BBA Certified Void Former: What Specifiers Need to Know in 2026

Written by Ashley Moscrop, Managing Director, Dufaylite Developments Ltd
Reviewed by Nigel West, Technical Manager, Clayboard
Last updated: 09 July 2026

Key takeaways

• A BBA certified void former is a clay-heave void former whose performance has been independently assessed and certified by the British Board of Agrément — many UK commercial and residential clay-belt specifications require this certification by name.
• The certified paper honeycomb choice is Clayboard KN30, holding BBA Certificate 98/3528, Issue 4, dated 27 June 2024 (originally certificated 29 October 1998), with scope across England and Wales, Scotland, and Northern Ireland.
• Clayboard supports construction loads at 30 kN/m² dry, then weakens when water is introduced via the Voidpak system after the slab sets — switching off its load-bearing strength (under 3 kN/m² wet) to absorb heave without transferring load to the slab.
• Clayboard panels are 30% thinner than the EPS equivalent for the same void (160mm panel gives a 150mm void vs 225mm of EPS) — a quantifiable saving in dig depth, spoil, and groundwork time.
• The 2026 backdrop is an active shrink year: the Environment Agency and National Drought Group flagged widespread drought risk after an exceptionally dry spring, leaving the South East and East Anglia clay belt at high shrink-swell risk.

A BBA certified void former is a clay-heave void former whose performance has been independently assessed and certified by the British Board of Agrément. For clay-heave protection, the certified choice is Clayboard KN30 (BBA Certificate 98/3528): paper honeycomb that supports construction loads while dry, then weakens when water is introduced after the slab sets — switching off its load-bearing strength to absorb heave at 30% less dig depth than an equivalent EPS void former.

This guide is written for the structural engineers, geotechnical engineers, architects, and groundworks contractors who specify or verify a BBA certified void former on a UK clay-soil site. It explains what BBA certification actually means for a void former, which grade holds it, why specifications mandate it by name, how a BBA certified void former works, and what separates the certified route from a product that is merely “tested.”

What does BBA certification mean for a void former?

A BBA certified void former is a product whose performance, durability, and fitness for purpose have been independently assessed by the British Board of Agrément and confirmed in a published Agrément certificate. The BBA is the UK’s leading construction certification body. An Agrément certificate is not a marketing badge a manufacturer awards itself — it is the outcome of independent testing, factory production control audits, and an assessment against the relevant Building Regulations.

For a BBA certified void former, that assessment covers the properties an engineer actually relies on: the compressive strength while the panel is dry and carrying construction loads, the way the panel behaves once the heave mechanism is engaged, the void it provides, and the conditions under which it must be installed. You can verify any current certificate directly on the BBA register.

The distinction that matters on site is between “tested” and “certified.” A manufacturer can commission a test report on a single batch and quote a strength figure. BBA certification is broader and ongoing: it assesses the product in use, audits the factory that makes it, and is reviewed and re-issued over time. A BBA certified void former carries an independent, traceable assurance that a one-off test report does not.

That difference becomes a compliance gate. Where a specification clause, a building control body, or a structural warranty provider requires a BBA certified void former, a product that is “tested but not certified” cannot be used — regardless of how good its test numbers look. This is why the credential question is the first one a careful specifier asks when selecting a void former for an expansive clay site.

Which void former is BBA certified for clay heave?

The BBA certified void former for clay heave is Clayboard KN30, manufactured by Dufaylite. It holds BBA Certificate 98/3528, Issue 4 — dated 27 June 2024, originally certificated 29 October 1998. That long certification history matters: it is a product that has been independently assessed and re-assessed across more than two decades of UK foundation use.

The certificate scope covers all three UK jurisdictions, which is why specifiers working across regional portfolios can rely on a single BBA certified void former rather than switching products by nation:

• England and Wales — Building Regulations 2010, Requirement A2(a)
• Scotland — Building Regulations 2004
• Northern Ireland — Building Regulations 2012

The certificate also confirms compliance with NHBC Standards 2024 (Chapters 4.2, 4.3, and 4.4), which is the reference most residential warranty providers point to. For a specification clause, it is the BBA certificate number — 98/3528, Issue 4 — not the brand name, that building control reviews when assessing whether the void former meets the requirement.

Clayboard is the BBA certified paper honeycomb void former in this category, and it is manufactured in the UK rather than imported. That gives a project a single accountable source for the product and the certificate, shorter lead times than overseas-sourced alternatives, and a BBA certified void former supplier that can support the specification directly — which matters when a contractor needs to verify a certificate before ordering, or an engineer needs the assessed data for a calculation.

