Written by Ashley Moscrop, Managing Director, Dufaylite Developments Ltd
Reviewed by Nigel West, Technical Manager, Clayboard
Last updated: 15 August 2026

Key takeaways

• Compressible void formers create a controlled void beneath a concrete slab or ground beam so that swelling clay expands into the void instead of pushing up on the structure.
• They work in the opposite way to rigid polystyrene void formers: a compressible void former yields so the slab never has to resist the full uplift of the clay, which can reduce the reinforcement steel a foundation needs.
• Paper honeycomb compressible void formers such as Clayboard are around 30% thinner than the EPS equivalent for the same void, cutting dig depth and groundwork cost.
• On certification, most commercial and many residential clay-belt projects require a BBA-certified void former — which is where Clayboard’s KN30 grade fits.
• The right choice comes down to soil expansivity, slab loads, the void depth required, and whether the project needs a paper-based, recyclable material.

Compressible void formers are the foundation detail that lets a building sit safely on expansive clay. They are installed beneath the concrete to create a void that absorbs clay heave — the upward movement of clay soil as it takes on moisture — so that the ground can move without transferring damaging pressure into the slab above. For any structure on high-plasticity clay, choosing the right compressible void former is one of the more consequential decisions in the foundation design.

This guide explains what compressible void formers are, how they differ from rigid alternatives, how they compare on cost, site handling and environmental impact, and when each is the right call.

What are compressible void formers?

A void former is a sacrificial insert placed beneath a concrete slab or ground beam to create a hollow zone between the soil and the structure. Compressible void formers are the type designed to give way under load once the structure is built — so when the clay swells, it expands into the void rather than lifting the slab.

The mechanism is what sets them apart. A compressible void former supports the concrete pour while dry, holding the slab firm and level as it cures. Once the structure is self-supporting and water reaches the material, its strength reduces and a void opens up. For a paper honeycomb void former, water weakens the paper core and switches off its load-bearing strength — creating the void that absorbs the heave. The slab above never feels the full force of the swelling clay.

That is fundamentally different from a rigid void former, which holds its shape and relies on the concrete structure being strong enough to resist the clay until the material eventually compresses.

You will hear compressible void formers described in a few different ways — as heave protection boards or clay heave boards — but they all refer to the same idea: a layer beneath the slab that is strong when you need it during construction and yielding when you need it afterwards. The common materials are paper honeycomb and, historically, various grades of polystyrene. What unites the compressible types is that they are engineered around the clay’s behaviour rather than fighting it, and that principle is the reason they have become the default approach to heave protection on modern foundations.

Compressible vs rigid void formers: the key difference

This is the single most important comparison to understand, because it changes how the whole foundation is designed.

With a compressible void former, the swelling clay meets a void, so the load is never transmitted upward. With a rigid EPS void former, the slab has to be engineered to resist the full heave force until the polystyrene gives way — which usually means additional reinforcement steel and, on some projects, a thicker concrete slab. That difference is not academic: on one major stadium project, specifying a compressible paper honeycomb void former in place of polystyrene saved over £1m in reinforcement steel alone.

For the structural engineer, this changes the design brief. When the void former does the work of absorbing movement, the slab can be designed for the loads it actually carries rather than for the worst-case uplift of the clay. That flows through to less steel, a potentially thinner slab, and a simpler, lower-carbon foundation.

When a rigid void former is used, the opposite is true — the design has to assume the material resists movement until it fails, so the structure is engineered to carry that force in the meantime. Two void formers that look similar on a data sheet can therefore lead to very different foundations, which is why the compressible-versus-rigid decision is best made early, alongside the structural design rather than after it.

Which is easier to work with on site?

Both types are lightweight and can be handled without lifting equipment, but there are practical differences on the ground. Paper honeycomb compressible void formers are rigid enough to walk on and to take the weight of the concrete pour, foot traffic and reinforcement steel while dry, then do their job once water is introduced through the installation system.

Because paper honeycomb is around 30% thinner than the EPS equivalent for the same void depth, there is less to dig out and less to handle. On a trench-by-trench basis that means shallower excavations, less spoil to remove, and panels that are quick to lay edge to edge across the prepared base. For groundworkers, a thinner, lighter, walk-on board is a straightforward material to install correctly.

Rigid polystyrene void formers are also light and easy enough to handle, so site handling alone rarely decides the choice. Where paper honeycomb compressible void formers pull ahead is the combination: the thinner profile that reduces dig depth, the walk-on strength during the pour, and the fact that a single, well-understood installation method works across residential and commercial jobs. Fewer variables on site tends to mean fewer opportunities for a detail to be built wrong, which matters when the void former is the thing standing between the building and years of heave damage.

What about cost?

The honest answer on cost is that the sticker price of a void former is only part of the picture. The bigger numbers sit in the groundwork it drives.

A compressible void former that reduces the reinforcement steel in the slab, and that is thinner so it needs less dig depth, can cut the total foundation cost even if its unit price is comparable to or slightly above a rigid alternative. The reinforcement saving in particular can be substantial on commercial slabs — steel is one of the more expensive and carbon-intensive parts of a foundation, and a detail that lets you use less of it pays back quickly. When comparing compressible void formers on price, always cost the whole foundation, not just the board.

What about the environmental impact of compressible void formers?

Material choice increasingly matters to specifiers working to net-zero and ESG commitments, and this is where paper honeycomb compressible void formers pull clearly ahead of polystyrene.

