Floor Levelling & Subfloor Prep

A poorly prepared subfloor is one of the commonest causes of premature flooring failure in UK renovation projects. Tiles crack at grout joints. Laminate develops hollow spots and clicking noises. LVT joins become visible under raking light. Engineered wood creaks. In almost every case, the root cause is a subfloor that was not flat, dry, or stable enough before the covering went down.

Floor levelling compound, also called self-levelling screed or floor smoothing compound, is the main tool for correcting minor unevenness in concrete and timber subfloors. But it is one step in a broader preparation process, not a substitute for understanding and addressing the specific problems your subfloor presents.

Acceptable Tolerances by Floor Covering

Different floor coverings have different tolerance for subfloor variation. The key measurement is deviation under a 2 m straightedge, how much the surface rises and falls over a 2 m span.

These tolerances apply to the finished, primed substrate, not the raw concrete or boards. The screed code BS 8204 grades flatness in three surface regularity classes, SR1, SR2 and SR3, allowing gaps of 3 mm, 5 mm and 10 mm under a 2 m straightedge.¹ They come from the tiling and flooring codes of practice rather than the Building Regulations. For ceramic and mosaic floor tiling the relevant part is BS 5385-3, whose current edition is 2024 and which now covers natural stone and terrazzo flooring too, the old Part 5 having been withdrawn in May 2024.²

Diagnosing Your Subfloor

Before buying any product, assess exactly what you are dealing with.

Concrete Subfloor

Age and condition: Older concrete ground floors may have no damp-proof membrane. Modern builds and extensions should have a polythene DPM under the slab, at least 300 microns (1200 gauge) with sealed joints, laid on a bed that will not damage the sheet.³ Without it, vapour transmission can be significant.

Moisture testing: The code of practice here is BS 8203, current in its 2017 edition.⁴ The limit flooring suppliers commonly set is a surface relative humidity of 75% or less, measured by the BS 8203 hygrometer test, before an impermeable covering goes down.⁵ Test using a calibrated hygrometer in an insulated, airtight hood on the slab surface, left until the reading stabilises (usually a few days), or a carbide (CM) test, taking the pass figure from the covering or adhesive manufacturer’s data since it varies with the type of base. Do not rely on the ‘back of hand’ test, it is unreliable.

Levels: Lay a 2 m straightedge across the floor in multiple directions. Mark all high and low spots. As a rough guide, variation of up to about 15 mm across the floor is correctable with levelling compound. Greater variation may require a full sand-cement screed or a floating screed system.

Bond: Check that any existing screed or render is bonded, tap with a steel bar and listen for hollow sounds. Unbonded sections must be removed before new compound is applied, or the whole build-up will fail.

Timber Subfloor

Movement and bounce: Walk the floor and identify any boards that flex, creak, or move relative to each other. These must be fixed, a flexible substrate cannot support rigid floor coverings including tile or rigid LVT.

Boards vs sheet: Original floorboards (typically 100–125 mm wide tongued-and-grooved softwood) should be overlaid with exterior-grade plywood, at least 15 mm and commonly 18 mm, before tiling or laying rigid LVT. The plywood is screwed down at close centres, commonly 300 mm or less, into the boards and joists (not into knots or near edges). This creates a continuous, stable sheet surface.

Gaps and voids: Gaps between floorboards should be filled before plywood is laid. On older properties, a ventilated void beneath the floor is normal and healthy, do not block air vents.

Moisture in timber: Timber subfloors should be dry before laying: check with a pin-type meter against the flooring manufacturer’s limit, which is often around 12% or less. Elevated readings suggest a plumbing leak, condensation, or a subfloor void with inadequate ventilation.

Products: Which Levelling Compound to Use

Primer

Most levelling compounds need the concrete primed first, though a few latex-based products are designed to go onto most substrates without priming.⁶ Primer seals porosity, prevents the compound drying too fast (which causes cracking and poor flow), and improves adhesion. Most manufacturers supply a matched primer; use it.

Apply primer by brush or roller at the manufacturer’s rate. For very porous or dusty concrete, two coats may be required. Allow it to dry for the time the data sheet gives before pouring compound.

