Written by Ashley Moscrop, Managing Director, Dufaylite Developments Ltd
Reviewed by Nigel West, Technical Manager, Clayboard
Last updated: 15 August 2026
Key takeaways
- Clayboard is a paper honeycomb void former that protects foundations on clay soils from ground heave — its KN30 grade is BBA-certified, so it can be specified by name on projects that require certification.
- Clayboard works by switching off: once water is introduced after the slab has set, the paper core weakens and its load-bearing strength gives way, opening a void that absorbs clay heave without loading the structure.
- Against rigid EPS void formers (such as Cordek), Clayboard is around 30% thinner for the same void — less dig depth — and because the slab doesn’t have to resist the full heave, it can reduce reinforcement steel.
- Clayboard is paper-based and recyclable, and Dufaylite is certified Carbon Neutral Britain™ and FSC® (C186880) — a clear advantage over polystyrene void formers for firms with net-zero targets.
- The right choice comes down to certification requirements, soil expansivity, slab loads, dig depth and sustainability priorities.
Clayboard is Dufaylite’s paper honeycomb void former, and the question this guide answers is a practical one: how does Clayboard compare with the alternative void formers a structural engineer might specify, and when is it the right choice? Getting that decision right matters, because the void former you choose shapes how the whole foundation is designed, costed and built on an expansive-clay site.
Below we set Clayboard against the two main alternatives — rigid polystyrene void formers and other compressible options — across the things that actually decide a specification: certification, thickness, cost, site handling and sustainability.
What is Clayboard?
Clayboard is a BBA-certified paper honeycomb void former, installed beneath concrete ground beams and suspended slabs on clay soils to protect against clay heave — the upward movement of expansive clay as it takes on moisture. It is a compressible void former, which means it is engineered to yield once the structure is built.
The mechanism is the heart of it. While dry during construction, Clayboard supports the concrete pour, foot traffic and reinforcement steel at its rated strength. Once the slab has set and is self-supporting, water is introduced into the paper core through the installation system. That water weakens the honeycomb and switches off its load-bearing strength, opening a void beneath the slab. When the clay later swells, it expands into that void rather than pushing up on the structure. The foundation never has to carry the full force of the heave.
Clayboard is made by Dufaylite — the only UK manufacturer of paper honeycomb — at St Neots, and comes in a range of thicknesses to provide different void depths. The panels are cut to standard site sizes and installed flat between the ground beams, so the material is straightforward to schedule and lay. That combination of a proven mechanism and simple site handling is a large part of why Clayboard has become a familiar specification on clay-belt projects.
The alternatives to Clayboard
When a structural engineer is specifying heave protection, Clayboard sits among a small set of options. The most common alternative is the rigid EPS (expanded polystyrene) void former, of which Cordek’s Cellcore is the best-known example. There are also other paper honeycomb void formers on the market. Broadly, the choice comes down to two things: the material (paper versus polystyrene) and the mechanism (compressible versus rigid).
That second point is where the biggest practical difference lies, so it is worth understanding properly before comparing products on price.
Clayboard vs EPS polystyrene void formers
Rigid EPS void formers and Clayboard solve the same problem in opposite ways, and the difference has real consequences for the foundation design.
| Clayboard (paper honeycomb) | EPS void former (e.g. Cordek) | |
|---|---|---|
| Mechanism | Strength switches off when wet; void absorbs heave | Slab must resist heave until the EPS compresses |
| Effect on the slab | Can reduce reinforcement steel | Often needs more reinforcement, sometimes a thicker slab |
| Thickness for the same void | ~30% thinner | Thicker — more dig depth |
| Material | Paper — recyclable, biodegradable | Polystyrene — not recyclable in standard streams |
| Certification | BBA-certified (KN30) | BBA-certified options available |
| Carbon | Carbon Neutral Britain™ certified business | Growing ESG liability |
With Clayboard, the swelling clay meets a void, so the uplift is never transmitted into the slab. With a rigid EPS void former, the slab has to be engineered to resist the full heave force until the polystyrene eventually compresses — which usually means more reinforcement steel and, on some projects, a thicker concrete slab. That is not a marginal detail: on one major stadium project, specifying a compressible paper honeycomb void former in place of polystyrene saved over £1m in reinforcement steel alone.