Paper honeycomb is made from paper and is recyclable, where EPS polystyrene is a petroleum-based plastic that is not recyclable in standard streams and carries growing regulatory and reputational liability in construction. A paper-based compressible void former also biodegrades naturally in the ground over the life of the structure. On top of the material itself, Clayboard is manufactured by Dufaylite — the only UK manufacturer of paper honeycomb — and the business is certified Carbon Neutral Britain™ and FSC (FSC® C186880). For a construction firm reporting on embodied carbon, swapping a polystyrene void former for a paper one is a simple, defensible improvement.

The carbon story compounds, too. Because paper honeycomb compressible void formers let the slab be designed with less reinforcement, they cut the embodied carbon of the steel as well as of the void former itself — steel being one of the most carbon-intensive materials in any foundation. So the environmental case for compressible void formers made from paper is not just about the board being recyclable; it is about a lighter, lower-carbon foundation overall. For specifiers who have to evidence sustainability decisions, that combination of a recyclable material and a reduced steel requirement is a genuinely strong position to report against.

When is a compressible void former the right choice?

A compressible void former is the right choice on the great majority of suspended-slab and ground-beam foundations on expansive clay, particularly where reducing reinforcement, minimising dig depth, or hitting a sustainability standard matters. The specific grade and void depth then follow from the soil’s plasticity index, the loads the slab will carry, and the ground heave risk band for the site.

Where a project mandates a BBA-certified void former — which covers most commercial and many residential schemes — the certified compressible option is the natural specification. For the fuller technical comparison against the alternatives, see Clayboard vs alternative void formers.

It is worth saying that compressible void formers are not required on every site. Where a building sits on stable, well-draining granular soils with no meaningful shrink–swell behaviour, heave protection may not be needed at all. The decision should always start with a geotechnical assessment of the soil rather than a default. But on the UK clay belt — London, the South East, the East Midlands and East Anglia — expansive clay is common enough that a compressible void former is the norm rather than the exception, and specifying one early is far cheaper than remediating heave damage later.

The Clayboard advantage

Clayboard is Dufaylite’s paper honeycomb compressible void former. Its KN30 grade is BBA-certified (certificate 98/3528) and compliant with NHBC Standards, which is what allows it to be specified by name on projects that require certification. It supports the concrete pour when dry, then weakens and switches off its strength once water is introduced — the mechanism that absorbs heave without loading the slab.

Because it is paper honeycomb, it is around 30% thinner than the EPS equivalent for the same void, recyclable, and made in the UK with the shorter lead times and lower transport carbon that brings. For specifiers on the clay belt, it combines the structural advantage of a compressible void former with the certification and sustainability credentials a modern foundation specification increasingly demands. Explore the full ground heave solutions range for grades and void depths.

How are compressible void formers installed?

Installation follows a clear sequence, and getting it right is part of what makes a compressible void former reliable. First, the ground beams are cast and the trench floor between them is prepared with a compacted, level base. The compressible void formers are then laid flat on that base, butted edge to edge as a single layer covering the trench floor wall to wall — never stacked, and never bridging across the top of the beams. While dry, the panels support groundworkers, the reinforcement steel and the concrete pour.

Once the concrete slab or ground beam has set and become self-supporting, water is introduced into the paper core through the installation system. This is the step that arms the void former: the water weakens the paper honeycomb, its load-bearing strength switches off, and the void opens up ready to absorb any future clay heave. Until that point the material must be kept dry, which is why panels are installed with polythene sheeting to manage water ingress during construction. Done correctly, a compressible void former needs no maintenance for the life of the structure.

What happens if you don’t use a compressible void former?

On a site with expansive clay, leaving out heave protection is a false economy. Without a void beneath the slab, swelling clay has nowhere to go and transfers its full uplift straight into the foundation. The result is the classic pattern of heave damage — cracked slabs and brickwork, doors and windows that jam, lifted floors, and structural repairs that can run to tens of thousands of pounds. Because heave is seasonal, the damage tends to worsen year on year as the clay swells and shrinks. Compressible void formers are a small proportion of the foundation cost but remove that risk at source, which is why they are standard practice rather than an optional extra on clay-belt sites.

Frequently asked questions

What is a compressible void former?

A compressible void former is an insert placed beneath a concrete slab or ground beam that yields once the structure is built, creating a void that absorbs clay heave. Swelling clay expands into the void instead of pushing up on the slab, which protects the structure from ground movement.

Are compressible void formers better than rigid ones?

For most clay-belt foundations, yes. A compressible void former means the slab does not have to resist the full uplift of the clay, which can reduce reinforcement steel. Paper honeycomb compressible void formers are also thinner than rigid EPS for the same void and are recyclable.

How thick does a compressible void former need to be?

The thickness depends on the void depth required, which is set by the soil’s plasticity and the heave risk band for the site. Paper honeycomb void formers deliver a given void in around 30% less thickness than the EPS equivalent, reducing dig depth.

Is Clayboard a compressible void former?

Yes. Clayboard is a paper honeycomb compressible void former. Its KN30 grade is BBA-certified, so it can be specified on projects that require certification, and it weakens once water is introduced to open the void that absorbs heave.

Are compressible void formers recyclable?

Paper honeycomb compressible void formers are paper-based and recyclable, and biodegrade in the ground over the life of the structure. Rigid EPS polystyrene void formers are not recyclable in standard streams.

Specifying a compressible void former?

If you are designing a foundation on clay and weighing up compressible void formers, the team can help you specify the right compressible void former — grade, void depth and material — for your specific soil conditions, slab loads and certification requirements. Contact Dufaylite to talk it through with a technical specialist.