For plywood-covered timber subfloors, use an appropriate primer formulated for sheet material, standard concrete primers may not bond properly.

Standard Self-Levelling Compound

A pumpable or hand-poured compound suitable for depths of approximately 2–50 mm (check the specific product data sheet, depths vary). Setting times vary: Ardex K 15, for example, is walkable after 2 hours and takes floor coverings the next day.⁷

Brands in this category are widely available from builders’ merchants, Ardex, F. Ball, Mapei, and Weber all produce well-regarded products. Mix to the manufacturer’s water ratio consistently, under-watering creates a stiff compound that will not flow to a level surface; over-watering causes a weak surface that dusts.

Supply cost: £3–£8/m² at typical 3–6 mm application depth.⁸

Fibre-Reinforced Compound

Contains polypropylene or glass fibres for improved crack resistance over timber subfloors. Manufacturers generally specify a fibre-reinforced compound, such as Ardex FA 20, when levelling over plywood or timber.⁹

Rapid-Setting Compound

Takes foot traffic and floor coverings within hours, so the covering can often go down the same day. More expensive than standard compound. Useful when programme is tight but more demanding to apply, there is little time to correct poured sections.

Supply cost: £6–£14/m².

Deep-Fill Compound

For thicker layers, up to 50 mm or more depending on the product. Typically pump-applied in professional applications. At these depths, a conventional cement-sand screed (typically 1:3 to 1:4.5 cement to sharp sand) or a calcium sulphate screed is often more cost-effective.¹⁰

Step-by-Step Application (Concrete Subfloor)

1. Clear the floor and remove all grease, laitance, paint, and adhesive residue. Mechanical preparation (floor grinder, shot blaster, or scarifier) gives the best bond. Grinding produces a smooth surface that receives compound well; shot blasting opens the pores of the concrete for maximum adhesion.

2. Fill large cracks and holes with a suitable repair mortar. Do not rely on self-levelling compound to bridge open cracks or voids, as it may crack on shrinkage.

3. Apply primer at the manufacturer’s specification. Ensure all edges and corners are coated. Allow to reach the correct tack.

4. Mix the compound in a clean bucket using a slow-speed drill with a paddle mixer. Pour in the correct weight of water first, then add powder. Mix for 2–3 minutes to a smooth, lump-free consistency.

5. Pour from a point at the back of the room and spread using a gauge rake or smoothing bar. Work quickly: working times are short, so check the data sheet before you mix. Pull towards the exit.

6. Use a spiked roller immediately after pouring to release trapped air bubbles from the surface.

7. Close the room against draughts and direct sunlight while curing. Draughts cause surface cracking.

8. Check flatness after curing using the 2 m straightedge. A second thin pour is acceptable if needed.

Damp Proofing Under the Levelling Compound

If your concrete subfloor fails the moisture test, an epoxy damp-proof membrane (DPM) must be applied before levelling compound. Epoxy DPMs are two-part coatings applied by brush, roller or squeegee, usually in two coats with a cure period between them that the data sheet sets. The levelling compound is then applied over the cured epoxy.

Do not use polythene sheeting as a DPM under levelling compound, it cannot bond to and will prevent the compound from keying to the slab.

Epoxy DPM supply and application: £10–£20/m².

Costs at a Glance

The cited prices in this table were published in 2023 and brought to 2026. What that means.

When to Call a Professional

DIY application of self-levelling compound is feasible in small areas (up to 10–15 m²) with good preparation and careful mixing. Larger areas, uneven substrates requiring deep fills, and floors with moisture issues are better handled by a specialist flooring contractor or screed layer.

A professional will also carry liability if the floor covering subsequently fails due to inadequate preparation, important when the floor covering itself is expensive. Get the preparation included in the flooring contractor’s written quote, with specific reference to the substrate standard (e.g., “surface to be prepared to BS 8203 flatness tolerance”) so expectations are documented.

The rules in this note are those that apply in England. Scotland, Wales and Northern Ireland have their own building standards and permitted development rights.