What is a void former used for?
Discover the role the void former plays in the construction process and get help choosing the right product type for your build
Written by Ashley Moscrop, Managing Director, Dufaylite Developments Ltd. Reviewed by Nigel West, Technical Manager, Clayboard. Last updated: 15 August 2026.
Key takeaways
- A void former is a sacrificial insert placed beneath a concrete slab or ground beam to create a compressible void that absorbs clay heave, protecting the structure from ground movement.
- Compressible void formers such as Clayboard weaken once water is introduced, so the slab never has to resist the full uplift of swelling clay — cutting the steel a project needs.
- Rigid EPS void formers work the opposite way: the slab must resist heave until the EPS gives way, usually needing more reinforcement and a thicker slab.
- Clayboard is a paper honeycomb void former, BBA-certified in its KN30 grade, and around 30% thinner than the EPS equivalent — less dig depth and lower groundwork cost.
- Choosing the right void former for concrete foundations comes down to soil expansivity, slab loads and the void depth required.
What are void formers?
Void formers are specially designed inserts, made from either degradable or permanent materials, that are placed beneath reinforced concrete slabs or structures. They create a hollow zone between the soil and concrete, which helps absorb ground heave and lighten structural weight. The common materials used to create them include paper honeycomb sandwich panels, EPS and plastics. The type commissioned will ultimately depend on the kind of soil present, the design of the slabs, and the goals for the overall build.Broadly, void formers for concrete foundations fall into two families. Compressible void formers, such as paper honeycomb, are engineered so their load-bearing strength switches off once the structure is built, opening a void that swelling clay can move into rather than pushing against the slab. Rigid void formers, typically EPS polystyrene, instead hold their shape until the pressure of the clay slowly compresses them. Both create the void former slab detail that protects the foundation, but they behave very differently under load — which is why the choice of void former matters as much as the decision to specify one at all.
You will also hear the same product described as a heave former, a clay heave board or a compressible former; in a foundation context these all refer to the same compressible layer of void former laid beneath the slab or ground beam.
How void formers work: the basics
In all cases, it supports the slab during curing but later allows soil movement to be absorbed without transferring pressure to the concrete.The sequence on site is straightforward. The void former is laid on the prepared ground before the concrete is poured, giving firm, level support while the slab or ground beam cures and gains strength. Once the structure is self-supporting, the void former’s job changes: it has to yield so that expanding clay has somewhere to go.
A compressible void former like paper honeycomb does this by design — when water reaches the paper core it weakens and switches off its load-bearing strength, opening a void that absorbs the heave without pushing up on the slab above. A rigid EPS void former works the other way around: the slab has to be engineered to resist the full uplift of the clay until the polystyrene finally compresses. That difference is the single most important thing to understand about how void formers work, because it flows straight through to how much reinforcement steel the foundation needs — and therefore what the groundworks cost.
How do void formers protect foundations and benefit your project?
They absorb soil heave and uplift pressure
Expansive soils, especially clay, can swell significantly when it’s wet, exerting upward pressure that risks cracking slabs. When properly installed, it serves as a temporary sacrificial formwork that takes the weight of wet concrete and steel reinforcement, but subsequently weakens to create a buffer zone. What happens is, the soil heave fills the void instead of pushing against the slab.
To activate Clayboard, water is introduced via the Voidpack pipe, irrigating the core. This weakens it, allowing it to give way under minimal load.
Plastic and polystyrene void formers are not activated in this way and require the slab to be designed to resist much greater uplift forces before it eventually gives way. This means more steel is needed to hold the slab down and overcome the void former’s fail load.
All these void former types are designed to be left in, with no need to remove the former, unlike a screw-jack method of forming a floor deck, which would require a tricky performance for someone to enter the void, undo and remove the jacks.
They reduce structural weight and material use
Void formers can lighten slabs and reduce material usage. This directly cuts costs, reduces carbon emissions, and eliminates the need for additional reinforcements.
They improve build efficiency
Ultimately, they lead to less excavation, faster installs, and reduced muck-away – all music to the construction team’s ears.
Not all void formers are created equal: why compressibility matters
As we’ve seen, void formers help protect against the effects of soil movement – but not all types perform equally well. Some void former materials, such as rigid EPS, must resist uplift forces until it gives way, often requiring more reinforcement and heavier slabs.
That’s where compressible void formers offer a smarter solution. Designed to compress or give way under soil pressure, they remove the need for slabs to ‘fight’ against soil heave. Products like Clayboard take it one step further by allowing controlled activation – combining structural support with engineered weakening, which helps you reduce steel requirements, simplify installation, and cut carbon.
Is it better to use compressible void formers?
Want to dive deeper into the benefits of compressible systems? Read our full blog: Is it better to use compressible void formers — and discover why Clayboard is fast becoming the go-to choice for sustainable, high-performance foundations.
The importance of Ground Movement Solutions
As you can see, ground movement solutions are far from decorative extras – they’re essential engineering tools that play a critical role in long-term foundation performance. These systems help to:- Minimise soil-induced slab damage
- Enable cost-effective, lightweight concrete designs
- Improve overall build efficiency
- Support sustainability and environmental goals
What happens if you don’t use a void former?
