Self-Levelling Compound: How to Get a Flat Floor Without Paying a Screeder

You've got your extension floor slab down, underfloor heating pipes laid, and now someone tells you the floor needs levelling before any tiles or vinyl can go on. You price up a professional screeder: £500 – £600 for a single room. Then you discover that a few bags of self-levelling compound and a Saturday morning could do the same job for under £100 in materials. The catch is that "self-levelling" is one of the most misleading product names in construction. Pour it wrong, skip the primer, or mix it too thick and you'll be chiselling it off the floor and starting again.

What it is and what it's for

Self-levelling compound is a cement-based powder that you mix with water to create a pourable liquid. You spread it across a floor and it flows to find a level surface, filling dips and smoothing undulations. Once set, it gives you a flat, hard base ready for tiles, vinyl, laminate, or engineered wood.

The "self-levelling" part is only half true. The compound does flow and settle under its own weight, but it won't travel across a room unaided. You need to pour it where it's needed, spread it with a steel trowel, and work it into low spots. Think of it like thick pancake batter, not water. It finds a local level within the area you pour it, but it won't magically flow from one end of a 5m kitchen to the other.

Most products contain latex (either as a separate liquid you add during mixing, or pre-blended into the powder). The latex gives the cured compound flexibility, which prevents cracking from minor floor movement. Fibre-reinforced versions add synthetic fibres for extra crack resistance and are the only type suitable for timber substrates.

You'll see this material called different things: floor leveller, floor compound, latex screed. They're all the same product category.

Types and when to use each

Not all self-levelling compounds do the same job. The product you need depends on three things: how deep you're pouring, what you're pouring onto, and how fast you need to walk on it.

The depth range matters more than anything else when choosing a product. Standard thin-coat compounds applied at 30mm will crack. Deep-base products applied at 2mm won't flow properly. Always check the datasheet.

The deep-fill trap

If your floor has dips of 30mm or more, self-levelling compound is the wrong product for the bulk of the fill. At those depths you're burning through bags at enormous cost. A 20kg bag of deep-base compound covers just 0.50m2 at 20mm thickness and 0.20m2 at 50mm. For a 24m2 kitchen floor with a 40mm dip across half the area, you'd need upwards of 24 bags at the deep section alone, costing over £500 in compound.

The trade approach is to use a sand-and-cement grano screed for the bulk fill (far cheaper per m2), let it cure, then apply a thin 3-5mm skim of self-levelling compound as the finishing layer. That finishing layer is what gives you the flat surface your tiles need.

How to work with it

This is where most DIY guides get vague. The actual process has five stages, and skipping any one of them is how floors fail.

1. Prepare the substrate

The floor must be clean, dry, and structurally sound. Sweep it, vacuum it, then sweep it again. Any dust, grease, or loose material will prevent the compound from bonding. Fill cracks wider than 2-3mm with rapid-set tile adhesive first. Grind down any high spots with an angle grinder fitted with a diamond cup wheel.

Check for moisture. Tape a piece of plastic sheeting (about 500mm square) to the concrete floor overnight. If there's condensation on the underside in the morning, you have a moisture problem. If relative humidity exceeds 75%, you need a liquid DPM (damp proof membrane) applied before the compound, not just a primer.

2. Prime the substrate

This is the step people skip, and the step that causes 90% of failures.

Priming does two things. On porous surfaces (concrete, sand-and-cement screed), it seals the surface so it doesn't suck the water out of your compound before it can level. On non-porous surfaces (power-floated concrete, existing tiles), it creates a key for the compound to grip.

Primer dilution depends on substrate porosity:

• Porous substrate (standard concrete, sand-and-cement screed): dilute SBR or acrylic primer with water, typically 1:2 or 1:3. Two coats, allowing the first to dry (about an hour at 20C) before applying the second.
• Non-porous substrate (power-floated concrete, existing ceramic tiles, terrazzo): use undiluted primer or a specialist grip primer like Mapei Eco Prim Grip Plus. Scuff glazed tiles first with 60-grit sandpaper to give the primer something to bite into.
• Timber/plywood: use a flexible, timber-specific primer. Standard acrylic primers are too rigid. The primer must accommodate wood's natural expansion and contraction. Only fibre-reinforced compound should be used over timber.