How a BBA certified paper honeycomb void former works

The mechanism is what makes a BBA certified void former different from a passive spacer, and it is the part engineers most want explained. A Clayboard panel does two opposite jobs at two different stages of construction, and the BBA certificate assesses both states of the BBA certified void former.

While dry, it carries construction loads. During the pour, the panel must support fresh concrete, foot traffic, and reinforcement steel. Clayboard KN30 is rated at 30 kN/m² dry compressive strength — enough to support concrete up to 1m thick during construction. The black-faced top surface is rated for groundworkers to stand and walk on, and the white-faced underside sits against the prepared base. At this stage the panel is a working platform, not yet a void.

Once the slab is self-supporting, its strength switches off. Water is introduced into the paper honeycomb core via the Clayboard Voidpak system after the concrete has set. The water weakens the core, switching off its load-bearing strength — it yields at under 3 kN/m² when wet (BBA-assessed). With the strength switched off, the void absorbs subsequent clay heave without transmitting any upward load to the slab above. The core then biodegrades in situ in the ground over the life of the structure; it is never recovered or extracted.

This is a critical engineering point, and the language is deliberate: the core weakens and its strength switches off. It does not fail suddenly or collapse. The transition is controlled, triggered by the Voidpak system at the right moment, and assessed by the BBA — which is precisely why a BBA certified void former gives the engineer the predictable behaviour a foundation design depends on.

The contrast with the EPS route is structural, not cosmetic. An EPS void former compresses under sustained load, but it does so by transferring the heave force into the slab until the polystyrene yields. The slab must therefore be designed and reinforced to resist that full heave force during the transition. A BBA certified void former made from paper honeycomb removes that load path entirely, because the strength switches off before any load can transfer.

BBA certified void former vs non-certified — the specifier’s risk

Choosing a non-certified void former where a BBA certified void former is required is a risk that sits with the specifier, not the supplier. The exposure runs across three fronts, and each one can surface long after the panels are in the ground.

Specification and tender risk. If the foundation specification or the employer’s requirements call for BBA certification and a non-certified product is substituted, the design no longer meets its own clause. That is a defect in the specification chain, and it is the named engineer who owns it.

Building control risk. Building control assesses compliance with the relevant Building Regulations requirement for heave on shrinkable clay. A BBA certificate is the cleanest evidence route — it maps the product directly to the regulation. A non-certified product forces a bespoke justification that building control may not accept, delaying sign-off.

Warranty risk. Structural warranty providers and NHBC-aligned schemes increasingly expect a certified ground heave void former on shrinkable clay. A non-certified substitution can put cover at risk on the foundation — the most expensive element to remediate if a claim is ever made.

The competitive context is straightforward and worth being explicit about. The dominant BBA-certified alternative to paper honeycomb is EPS polystyrene — typically Cordek Cellcore HX, which also holds BBA certification. That gives a specifier two genuinely certified routes to compliance: Clayboard KN30 (paper honeycomb — strength switches off when wet, no load transfer to the slab) and an EPS product (the slab must be reinforced to resist heave until the polystyrene yields).

Both clear the certification gate; the decision between them then turns on the mechanism, the dig depth, and the downstream steel. Positioning a BBA certified void former against the EPS material category — rather than against unbranded or uncertified imitations — is the honest comparison, because EPS is what most specifiers are weighing a BBA certified void former against.

BBA certified void former vs EPS — the comparison table

The table below compares the two BBA-certified routes a UK specifier is most likely to evaluate. It is the comparison most engineers want on a single screen before they commit a clause.

The thickness line is the one that converts to money on a large footprint. A 160mm Clayboard panel delivers a 150mm void where roughly 225mm of EPS is needed for the same result — around 65mm less dig depth across the whole foundation. That is less excavation, less spoil to cart away, and a shorter groundwork programme. For a deeper read on how paper honeycomb compares against EPS in this application, our technical piece on Clayboard vs alternative void formers walks through the specification differences in detail.

The steel line is the one that converts on heavily loaded slabs. Because an EPS void former requires the slab to resist heave force until the polystyrene yields, EPS-based designs typically carry additional reinforcement steel and, on the most heavily loaded slabs, a thicker concrete section. On a major commercial project in London, specifying paper honeycomb over a polystyrene alternative saved the scheme over £1m in reinforcement steel alone — a single-line benefit attributable to the load-transfer mechanism, not the material on its own.