There is a thickness advantage too. Because Clayboard is paper honeycomb, it delivers a given void in around 30% less thickness than the EPS equivalent — for example, a 160mm Clayboard provides the same void as roughly 225mm of EPS. Less thickness means less dig depth, less spoil to remove, and a lower groundwork cost. For the fuller picture on the compressible mechanism, see our guide to compressible void formers.
It is worth being fair to EPS here: rigid polystyrene void formers are well established, widely available, and there are BBA-certified options, so they remain a valid choice on many projects. The point is not that EPS does not work — it is that the two approaches lead to different foundation designs. If the structural design is already committed to resisting heave with reinforcement, an EPS void former fits that approach.
But if the goal is to design the movement out — using less steel, a thinner build-up and a recyclable material — the compressible mechanism that Clayboard uses is what delivers it. The best time to weigh that up is at the start of the structural design, not once the slab has been detailed.
Clayboard vs other paper honeycomb void formers
Clayboard is not the only paper honeycomb void former, but two things set it apart. The first is certification: Clayboard’s KN30 grade holds BBA certificate 98/3528 and is compliant with NHBC Standards, which is exactly what most commercial and many residential specifications require. Not every paper honeycomb void former carries BBA certification, and on a project where it is mandated, that is the difference between a product that can be specified and one that cannot.
The second is manufacturing. Dufaylite makes the paper honeycomb itself in the UK, rather than buying in the core, which means consistent quality, shorter lead times and lower transport carbon, backed by 70 years of manufacturing heritage in the material. When you are relying on a void former to protect a structure for its whole life, the provenance of the product matters.
Technical specifications and grades
Clayboard is available in a range of panel thicknesses, each providing a different void depth, so the specification can be matched to the site. The depth of void required is driven by the soil’s plasticity and the ground heave risk band — commonly assessed against NHBC’s low, medium and high movement bands.
The KN30 grade is the BBA-certified option, rated to support the loads of construction while dry and then to switch off its strength once water is introduced. Choosing the correct thickness and grade is a technical decision best made with the soil report and slab loads to hand, which is why specifiers work it through with a technical specialist rather than picking from a shelf.
Site handling and installation
On site, Clayboard behaves like the rigid, walk-on board it is during construction. Panels are laid flat on the prepared base between the ground beams, butted edge to edge as a single layer covering the trench floor, and they carry foot traffic and the concrete pour without issue. Because paper honeycomb is thinner than the EPS equivalent, there is less excavation and less material to handle.
Once the concrete has set, water is introduced through the installation system to arm the void. Until then the panels are kept dry, installed with polythene sheeting to manage water ingress. It is a well-understood method that works consistently across residential and commercial jobs — one of the quiet advantages of specifying a mature, certified product.
Installation quality still matters, and it is worth getting right. The panels should sit directly on the prepared base in a single layer, butted tightly edge to edge and up to the beam sides so the void is continuous across the trench floor — not stacked, and not bridging the tops of the beams. Because Clayboard carries foot traffic and the pour while dry, groundworkers can move across it freely during construction.
The one non-negotiable is keeping it dry until the structure is self-supporting, since water is what triggers the mechanism. Get those basics right and the void former does its job silently for the life of the building; get them wrong and even a well-specified product can underperform. This is another reason specifiers value a certified, well-documented system with clear installation guidance behind it, rather than an unbranded board with little support — the certainty is part of what you are buying.
Cost: comparing Clayboard with the alternatives
On cost, the sticker price of the board is the least important number. The bigger figures sit in the groundwork and the structure. Because Clayboard can reduce the reinforcement steel a slab needs and is thinner so it cuts dig depth, it frequently lowers the total foundation cost even where its unit price is comparable to an EPS alternative. Reinforcement steel is expensive and carbon-intensive, so a detail that lets you design it out pays back quickly.