One exception: Ardex Arditex NA requires no priming at all. It's a two-part product (powder + latex liquid in the same box) designed to bond directly to almost any substrate including tiles, plywood, and even bitumen. It costs more per bag, but when you factor in the primer cost and extra preparation time, it's competitive for smaller areas.

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3. Mix correctly

Add the powder to the water, not the water to the powder. This sounds pedantic but it matters. Adding water to powder creates lumps that won't break down, even with a paddle mixer. Start with the water in the bucket, then gradually add the powder while mixing.

You need a drill with a paddle mixer attachment. Mixing by hand won't work. The compound needs to reach a smooth, lump-free consistency that pours freely, often described as "cold runny porridge." Too thick and it won't flow. Too thin and it will be weak when cured, with surface cracking. Follow the water ratio on the bag exactly. For Mapei Ultraplan 3240 (a common choice), that's 4.25-4.5 litres per 25kg bag. For Bostik Cempolay Ultra Strong, it's 3.8 litres per 20kg bag.

4. Pour and spread

Start at the farthest point from the door and work backwards. Pour the compound in continuous ribbons across the floor, not in a single pile. Use a steel trowel or a gauge rake to spread it into low spots and guide it to the edges.

Maintain a wet edge. Each new bucket of compound must overlap with the previous pour while it's still liquid. If the edge of your first pour has started to skin over before you pour the second, you'll get a visible ridge at the joint that's almost impossible to sand flat.

One builder's trick from the forums: for floors with a known slope, screw a grid of screws into the substrate at 500mm centres, all set to the exact finished height using a laser level. The screw heads act as depth guides as you pour. Pull them out once the compound has set to trowel-height consistency but before it fully cures.

5. Roll out the air

On standard (non-fibre-reinforced) compound, push a spiked roller across the surface within a few minutes of pouring. The spikes pop air bubbles trapped in the mix. Skip this and you'll get pinholes in the surface, tiny craters that telegraph through vinyl flooring.

Environmental conditions

Self-levelling compound is sensitive to temperature. Apply between 5C and 25C, both air and substrate temperature. Below 5C, the cement reaction slows dramatically and the compound may not cure properly, leading to a soft, powdery surface. Above 25C, it sets too fast for you to work it.

Close windows and doors during application and for at least 24 hours after. Draughts dry the surface before the bulk of the compound has set, causing surface cracking and curling at the edges. Do not turn on underfloor heating to "help it dry." The heat causes rapid surface drying while the interior is still wet, and the result is a cracked, delaminated floor.

How much do you need

Coverage depends entirely on depth. A 25kg bag of Mapei Ultraplan 3240 covers about 6m2 at 3mm, but only 1.5m2 at 12mm. A 20kg bag of Setcrete Deep Base covers 2.15m2 at 5mm, dropping to 0.50m2 at 20mm. The relationship between depth and coverage is not linear: doubling the depth more than halves the coverage because the compound is denser than you'd expect.

Worked example: 24m2 kitchen floor, 5mm average depth

A typical extension kitchen floor is about 24m2 (roughly 4m x 6m). If your floor is reasonably level and you're applying a 5mm skim to smooth minor undulations:

• Using Mapei Ultraplan 3240 (25kg bags): 6m2 per bag at 3mm, so roughly 3.6m2 per bag at 5mm. You need approximately 7 bags.
• Using Setcrete Deep Base (20kg bags): 2.15m2 per bag at 5mm. You need approximately 12 bags.
• Using No Nonsense L12 (20kg bags, Screwfix): 4.2m2 per bag at 3mm, so roughly 2.5m2 per bag at 5mm. You need approximately 10 bags.

Add 10-15% for wastage (compound left in buckets, uneven substrate eating more material in the dips than you estimated). That takes 7 bags to 8, or 12 bags to 14.

Underfloor heating: a different calculation

If you're encapsulating underfloor heating pipes, you need enough depth to fully cover the pipes with at least 5-10mm of compound above the top of the pipe. UFH pipes are typically 16mm diameter, so you're looking at a minimum pour depth of 25-30mm.

