
- If a hollow-sounding area develops, how would it be investigated?
- Would repairs need the same product, and has the product used been recorded?
- If the covering is replaced later, can the compound layer be reused as it is?
- How would a moisture problem beneath the layer be identified after the covering is down?
- Is there a risk that removing the covering damages the levelling layer underneath?
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- Preparation usually takes longer than the pour itself and determines whether it works.
- The room has to be closed off while the compound sets, because footprints cannot be removed.
- Mixing equipment and enough labour are needed to keep a continuous pour going.
- Doorways, perimeters and level changes need forming or damming before material is poured.
- Waiting periods before the floor covering is laid come from the manufacturer, not from the programme.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- The manufacturer's data sheet for the specific compound and its matching primer.
- Confirmation of which substrates the product accepts and what preparation it requires.
- The stated waiting period before floor coverings may be laid over it.
- Any moisture assessment carried out on the base before the pour.
- The floor covering manufacturer's requirements for the surface beneath it.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Is a levelling compound the right answer here, or does this floor need a screed?
- What is causing the unevenness, and does it need addressing before smoothing over it?
- Which product and primer are being used, and do they suit this substrate?
- How much build-up is possible without exceeding what the product allows?
- How will the base moisture be assessed before we pour over it?
- How flat does the surface need to be for the covering we have chosen?
- How long will the room be out of use, and when can the covering be laid?
- What happens at doorways and where the floor meets other levels?
Blind spots
Commonly overlooked points
- Smoothing a floor does not fix what is under it, and movement or moisture comes back through.
- A levelling compound cannot form a fall, so wet-room floors are a different conversation entirely.
- Primer is not an optional preliminary, and skipping it is a common cause of debonding.
- Even a thin layer changes the floor height, which affects doors and thresholds.
- Timber floors need particular products, and a compound made for concrete may not tolerate the movement.
What this page does not do
- This entry does not state that a levelling compound suits any specific floor, which depends on the substrate, the product and a specialist assessment.
- Nothing here should be read as guidance on moisture in a floor, which needs investigation of the specific building.
- Mixing proportions, primers and waiting periods come from the manufacturer's documentation rather than from a reference page.
- Build Design Hub does not supply, test, approve or endorse levelling products or those who install them.
Related entries
Related materials
Materials from the same family, an adjacent role in the assembly, or a shared application.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
Concrete & Cement Materials
Reference entries for cement-based materials — ready-mixed and precast concrete, screeds, levelling compounds, renders of the cementitious family and the admixtures that modify them.
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