Clay heave void former

Dufaylite’s Clayboard is a sustainable construction void former which gives virtually no uplift, meaning lighter slabs and less reinforcement steels.

Up to 30% lighter than other void formers on the market, using Clayboard means less deliveries to site and far less muck away, resulting in further substantial savings on your construction projects.

Dufaylite Clayboard void former

How it works

Due to it’s unique make-up, Clayboard is the perfect, eco-friendly solution to creating an expansion void.

With virtually no uplift from moving ground the Clayboard void former allows the use of thinner, lighter concrete slabs, less reinforcement steel and requires a much smaller working void area than traditional products.

When dry Clayboard has a superb strength to weight ratio. However, after the introduction of water the honeycomb core looses it’s strength, allowing it to succumb to any subsequent soil heave without transmitting any pressure to the foundations or structure above.

Showing how Dufaylite Clayboard works before and after water is introduced
Showing how Dufaylite Clayboard works before and after water is introduced
Showing how Dufaylite Clayboard works before and after water is introduced

Frequently Asked Questions

  • The forecast is for rain, can I lay Clayboard?

    The key thing to remember is that Clayboard is designed to collapse under pressure when wet so it must be kept dry while supporting the weight of steel and wet concrete. The fact that it may rain when you have planned to install Clayboard does not mean you should avoid it but will require some vigilance. If there is standing water, Clayboard should not be laid. Any surface water should be removed. Pooling in trenches may require a sump and pump arrangement. Instead of sand, some users prefer a pea gravel blinding, as wet sand can hold the water longer.

    The time between laying the product and pouring the concrete should be kept to a minimum to reduce the risk of Clayboard getting wet. Over large areas, consider wrapping in smaller sub-sections. Take care in carrying mesh, re-bar etc in case this rips the poywrapping.

  • My design incorporates layer of insulation beneath the slab. Will it stay in place after the Clayboard collapses under heave?

    While the Clayboard remains intact and under heave, that insulation will be held in place. There is a concern that if the ground subsequently dries out and shrinks away, what is holding the insulation in place. Two methods used are 1) to poke wire ties of suitable length through the insulation so it is standing proud on the upper face and attached to mesh or rebar or 2) use wide helix plastic screws in the upper face of the insulation, half embedded and half exposed to key into the concrete when poured.

  • How many Voidpak pipes do I need?

    The general rule of thumb is a minimum of one Voidpak per 24m2. Where internal beams subdivide the building footprint, then use at least one per enclosed area (more if each enclosed area is >24m2) of Clayboard.

  • So I use the Voidpak pipe to insert water and dissolve the core?

    No, the Clayboard is not removed by the irrigation method and remains in situ. The aim of the Voidpak pipe to introduce water is purely to weaken the honeycomb core. This will ‘switch off’ its compressive strength, taking it from 30kN/m2 down to a nominal 2-3kN/m2. This ensures that, when the ground heaves, the Clayboard yields with no resistance. This is in contrast to polystyrene voidformers with a narrow safe load/fail load window requiring the slab to be designed to resist uplift.

  • I am using Clayboard in pre-wrapped sections. Does this affect the number of Voidpaks needed?

    Yes. If we are inserting Clayboard into sections of lay flat tubing, a max number per section is 3 panels, due to practicalities of handling. In this case, one Voidpak pipe is required per bagged section, rather than one per 24m2.

  • Your installation guide advises a sand blinding – I have specified a concrete blinding, is this a problem?

    No. The placement of a sand blinding is to provide a smooth, flat surface to avoid any point loadings to the underneath of Clayboard when applying the load of steel and wet concrete. Sand is preferred as it allows for draining of water, or sometimes a pea gravel is used. After the irrigation process, use a steel rod poked down the opening of each Voidpak pipe and break through the bottom skin to allow any excess water to drain through the blinding. If you have used a concrete blinding, you just need to go a bit further in knocking a hole through that as well.

  • How long after the pour do we irrigate the Clayboard?

    This will always depend on the concrete mix, depth of pour etc The general rule of thumb is how long it takes for the concrete to achieve self-supporting strength: when this point is reached, the Clayboard is now redundant, as the slab is supported by piles and beams.


Over 60 years’ experience resulting in a unique, environmentally friendly void former

At Dufaylite, we have independently manufactured the best quality recycled paper honeycomb since 1955, and through continued investment and product development it has made us one of the leading sustainable suppliers in Europe.

Our specialists have worked tirelessly to understand and explore the properties of honeycomb, enabling our business to extend into a broad range of markets and industries.

With over 60 years of industry expertise, Dufaylite offers exceptional products that perform to the highest of standards. Our unique sustainable, 100% recyclable honeycomb board creates a technically sound void, solving ground movement issues and preventing structural damage caused to foundations by clay heave. Accredited by the BBA and specifically designed to work in both commercial and domestic environments, the techincal properties of our Clayboard system means significant savings in both time and money.

Key features and benefits include:

  • Sustainable, manufactured from recycled materials
  • Superb strength to weight ratio
  • Easy on-site applications
  • Increased cost savings
  • Fast delivery times
Dufaylite at the forefront of technical excellence