At 30mm over 24m2, the material cost escalates fast. The graph relationship for this task suggests 20-30 bags. At that depth, a deep-base product or even a liquid screed pumped in by a contractor is usually more cost-effective than self-levelling compound.

Cost and where to buy

Self-levelling compound ranges from around £15 to £40 per bag depending on brand, formulation, and bag size. Budget products do the job for simple concrete smoothing. Premium products justify their price when you need deeper pours, faster set times, or compatibility with difficult substrates.

Primer adds to the total cost. Setcrete High Performance Acrylic Primer is around £20 for 2.5 litres. Wickes SBR costs around £8 – £12 for 5 litres. Diluted at 1:3 or 1:4, a 5-litre bottle of SBR is enough for 30-40m2 across two coats, making it the cheapest priming option.

For that 24m2 kitchen at 5mm depth, your total material cost looks like:

• Budget route (No Nonsense L12 + SBR primer): 10 bags at £16 + primer £10 = around £170
• Mid-range route (Mapei Ultraplan 3240 + Mapei primer): 8 bags at £23 + primer £20 = around £204
• Premium route (Ardex Arditex NA, no primer): 8 bags at £40 = around £320

Professional application for the same area would cost £400 – £600 including materials and labour.

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All of these products are stocked at UK builders' merchants (Travis Perkins, Jewson) and the major retail chains (Screwfix, Toolstation, Wickes). Screwfix often has multi-buy deals, particularly on the Mapei Ultraplan 3240 (4 bags for £80 at the time of writing). For large orders, check builders' merchant trade accounts for better pricing.

Alternatives

Self-levelling compound isn't always the right answer.

Traditional sand-and-cement screed is far cheaper for deep fills (30mm+). A 1:3 or 1:4 cement-to-sand mix laid by a labourer costs a fraction of the equivalent depth in self-levelling compound. The trade-off is that screed takes days to cure, needs skill to get flat, and doesn't achieve the same surface regularity as compound.

Liquid (anhydrite) screed is pumped in by specialist contractors and gives a superb finish. It's the professional choice for large areas with underfloor heating, where the compound flows around the pipes far better than anything applied by hand. Expect to pay £15 – £25 per m2 for supply and installation, which becomes competitive with DIY self-levelling compound at depths over 30mm.

Plywood overlay is an alternative for timber floors where the existing boards are sound but uneven. Screw 6mm or 9mm plywood across the floor to create a flat surface. No wet trades, no drying time. Won't work if the floor needs to come up in level by more than a few millimetres.

Where you'll need this

• Screeding - applied over the floor slab as an alternative to traditional sand-and-cement screed, particularly suitable for thin finishing layers
• Underfloor Heating - poured over UFH pipes to encapsulate them and create a level floor surface
• Flooring - applied over existing screed to correct minor undulations before laying vinyl, laminate, or thin tiles

Common mistakes

Using PVA as primer. Already covered above, but it bears repeating because it's the most common cause of failure. SBR or proprietary acrylic primer only.

Pouring too thick for the product. A compound rated for 1-10mm will crack at 20mm. Deep dips need a deep-base product or a screed-and-compound two-stage approach.

Mixing too wet. Adding extra water makes it flow further, which feels like a win until it cures with a weak, dusty surface that crumbles under foot traffic. Follow the water ratio on the bag. Exactly.

Working alone on a large area. For anything over 15-20m2, you need a second person. One mixes while the other pours. If you stop to mix a new batch, the edge of your last pour skins over and you get a visible ridge.

Forgetting the spiked roller (on non-fibre products). Those tiny air bubbles trapped in the mix create pinholes in the surface. They're invisible until you lay vinyl and every single one telegraphs through as a tiny bump.

Confusing walk-on time with ready-to-tile time. A compound that's walkable in 3 hours still needs 24 hours minimum before you lay tiles, and several days before wood or laminate. The surface feels hard but moisture is still escaping from the interior of the pour.

Applying in cold weather without checking the forecast. The compound needs sustained warmth to cure properly. A morning pour that freezes overnight will give you a floor that crumbles underfoot.