Failing to incorporate one when needed can lead to several structural and economic consequences. Remember, soil pressure cracks slabs! Expansive soil will have nowhere to go and will push directly on the slab, leading to deformation or structural failure. In short, without installing void formers, you lose control over the structural integrity, future longevity, and cost-effectiveness of your project.How to choose the right material and grade of void former for your project
When considering which void former is the best fit for your groundworks, you’ll need to think about:- The soil type and expansivity
- The expected slab thickness and loads
It is also worth matching the void former to the certification the project demands. On most commercial and many residential schemes the specification calls for a BBA-certified void former, which is where Clayboard’s KN30 grade fits; a higher-capacity grade covers heavier commercial loads. Because paper honeycomb is around 30% thinner than the equivalent EPS void former for the same void depth, choosing it can reduce dig depth and groundwork cost — a practical saving that often outweighs any difference in unit price.
If you are unsure which grade of void former for concrete foundations your site needs, the safest route is to share the soil report and slab loads with a technical specialist, who can specify the right void depth and strength band rather than leaving it to guesswork. Getting the void former specification right at design stage is far cheaper than remediating heave damage later.
How does Clayboard compare to other sustainable solutions?
A standout choice in compressible void formers is Clayboard, a honeycomb paper-based product pioneered by the team here at Dufaylite and designed for engineered groundworks.
It’s one of the many eco-friendly construction methods that’s transforming the construction industry and paving the way for more sustainable practices without compromising on results.
The key benefits of Clayboard in sustainable construction projects
- Engineered weakening: Under controlled water introduction via Voidpack pipes, Clayboard reduces strength significantly from residential/commercial grades (KN30, KN90) down to a nominal 2–3 kN/m², allowing soil heave to fill the space without stressing the slab.
- Lighter slabs: Because Clayboard relinquishes upward pressure once activated, it reduces the amount of reinforcement required.
- Environmental sustainability: Made from recycled paper honeycomb, it helps construction professionals mitigate plastic waste concerns and reduce their project’s carbon footprint.
- Ease of handling and installation: Lightweight panels make transport and placement simpler. Standardised sizes (for example, 60 mm, 85 mm, 110 mm, and 160 mm) suit various soil plasticity indices and slab depth needs.
- Reduced muck-away and labour costs: With less excavation, projects see fewer spoil removals and quicker timelines.
- Permanent void without removal: After concrete cures and activation occurs, Clayboard stays in situ and weakens, so there’s no need to extract anything from beneath the slab.
If you’re interested in how solutions like Clayboard are shaping the future of greener building practices, read our blog on combatting sustainable construction challenges.
Clayboard stands head and shoulders above other ground engineering solutions because it:
- Weakens precisely when needed
- Eliminates soil heave pressure without resisting uplift
- Reduces the need for costly steel reinforcement
- Offers an environmentally responsible and installation-friendly alternative
Are there any downsides to Clayboard?
We must note that Clayboard requires careful irrigation to function as intended. Panels must be kept completely dry prior to pouring. Low-pressure water is introduced via Void Pack pipes. Additionally, Clayboard is best suited for soils prone to heave, most commonly clay, although other types can also prove susceptible, including shales and brownfield sites, such as former steelworks. Because Clayboard’s compressive strength can be switched off at the desired moment, it also lends itself to non-heave applications, such as an anti-vibration measure for deflection or for transfer slabs.Explore Clayboard in action
It always helps to understand how a product works in context.
Keybridge House, London
In our case study outlining our contribution to Keybridge House, a 415-unit development that also included retail units, office space, and a brand-new primary school, we explained how we delivered a tailored void former solution to the client for maximum effectiveness. 4,000 m² of Clayboard panels were shipped for use on this particular project, which demonstrates our ability to cater to both larger-scale and smaller builds.
Brooklyn Redevelopment, New York
While Clayboard is widely used in UK construction, its benefits are increasingly being recognised internationally. One standout example is a redevelopment project in Brooklyn, New York, where a former car park site was being transformed into residential condos and a hotel. The challenge? A live subway tunnel ran directly beneath the development.
Dufaylite was brought in to recommend a solution. We proposed our commercial-grade Clayboard to mitigate the structural load on the tunnel roof and reduce vibration transfer to the buildings above. A supporting Voidpak system was also suggested to help create the required void beneath the slab once the concrete was self-supporting.
After a successful trial, the client requested slight modifications to optimise shipping. We worked closely with their engineering teams, refining specifications and supplying the customised Clayboard system — all within a fortnight.
Want the full story? Read the full case study: Clayboard Void Former Protects New York Subway.
Queensferry Crossing, Scotland
Clayboard has also been used in a range of civil engineering projects over the decades. One high-profile case that springs to mind is that of the Queensferry Crossing in Scotland, which spans the River Forth and connects Edinburgh to areas in the north of the country.
Transport Scotland noted that the replacement bridge was the biggest infrastructure project in Scotland of its kind for a generation. You can learn more about this iconic structure and our involvement in the build here.
Void former FAQs
What is a void former used for?
A void former creates a compressible void beneath a concrete slab or ground beam so expansive clay can heave into the void rather than cracking the structure. It supports the load while you build, then yields to absorb ground movement.
What is a compressible void former?
A compressible void former is designed to give way under ground pressure, so the slab doesn’t have to resist the full force of clay heave. Clayboard is a compressible paper honeycomb void former whose core weakens when water is introduced after the concrete sets.
Is a paper void former better than an EPS one?
For heave protection on clay, a paper honeycomb void former like Clayboard weakens on activation, so the slab needs less reinforcement. It’s also around 30% thinner than the EPS equivalent, recyclable, and BBA-certified in its KN30 grade.
Do you remove a void former after the concrete sets?
No. Void formers are designed to stay in the ground for the life of the foundation. Once the concrete is self-supporting and the void has formed, there’s nothing to remove.