The dig-depth saving compounds the point. Every extra millimetre of void former thickness is a millimetre deeper the trenches have to be excavated, which means more spoil to cart away, more disposal cost, and more time on the groundworks programme. A void former that is 30% thinner for the same void takes cost out of all of those lines at once. When comparing Clayboard against alternative void formers on price, then, the only meaningful comparison is the whole foundation — the board, the steel, the dig and the disposal — rather than the board on its own. Costed that way, the compressible, thinner option often comes out ahead even before its sustainability benefits are counted.
Sustainability: where Clayboard pulls ahead
For firms working to net-zero and ESG commitments, the material choice is increasingly decisive. Clayboard is paper honeycomb — recyclable, made from paper, and biodegradable in the ground over the life of the structure — where EPS void formers are petroleum-based polystyrene that is not recyclable in standard streams. Dufaylite is certified Carbon Neutral Britain™ and FSC (FSC® C186880). Combined with the reduced reinforcement steel a compressible void former allows, choosing Clayboard lowers the embodied carbon of the foundation on two fronts at once — the void former itself and the steel it saves.
When should you choose Clayboard?
Clayboard is the natural choice on most suspended-slab and ground-beam foundations on expansive clay — particularly where the project mandates a BBA-certified void former, where minimising dig depth or reinforcement matters, or where sustainability credentials need to be evidenced. On a stable, well-draining site with no shrink–swell risk, heave protection may not be needed at all, so the decision should always start with a geotechnical assessment. But across the UK clay belt, Clayboard combines the structural advantage of a compressible void former with the certification and sustainability profile a modern specification demands. Browse the full ground heave solutions range for thicknesses and void depths.
What happens if you choose the wrong void former?
Getting the void former choice wrong is an expensive mistake to make, because it is buried under a finished building by the time the consequences show. Specify a void former with too little strength for the pour and it can fail during construction; specify one that does not open a proper void, or use a rigid product where the slab was not designed to resist the heave, and the swelling clay transfers its uplift straight into the structure. The result is the familiar pattern of heave damage — cracked slabs and brickwork, jammed doors, and structural repairs that dwarf the original saving.
Certification exists precisely to remove this risk: a BBA-certified void former like Clayboard has been independently assessed to perform as described, which is why it is mandated on so many schemes.
There is also a compliance dimension. On projects that require a certified void former, using an uncertified alternative is not simply a technical risk — it can hold up building control sign-off and create problems at warranty stage. Choosing Clayboard, with its BBA certificate and NHBC compliance, keeps that side of the project clean.
Frequently asked questions
What is Clayboard?
Clayboard is a BBA-certified paper honeycomb void former made by Dufaylite. It is installed beneath concrete slabs and ground beams on clay soils, where it supports the pour while dry, then weakens once water is introduced to open a void that absorbs clay heave — protecting the structure from ground movement.
How does Clayboard compare with EPS void formers?
Clayboard switches off its strength so the void absorbs the heave, whereas a rigid EPS void former requires the slab to resist the heave until the polystyrene compresses — often needing more reinforcement. Clayboard is also around 30% thinner for the same void and is recyclable, where EPS is not.
Is Clayboard BBA-certified?
Yes. Clayboard’s KN30 grade holds BBA certificate 98/3528 and complies with NHBC Standards, which is why it can be specified by name on projects that require a certified void former.
Is Clayboard more sustainable than polystyrene void formers?
Clayboard is paper-based, recyclable and biodegradable, and Dufaylite is certified Carbon Neutral Britain™ and FSC®. EPS polystyrene void formers are petroleum-based and not recyclable in standard streams, so Clayboard is the stronger choice for firms reporting on embodied carbon.
How thick does Clayboard need to be?
Clayboard comes in a range of thicknesses providing different void depths. The depth required depends on the soil’s plasticity and the site’s heave risk band, so the right thickness and grade are specified against the soil report and slab loads.
Specifying Clayboard for your project
If you are weighing up Clayboard against the alternative void formers on the market, the team can help you specify the right grade, thickness and void depth for your particular soil conditions, slab loads and certification requirements. Contact Dufaylite to talk it through with a technical specialist.
