Floor Levelling Compound: When Do You Need It?

Floor levelling compound (also called self-levelling compound or screed) is a cement-based liquid mix poured over an uneven subfloor to create a smooth, flat surface before new flooring is laid. You need it whenever your subfloor's unevenness exceeds the flooring manufacturer's stated tolerance — which is more often than most people expect, especially in older properties.

How do I know if I need it?

Most flooring manufacturers specify a maximum tolerance, often checked by placing a straight edge (commonly around 2-3 metres long) across the floor and measuring the largest gap underneath it — if the gap exceeds their stated tolerance (often a few millimetres over that distance), levelling is needed before their flooring can be fitted, and skipping it can void the product warranty as well as risk the practical problems covered in our guide to subfloor preparation.

What causes an uneven subfloor?

• Old concrete slabs that were never perfectly flat to begin with, or have settled unevenly over time
• Repairs or patches to the subfloor over the years, at a slightly different level to the surrounding area
• Removal of old flooring that leaves patches of adhesive residue or damage at different heights
• Timber subfloors with boards at slightly different heights, or dips between joists

How levelling compound is applied

The subfloor is first cleaned thoroughly and usually primed with a compatible primer to help the compound bond. The self-levelling compound is then mixed and poured, and largely levels itself under gravity, though it's typically spread with a trowel or spiked roller to help it flow evenly and release trapped air. Drying times vary by product and depth applied, but many are walkable within hours and ready for flooring within a day or two — always check the specific product's guidance, since thicker applications take considerably longer to fully cure.

DIY or professional?

Small areas and shallow levelling jobs are a realistic DIY task for a careful homeowner, since the compound does most of the levelling work itself. Larger areas, significant unevenness, or rooms where getting it wrong would be expensive to fix (before an expensive flooring type goes down) are often better left to a professional, particularly because self-levelling compound sets relatively quickly once mixed, leaving little room to correct mistakes.

Common mistakes

Skipping the primer step is a common cause of the compound not bonding properly to the subfloor beneath, leading to cracking or delamination later. Applying it too thin in places (rather than achieving the full stated minimum depth) is another frequent issue that leaves the levelling compound itself prone to cracking under the new flooring. Working in sections that are too large for the working time of the specific product can also lead to visible ridges where one pour meets the next.

Types of levelling compound

• Standard self-levelling compound: suitable for most general levelling jobs on concrete subfloors ahead of rigid flooring types.
• Fibre-reinforced compound: contains reinforcing fibres for extra strength, often used for thicker applications or where some flexibility is needed, including some timber subfloor applications.
• Rapid-set compound: cures faster than standard products, useful when the project timeline is tight, though it typically costs more and can be less forgiving to work with due to the shorter working time.

Can you level a floor without compound?

For very minor unevenness, some flooring types (particularly click-together laminate and LVT on a suitable underlay) have a small amount of built-in tolerance and don't strictly require levelling compound for negligible dips. However, this is genuinely marginal — most professionals would rather level a borderline subfloor properly than risk the flooring performing poorly, since the cost of levelling compound is small relative to the cost of relaying flooring that's failed prematurely due to movement over an uneven base.

Levelling compound and old adhesive residue

Levelling compound generally needs to be applied to a clean, sound surface, so old flooring adhesive or residue usually needs removing first rather than levelling compound simply poured over the top — pouring over unsuitable residue risks the compound not bonding properly and cracking or delaminating once new flooring is laid on top and put into use.

If you're unsure whether your specific flooring choice needs levelling compound at all, it's worth asking the flooring supplier directly — they'll know the exact tolerance their product needs and can save you the cost of levelling compound in cases where your subfloor is already within an acceptable range.

Always check the manufacturer's guidance on minimum and maximum application depth for the specific product you're using, since applying levelling compound outside its intended depth range can affect how well it cures and performs.