How to level a concrete floor: grind, compound or new screed?

By K & B Screeding Ltd, floor screeding contractors in Slough. Published 23 October 2026. How we check our facts.

Most people searching for how to level a concrete floor start by looking at bags of self levelling compound. That is the right answer for some floors and a costly mistake on others. The deciding factor is a single number: how far the floor is out, measured properly. Get that number first and the method more or less picks itself.

Short answer: lay a 2 metre straightedge across the floor and measure the biggest gap. Under about 3mm with the odd ridge, grind the high spots. From about 3mm to 10mm, pour a smoothing compound over a clean, sound base, primed where the product needs it. From 10mm to 40mm, use a deep base compound or a bonded screed. Beyond that, or if the slab is weak or damp, lay a new screed.

The four methods at a glance

The bands below are our working guide, set against the limits UK manufacturers and the Mineral Products Association (MPA) publish. The product named for each band is an example, not a recommendation, and its own data sheet always takes priority.

Swipe the table sideways to see every column.

Two points sit behind this table. First, every method except the last relies on bonding to the slab, so the slab must be sound and clean. Second, the deeper you go, the more height you add to thresholds, doors and the bottom stair.

How to measure how far out the floor is

Short answer: use a 2 metre straightedge for flatness and a laser or long level for overall slope, and record the worst gap in millimetres.

Flatness and level are different things. A floor can be flat but slope 20mm from one wall to the other, or be level overall with dips and ridges. You need to know both.

1. Sweep the floor clean. Grit under a straightedge gives false readings.
2. Check flatness. Lay a 2 metre straightedge in several directions: along the room, across it and diagonally. Slide a tapered gauge, ruler or stacked feeler strips under the lowest point and note the biggest gap.
3. Mark the floor. Chalk the high spots with a cross and outline the hollows. This map is what a contractor needs to price the job.
4. Check overall level. A rotating laser or a long spirit level shows whether one side of the room sits lower than the other. Note the difference from the highest point to the lowest.
5. Note fixed heights. Measure what height you can add before doors, thresholds, the kitchen plinth or the bottom stair become a problem.

What the SR classes mean

BS 8204, the British Standard for screeds and concrete bases, sets three surface regularity classes measured under a 2 metre straightedge: SR1 allows a gap of no more than 3mm, SR2 no more than 5mm and SR3 no more than 10mm. Thin glued finishes such as sheet vinyl and LVT usually need SR1.

Flooring makers set their own limits too, and some measure differently. Karndean's January 2024 rigid core guide asks for unevenness of no more than 5mm below level over 3m in any direction, says isolated high points and ridges should be removed first, and says larger differences may need levelling by filling the entire floor. Our guide to screeding before flooring compares limits for other finishes.

Under 3mm: grinding and a thin skim

Short answer: if the gaps are small and the problem is a few ridges, trowel marks or a lip between bays, grind them down rather than filling around them.

Diamond grinding cuts the top of the slab away in thin layers. It takes down high spots, and at the same time removes laitance, old adhesive and paint that would stop a compound bonding. What it cannot do is fix a slope or a wide dip: grinding the whole high side down to meet the low side means removing a lot of concrete from a large area, which is slow and costly.

After grinding, a floor within 3mm may be ready as it is for carpet or a tolerant click floor. For sheet vinyl or glued LVT, a skim of smoothing compound a few millimetres thick gives the smooth surface those finishes show every flaw through. Stopgap 300, for example, is listed for 2mm to 20mm, walk on in 90 minutes at 20°C and floor coverings from 6 hours on absorbent surfaces.

Grinding concrete creates silica dust. The HSE states that silica is a major constituent of concrete, mortar and tiles, that grinding generates dust from them, and that heavy and prolonged exposure can cause lung cancer and other serious respiratory disease. Do not dry grind indoors without proper extraction. Our concrete floor grinding service is carried out with dust extraction; we do not sell or hire grinders.

3mm to 10mm: levelling compound

Short answer: this is what smoothing compound is made for, provided the slab is sound, clean and primed.

The steps that decide whether it works:

1. Prepare the surface. Remove dust, laitance, paint and loose material, by grinding where needed. Compound bonds to whatever it is poured on, including dust.
2. Repair first. Tap the slab for hollow areas and fill cracks. A compound does not hold together a broken base.
3. Prime if the product needs it. Use the primer the compound maker names for your base. F. Ball, for example, says Stopgap Green Bag mixed with 114 Liquid needs no primer, but mixed with 128 Liquid it needs Stopgap P131 or P121 first.
4. Set the depth. Work out the depth needed at the lowest point, and check it is inside the product's stated range.
5. Mix and pour. Mix to the stated water ratio, pour in one continuous operation and spread to depth.
6. Spike out air. A spiked roller releases trapped air that would leave pinholes.
7. Let it dry. Follow the data sheet before laying the finish, and allow longer in a cold room.

