Written and reviewed by the Apolodor LTD concrete repair team

A concrete slab that has dropped out of level is more than a cosmetic issue. Gaps open up under skirting boards, doors start to catch, joints step, water ponds where it should drain away, and on external areas and commercial floors, uneven slabs become trip hazards and damage equipment.

The good news is that most sunken slabs can be fixed without demolishing the whole floor. The key is understanding why the slab moved in the first place, because a slab that is relevelled without dealing with the cause will usually sink again. That principle shapes every slab repair we carry out at Apolodor LTD.

Quick answer: A sunken or settled concrete slab is fixed by first finding the cause of the movement, such as poorly compacted fill, leaking drains or clay shrinkage, then filling the voids beneath the slab with cementitious grout, lifting it back towards level with injected expanding resin, or breaking it out and recasting it on a properly prepared sub-base. Cracks and joints are then repaired and the cause is corrected so the movement does not return.

Why Do Concrete Slabs Sink?

Ground-bearing slabs rely on the fill and sub-base beneath them for support. When that support is lost or compresses, the slab follows it down. Common causes include:

• Poorly compacted fill. Hardcore or soil placed in thick layers without proper compaction continues to consolidate for years after construction
• Leaking drains and water mains. Escaping water washes fine material out from beneath the slab, leaving voids
• Clay shrinkage. Shrinkable clay soils lose volume in dry periods, especially near trees and large shrubs
• Decaying organic material. Timber, topsoil or rubbish left in the fill breaks down over time
• Flooding and scour. Moving water can remove fill from beneath external slabs and ground floors
• Overloading. Heavy racking, plant or vehicles on a slab designed for lighter use

How Serious Is a Sunken Slab?

Most settled slabs are a serviceability problem rather than a structural emergency, particularly where the slab is a separate ground-bearing floor that does not support walls. Warning signs that need prompt attention include:

• Gaps that keep widening between the floor and the walls
• Cracking in walls as well as in the floor
• Doors and windows that have started to stick
• Slabs that rock or move underfoot or under wheels
• Water ponding on external slabs next to the building

If walls or foundations show signs of movement, a structural engineer should be involved before any slab repair is carried out. Diagnosing building cracks properly, by reading their pattern, width and direction, helps separate slab settlement from foundation movement.

Step 1: Find the Cause

Investigation usually includes:

• A level survey to map how far and where the slab has moved
• A CCTV survey of nearby drains to check for breaks and leaks
• Checks for trees and vegetation close to the affected area
• Trial holes or cores through the slab to inspect the fill and look for voids
• Monitoring over time where it is unclear whether movement is still happening

Where trees are involved on clay soil, specialist advice is essential. Removing a large, established tree can cause the clay to swell as it rehydrates, which can lift slabs and foundations. The same caution applies when repairing concrete affected by tree root damage, where the tree and the concrete need to be considered together. On our projects, we will not recommend a repair method until we know why the slab moved.

Step 2: Choose the Right Repair Method

Void Filling With Grout

Where the slab is still reasonably level and in good condition but has voids beneath it, holes are drilled through the slab and a cementitious grout is pumped in to fill the gaps and restore full support. This stops further movement and rocking, but does not lift the slab significantly.

Resin Injection Lifting

Expanding resin is injected through small holes in the slab. As it expands, it fills voids, compacts weak ground near the surface and can raise the slab gradually back towards level. Movement is monitored with lasers or levels during injection. The method is quick and causes little disruption, but it relies on the ground beneath being stable once the cause has been dealt with.

Break Out and Recast

Where the slab is badly cracked, the fill has failed over a large area, or the floor needs to be rebuilt to a new specification, the affected area is broken out, the sub-base is excavated and replaced with properly compacted material, and new concrete is laid. Weighing up concrete resurfacing versus replacement helps decide whether this is justified or whether a less disruptive option will do.

When It Is a Foundation Problem

If the movement involves walls and foundations rather than just the floor slab, the repair is a structural matter. An engineer will assess the need for foundation strengthening, which goes beyond slab repair.

Step 3: Repair Cracks and Joints

Once the slab is supported and level, cracks are filled or injected, and joints are reformed and sealed so water cannot reach the sub-base again. Properly filling and sealing expansion joints is a small cost that protects the whole repair.

Step 4: Stop It Happening Again

• Repair leaking drains and water supplies
• Improve drainage around concrete foundations so surface water runs away from slabs
• Manage trees and vegetation with specialist advice
• Keep loads within what the slab was designed for

How Apolodor LTD Fixes Sunken Slabs

We start by investigating the cause, then choose the least disruptive method that will actually last, whether that is void filling, lifting or recasting part of the slab. Where walls or foundations are involved, we bring in a structural engineer before any work is agreed.

We also explain the options plainly, including what each one will and will not achieve, so the client can decide with a clear understanding of cost, disruption and long-term performance.

Towns and Cities We Cover

We investigate and repair sunken slabs as part of our concrete repair services in:

• London
• Manchester
• Birmingham
• Liverpool
• Leeds
• Nottingham
• Bristol
• Brighton
• Cardiff
• Plymouth
• Luton
• Reading
• Norwich
• Swindon
• Portsmouth
• Oxford
• Ipswich
• Maidstone
• Cambridge
• Southampton
• Slough
• Warrington
• Sheffield
• Leicester
• Coventry
• Milton Keynes
• Northampton
• Derby
• Stoke-on-Trent
• Wolverhampton
• Hull
• Exeter
• Gloucester
• Sunderland
• York
• Peterborough
• Chelmsford
• Newcastle
• Bradford
• Bournemouth
• Swansea
• Newport
• Middlesbrough
• Preston
• Blackpool
• Bolton
• Stockport
• Huddersfield
• Lincoln
• Worcester
• Colchester
• Canterbury

Key Takeaways

• Sunken slabs are usually caused by loss of support from the fill or ground beneath
• Finding and fixing the cause is essential, or the slab will settle again
• Void filling restores support, resin injection can also lift, and recasting suits badly failed slabs
• Movement in walls or foundations needs a structural engineer
• Drainage, joint sealing and tree management help prevent future settlement

Frequently Asked Questions

Can a sunken concrete slab be lifted back to level?

Often, yes. Resin injection can raise many slabs back towards their original level, provided the slab is in reasonable condition and the ground beneath is stable once the cause is dealt with.

Is a sunken internal floor the same as subsidence?

Not necessarily. A ground-bearing floor slab can settle on its own because of poor fill, without the building’s foundations moving. An engineer can confirm which is happening, and it is worth checking your insurance position before work starts.

How long does slab lifting take?

Many resin lifting and void filling jobs are completed within a day or two, and the area can often be used again shortly afterwards.

Get Expert Advice

Apolodor LTD investigates and repairs sunken and settled concrete slabs on residential, commercial and industrial sites across the UK. Call 07808 709670 or contact us here.