Self Levelling Compound: UK Guide

What is self levelling compound?

Self levelling compound (also called self levelling screed, floor leveller, or SLC) is a cement-based powder mixed with water to create a pourable liquid that spreads across a floor and finds its own level. The surface cures to a smooth, flat finish ready for final flooring.

It differs from traditional sand-cement screed in two ways. First, it flows rather than needing to be spread and trowelled. Second, it works at much thinner depths (3-10mm vs 50mm+ for screed). You pour it, give it a light rake, and gravity does the rest.

Most UK products are polymer-modified, meaning they include additives that improve flow, adhesion and crack resistance. Common brands include Bostik Cempolay, Mapei Ultraplan, and Bond It Self Level 30.

When to use self levelling compound

Use self levelling compound when you need a smooth subfloor but the existing surface has minor imperfections. Typical scenarios:

• Vinyl or laminate prep: These finishes telegraph every bump. A 5mm layer of SLC gives you a perfectly flat base.
• Uneven concrete: Old garage or basement floors with shallow dips and ridges. SLC fills the lows without building up the highs.
• Over existing screed: If the original screed is sound but has surface defects (minor cracking, dustiness, slight slope), a thin SLC coat repairs it.
• Underfloor heating install: Many installers pour SLC over the heating pipes to encapsulate them and provide a smooth finish. Check the product is rated for UFH (most are).

Don't use SLC if the floor is structurally unsound, has major movement, or needs more than 10mm build-up in standard depth. For that, use traditional screed or repair the structural issue first.

Types of self levelling compound

Standard SLC (3-10mm)

Most common type. Applied at 3-10mm depth. Walk-on after 2-4 hours, flooring after 24 hours. Good for general floor prep. Costs around £6-8 per 20kg bag.

Rapid set SLC

Faster curing formula. Walk-on in 1 hour, flooring in 3-4 hours. More expensive (£10-12 per bag) but saves downtime on commercial jobs or quick turnarounds. Shorter working time means you need to mix and pour faster.

Deep fill SLC (up to 50-60mm)

Specialist product for areas needing more build-up. Can be poured up to 50-60mm in one layer. Takes longer to cure (48-72 hours). More expensive and not always necessary. If you need more than 60mm, traditional screed is more economical.

Flexible SLC

Contains additional polymers for slight flex. Used over timber subfloors or areas with minor movement. Costs slightly more. Not suitable for heavy traffic or areas with significant deflection.

How to apply self levelling compound

1. Prepare the subfloor

The subfloor must be clean, dry, and primed. Sweep and vacuum all dust. Repair any cracks wider than 3mm with rapid-set repair mortar. If the concrete is porous or dusty, apply a diluted PVA primer or acrylic primer per the SLC manufacturer's instructions. Let the primer dry to tacky (usually 30-60 minutes).

2. Mix the compound

Pour clean cold water into a mixing bucket first, then add the powder. Typical ratio is around 5-6 litres of water per 20kg bag (check the bag). Mix with a paddle mixer on a slow-speed drill for 2-3 minutes until smooth and lump-free. Don't over-mix or add extra water beyond the stated ratio.

3. Pour and spread

Pour the mixed compound onto the floor, starting at the furthest corner. It will flow, but give it gentle encouragement with a smoothing rake or plasterer's trowel to push it into corners and edges. Use a spiked roller to release any trapped air bubbles. Work quickly because working time is limited (15-30 minutes depending on product and temperature).

4. Allow to cure

Keep the room well-ventilated but avoid draughts that cause too-rapid drying. Don't walk on it until the stated foot-traffic time (check the bag). Don't lay final flooring until fully cured. For most products that's 24 hours at 3-5mm depth, 48 hours at 10mm.

Coverage and quantities

A 20kg bag of standard SLC typically covers:

• 10m² at 3mm depth
• 5m² at 5mm depth
• 3m² at 10mm depth

To calculate bags needed: measure the floor area in m², decide on depth in mm, then use the bag coverage chart. Example: 15m² floor at 5mm depth needs 3 bags (15 ÷ 5 = 3).

Always buy 10% extra to account for wastage, uneven depths, and spillage.

Drying times explained

Self levelling compound dries from the surface down. The top feels solid after a few hours, but moisture is still evaporating from the lower layers. Three key milestones:

• Foot traffic: 2-4 hours (standard), 1 hour (rapid set). You can walk on it but don't drag heavy items.
• Light loads: 12-24 hours. You can move furniture carefully but don't install flooring yet.
• Full cure: 24-48 hours (standard), 3-4 hours (rapid set). Safe to install vinyl, tiles, laminate, carpet.

Drying time increases with depth, humidity, and poor ventilation. A 10mm layer in a damp basement might take 72 hours. A 3mm layer in a warm, ventilated room might be ready in 18 hours.

Use a moisture meter if you're unsure. Most vinyl and laminate manufacturers specify maximum subfloor moisture content (often 75% RH). Test before laying the floor or you risk adhesion failure and bubbling.

Common problems and how to avoid them

Cracking

Happens when the mix is too wet, the subfloor wasn't primed, or the layer is too thin in places. Stick to the water ratio on the bag. Prime porous surfaces. Aim for consistent depth across the floor.

Poor levelling

Usually caused by not spreading the compound into corners or pouring too slowly (it starts setting before you finish). Work quickly and use a rake to push material where needed. Have a helper mix the next batch while you pour the first.

Delamination

The SLC lifts away from the subfloor. Almost always a priming failure. Either the primer wasn't applied, wasn't left to dry to tacky, or the subfloor was dusty/contaminated. Clean and prime properly.

Surface bubbles

Trapped air rises and leaves small craters. Use a spiked roller immediately after pouring to release air. Walk through the wet compound in spiked shoes (专业installers use golf shoes).

Cost and where to buy

Expect to pay:

• Standard SLC: £6-8 per 20kg bag (Wickes, Screwfix, Toolstation)
• Rapid set: £10-12 per 20kg bag
• Deep fill: £12-15 per 20kg bag
• Primer: £15-25 per 5L (covers 25-40m² depending on porosity)

A typical small bathroom (5m²) at 5mm depth needs 1 bag of SLC and half a litre of primer, total cost around £10. A large open-plan room (40m²) at 5mm needs 8 bags and 2L primer, total cost around £65.

Buy from trade counters (better quality, advice available) or DIY sheds (convenient, more expensive). Bostik and Mapei are premium brands used by professionals. Own-brand versions from Wickes or Screwfix are fine for DIY.

Self levelling compound vs traditional screed

Use SLC for topping and smoothing existing subfloors. Use traditional screed when building a new floor from scratch or needing significant depth.

Related guides

• Floor Screed: Complete Guide
• Self Leveling Concrete Explained
• Liquid Screed vs Traditional Screed
• Concrete Screed Mix Ratios