We use the same approach on our floor levelling work, where compound is usually laid from about 2mm up to around 10mm. If you are unsure whether you need a latex or a water mix product, our guide to latex, self levelling compound or a new screed explains the difference.

10mm to 40mm: deep base compound or bonded screed

Short answer: for this range, use a product designed for depth, or a bonded screed, not a thick pour of a standard smoothing compound.

There are three realistic options:

• Deep section base compound. F. Ball lists Stopgap 600 at 5mm to 75mm in one application, walk on from 90 minutes and floor coverings from 6 hours. It is a base, not a finish: F. Ball says it must be overlaid with a smoothing compound such as Stopgap 300 or 1200 before floor coverings go down.
• Rapid bonded screed. ARDEX states that A 38 can be laid 15mm to 40mm bonded over a properly prepared concrete base, using a slurry bonding coat applied fresh in fresh. It is walkable in 3 hours, takes tiles after 4 hours, and carpet, wood and vinyl after 48 hours.
• Traditional sand and cement bonded screed. The MPA says a bonded screed should be at least 25mm thick, that 40mm should normally be specified to allow for deviations in the base, and that the hardened concrete must be prepared by mechanical methods to maximise bond. It is the slowest option to dry: our working rule is about a day per mm for the first 40mm, then slower.

A bonded screed is our usual answer where the floor carries heavy loads or a rigid finish such as large tiles, and time allows it to dry. See bonded screed for how we prepare and lay one.

Can you level a floor with concrete?

People often ask whether they can simply pour new concrete on top. A thin layer of ordinary concrete over an old slab tends to crack and lose its bond, because it has neither the depth to stand alone nor the preparation to bond. The bonded screeds and deep base products above exist for this job. Where the existing slab itself is cracked, sinking or structurally unsound, that is a question for a structural engineer, not a levelling product. We do not pour structural slabs.

Over 40mm: a new screed

Short answer: past about 40mm, or where you cannot bond to a sound base, you need a new screed at full depth.

That applies when the slab is contaminated or hollow, when a damp proof membrane has to go on top, or when insulation or underfloor heating is going into the floor. The MPA gives an unbonded screed a 50mm minimum, with 70mm specified, and on our own work a floating sand and cement screed is 65mm in homes and 75mm in commercial buildings. ARDEX gives its rapid screed the same 65mm domestic floating minimum, so a fast product does not let you go thinner over insulation.

Common mistakes when levelling concrete

• Buying product before measuring. A 20mm problem needs a different product from a 3mm one.
• Pouring past the stated depth. A general purpose compound poured deep to save money can crack and debond.
• No primer, or the wrong one. The compound dries too fast or does not bond.
• Pouring over dust, paint or laitance. The compound bonds to the contamination, which then lets go.
• Levelling a damp floor. No compound or screed cures damp. Test first, and use a membrane if needed.
• Ignoring height. Every millimetre raises door bottoms, thresholds and the bottom step.
• Grinding a slope. Grinding fixes ridges, not a floor that falls across the room.

What it costs

We have not found a published 2026 UK price per m² for smoothing or deep base compounds, so we do not quote one. For screeds, trade2base (June 2026) puts a bonded screed at £12 to £18 per m² for 25mm to 40mm, and MyJobQuote (August 2026) puts a fast drying screed at £20 to £28 per m², both installed. Preparation, such as grinding and crack repairs, can cost as much as the levelling itself. Our floor screeding cost guide explains how prices are built up.

Get a fixed price for your floor

Measure the worst dip with a straightedge, send photos and the room size, and we will tell you the method and a fixed price.

To price it accurately, send us:

• The area in m², or the room sizes.
• The biggest gap under a 2 metre straightedge, and any fall across the room.
• The height available before doors or thresholds become a problem.
• What is on the slab now: bare concrete, paint, adhesive, old tiles.
• What goes on top: LVT, vinyl, tiles, wood or carpet.
• Your postcode.

Use our contact form or call 07466 650936. We call back the same day, send a written quote within 24 hours and agree a fixed price before any work starts.

Sources

• BS 8204-1:2003+A1:2009, concrete bases and cementitious levelling screeds to receive floorings, BSI, accessed October 2026
• Mortar data sheet 22: Screeds, Mineral Products Association, accessed October 2026
• Stopgap 300 heavy duty smoothing compound, F. Ball and Co., accessed October 2026
• Stopgap Green Bag general purpose smoothing compound, F. Ball and Co., accessed October 2026
• Stopgap 600 deep section base compound, F. Ball and Co., accessed October 2026
• ARDEX A 38 rapid drying screed, ARDEX UK, accessed October 2026
• Rigid core installation guidelines, UK, Karndean Designflooring, January 2024
• Silica dust in construction, Health and Safety Executive, accessed October 2026
• Floor screed costs UK, trade2base, June 2026
• Floor screeding cost guide, MyJobQuote, August 2026