Why clay heave makes a certified void former a 2026 priority

The timing sharpens the case. Expansive clay does not just shrink in a dry spell — it swells, and heaves, when it rewets. That shrink-swell cycle is the core engineering reason a BBA certified void former exists, and 2026 is a shrink year. The Environment Agency and the National Drought Group flagged widespread drought risk across England following one of the driest springs the Met Office has recorded, and the clay belt that defines this problem — the South East, East Midlands, and East Anglia — sits in the highest-risk zone. (Source: Environment Agency / National Drought Group / Met Office — confirm the latest published figures before relying on any specific number.)

A site that has been desiccated through a dry summer is primed to heave hard once the moisture deficit closes. A foundation bearing onto, or spanning over, that rehydrating clay has nowhere to send the force unless a compressible void sits beneath it. Without that void, the heave pressure transmits straight into the concrete — cracking ground beams, distorting suspended slabs, and in severe cases lifting the structure off its bearings.

A BBA certified void former for clay heave is the engineered component that decouples the swelling ground from the structure above. The drought-driven shrink-swell context is exactly the condition a certified ground heave void former is specified to manage, which is why clay-belt specifiers who once treated heave as a rare edge case now design for it as a default — and reach for a BBA certified void former to evidence the design.

How to specify a BBA certified void former

Getting the specification clause right is a five-step sequence. Following it produces a clause that building control, the warranty provider, and the contractor can all act on without ambiguity.

1. Confirm the heave risk and clay type from the ground investigation. A Phase 1 desk study and Phase 2 intrusive investigation establish the plasticity index, the soil moisture state, and the predicted heave magnitude. This data drives every downstream decision, including whether a BBA certified void former is needed at all.

2. Check the specification and warranty BBA requirement. Confirm whether the project specification, building control body, or warranty provider mandates BBA certification. Where they do, the product must carry a current BBA certificate — a test report is not a substitute.

3. Select the void depth, then the panel. Set the required void from the predicted heave plus a working tolerance, then read the panel thickness from the manufacturer’s fixed void-to-thickness table. Clayboard offers a 60mm panel (50mm void), 85mm panel (75mm void), 110mm panel (100mm void), and 160mm panel (150mm void); panel thickness equals the void required plus a 10–15mm minimum allowance.

4. Verify the BBA certificate number and issue. Reference the certificate explicitly in the clause — for example, “BBA Certificate 98/3528, Issue 4 — Clayboard KN30.” Naming a BBA certified void former by brand alone, without the certificate reference, leaves the specification weaker than it needs to be, because building control reviews the certificate, not the brand.

5. Confirm the Voidpak detailing and keep-dry sequence. Specify polythene sheeting above and below the panels, the Voidpak water-introduction sequence after the concrete has set, and the requirement to keep the panels dry until the slab is self-supporting. Omitting the polythene is not a saving — premature wetting can weaken the core before the slab is ready to carry the load on its own.

A short list of design constraints to confirm at the same time: the maximum installation depth is 2m below finished ground level; the panels must not be installed below the water table; and they must not be used where soil gas protection is required (radon or methane membranes are a separate specification item). A certified ground heave void former is selected to the predicted heave, never trimmed to save dig depth — under-sizing leaves residual heave that loads the slab.

Talk to the Clayboard team — and book a lunch-and-learn

A BBA certified void former is a structural specification, not a procurement preference. The decision sits with the engineer, and the consequences of getting it wrong — slab cracking, foundation distortion, warranty exposure — are not recovered cheaply. The decision comes down to the evidence on the table: the BBA certificate, the load-transfer mechanism, and the downstream cost for reinforcement steel and dig depth.

The fastest way to bring a consultancy team up to speed is an in-person session led by Nigel West, Dufaylite’s Technical Manager for Clayboard. Nigel runs a 30–45 minute lunch-and-learn for structural engineers, architects, geotechnical engineers, and graduates — a technical primer on clay shrink-swell behaviour, how to read a geotechnical report for heave, the two void former mechanisms and what each does to the slab design, and how to write a defensible specification clause. The session is technical, not promotional, and we bring the lunch.

Specifying with confidence

The certified void former decision is one of the few foundation choices where the right answer is also the documented one. A BBA certified void former gives the engineer independent assurance, building control a clean compliance route, and the warranty provider the evidence it expects. On clay-belt sites in a 2026 shrink year, that combination of certification, a strength-switches-off mechanism, and 30% less dig depth is the case worth putting in front of the design team.

Talk to the Clayboard team via our contact page to book the lunch-and-learn, or to request BBA Certificate 98/3528 (Issue 4) and the Clayboard technical specification pack. You can also read more about our verified credentials — Carbon Neutral Britain™ certification and FSC certified responsible sourcing across recycled and virgin fibres — on our sustainability and certifications page, which sits alongside the Carbon Neutral Britain™ programme detailed at carbonneutral.com.