A void beneath a floor slab is usually invisible until something goes wrong. Finding it early means a targeted repair rather than a failed floor.

What causes voids?

The most common causes are:

• Leaking drains and water mains washing fine material out of the ground beneath the slab, sometimes over many years
• Poorly compacted fill settling after construction
• Ground shrinking as it dries out, often worsened by nearby trees
• Old features such as backfilled trenches, soakaways or former cellars

Warning signs

Look out for:

• Cracking in floor slabs, tiles or screeds
• Floors that sound hollow when tapped or feel springy underfoot
• Sloping or sinking floors, or gaps opening under skirting boards
• Cracks in walls near floor level, or doors that start to stick
• Damp patches or unexplained water loss from the supply

How we find them

We use 3D ground penetrating radar to scan through the slab and the ground below, locating voids, washouts and areas of poor compaction without breaking anything open. Findings are marked on the floor and mapped so their position and extent are clear.

Where needed, we confirm results with small drilled holes and a borescope camera to see inside the void directly. A drainage survey can also identify leaking pipes, so the cause is fixed as well as the damage.

Why it matters

Filling a void without fixing its cause, or without knowing its full extent, often means the problem comes back. A proper investigation targets repairs such as grouting, slab replacement or underpinning exactly where they're needed.

Void detection in London and across the UK

ACUO Structural Insights provides void detection, structural investigation and drainage surveys for homeowners, managing agents, insurers and commercial clients across London, Kent, Essex and the South East.

For full details on what the service covers, visit our void detection service page. For a broader look at what GPR can detect beneath a slab, see our article on what GPR can actually see in concrete. Where cracking accompanies voiding, our crack and subsidence investigation service can assess the full picture.

Worried about a floor? Call 07340 549892 or email [email protected].