Structural and base materials · Concrete & Cement
Compressible Void Former
Explains a product family that is ordered to be destroyed by the structure above it, and why an insulation board pressed into the same position is not the same thing.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What void former is
A compressible void former is a manufactured unit placed in the ground beneath a concrete element so that the ground below cannot press up against it. It is supplied as boards, blocks or moulded cellular units, laid on the formation or against the formwork before reinforcement and concrete go in, and it stays buried for the life of the building.
What sets the family apart from anything else in the library is that the product is bought in order to stop performing. It is specified because it will yield: either by compressing under a stated pressure so that ground movement is taken up in the material rather than transmitted to the concrete, or, in a different sub-family, by breaking down chemically or biologically after the concrete has cured so that an actual air gap is left behind. The two are not interchangeable. One is a permanently present but crushable layer; the other is a temporary former that is intended to disappear.
The reason either is used is ground behaviour. Shrinkable clays swell when they regain moisture, notably where trees have been removed or where a new structure changes the moisture regime, and that swelling can lift a slab or a ground beam. A void or a crushable layer is the space that movement is allowed to occupy. The depth of ground involved, the pressure to allow for and whether a void is needed at all are geotechnical and structural determinations made before anything is ordered.
Expanded polystyrene board is frequently seen in this position on site and is a different product with a different purpose. It is made to insulate and to stay intact, not to be crushed to a controlled degree or to decompose, and using it as a stand-in changes what the buried layer actually does.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- Void former
- Compressible filler board
- Clayboard
- Heave protection board
- Collapsible former
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Beneath ground beams and pile caps in shrinkable clay, where upward ground movement is being allowed for.
- Under suspended ground-bearing slabs where the design calls for a gap between slab and ground.
- Against the vertical faces of ground beams and pile caps where lateral ground movement is a concern.
- Around pile shafts where a sleeve or collar is used to isolate part of the shaft from the surrounding ground.
- Beneath external slabs and hardstandings adjacent to buildings where differential movement is anticipated.
- Alongside foundations near retained or removed trees where moisture change in the ground is expected.
Consideration
Where it is commonly considered
- Where a site investigation has identified shrinkable soils and the engineer has raised heave.
- Where trees have been removed, are to be removed, or are close enough to affect ground moisture.
- Where the design separates a concrete element from the ground rather than bearing on it.
- Where a piled solution means loads are carried below the level of the moving ground.
- Where existing structures nearby have a history of movement associated with the ground.
- Where a designer has specified a void beneath an element and the means of forming it is being chosen.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- Where no geotechnical assessment has been carried out and the ground behaviour is assumed rather than known.
- Where the product is being chosen on site as a substitute for something specified, without reference back to the designer.
- Where an insulating board is being used in the position because it was available, rather than because it is the specified product.
- Where the element is structural and the pressure the former is expected to take has not been set by the engineer.
- Where groundwater or standing water is present in the excavation and the effect on the product has not been considered.
- Where the two sub-families are being treated as one and a break-down former is proposed where a permanent crushable layer was intended.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Installation dependency
- The product goes in before reinforcement and concrete and cannot be reached afterwards. Laying, jointing, support against the formwork and protection from site traffic all happen in a short window during groundworks.
- Substrate dependency
- It sits on a prepared formation, and how level, clean and stable that formation is affects whether the units lie as intended and whether concrete finds its way underneath them.
- Moisture behaviour
- The product is buried in wet ground. How a particular material behaves when saturated, and whether water in the excavation changes it before the pour, are questions for the manufacturer's information rather than general knowledge.
- Documentation
- The whole reason for the product is a behaviour under pressure that cannot be seen. The manufacturer's technical information and the engineer's specification are the only record of what was intended, and both are worth keeping.
- Repairability
- Once the concrete is cast and the ground backfilled, the layer cannot be inspected, adjusted or replaced. Anything that went wrong during placing is permanent, which raises the value of checking before the pour.
- Thermal behaviour
- This is not an insulation product, even where it resembles one. Where a thermal layer is also required beneath a slab, that is a separate element of the build-up with its own material and its own position.
- Maintenance
- There is nothing to maintain, because nothing is accessible. What can be maintained is the ground and drainage conditions around the structure that the design assumed.
Exposure
Exposure and environmental conditions
- What did the site investigation find about the soils beneath this element?
- Are there trees on or near the site, and is their removal or growth part of the assessment?
- Is groundwater present in the excavation, and how will that affect the product before the pour?
- Will the excavation stand open in rain or frost before the concrete goes in?
- Does drainage or a soakaway nearby change the moisture regime in this ground?
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- The formation has to be prepared and stable before units are laid, since the pour follows immediately after.
- Reinforcement, spacers and starter bars are fixed over or beside the former without displacing it.
- Formwork positions and the way units are held against vertical faces are worked out before the pour.
- Any damp-proof membrane, gas barrier or separating layer in the build-up has its own position relative to the former.
- Backfilling and compaction around the element must not disturb what has been placed against its faces.
- Where a thermal layer is also part of the floor build-up, it is a separate layer in a separate place.
Appearance
Appearance and finish considerations
- Nothing about this product is visible in the finished building, so appearance is not a selection criterion.
- The visible concrete face can be affected if units move or float during the pour and grout escapes past them.
- Units left proud or misaligned can show as an irregular line at the edge of an exposed ground beam.
- Photographic records taken before the pour are usually the only visual record that exists of the layer.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- What pressure behaviour did the engineer ask for, and does the product supplied match that requirement?
- Is this a compressible layer that stays in place, or a former intended to break down after casting?
- How long can the product stand in an open excavation before the concrete is placed?
- What happens to the material if the excavation floods before the pour?
- How was the layer checked and recorded before it was covered?
Upkeep
Maintenance and repair considerations
- If ground movement were suspected later, how would anyone establish what is beneath the element?
- What record exists of what was placed, where, and to what specification?
- Are there drainage or landscaping changes around the building that the design assumed would not happen?
- Who would be the right professional to consult if movement appeared in the structure above?
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- Units are laid, jointed and protected within the groundworks sequence, immediately before reinforcement and concrete.
- Fixing or propping is needed where units are placed against vertical formwork so they are not displaced by wet concrete.
- Joints between units are closed so that concrete does not run between or beneath them.
- Site traffic, plant tracking and material storage over a laid area can damage it before the pour.
- A pre-pour inspection is the last opportunity to see the layer at all.
- Weather in the excavation influences both the product and whether the pour proceeds.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- The manufacturer's technical information for the specific product, describing how it behaves under load.
- The engineer's specification stating what is required beneath and beside the element.
- Confirmation of which sub-family was supplied — a permanently crushable layer or a break-down former.
- Photographic records of the layer taken before reinforcement and concrete went in.
- Delivery records showing that what arrived matches what was specified.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What did the ground investigation conclude, and why is a void or a compressible layer needed here?
- Is the design relying on a permanent crushable material or on a void that appears after casting?
- Which faces of the element need the product, and how deep does it go?
- How is the product held in place while the concrete is poured?
- Who inspects the layer before the pour, and is that recorded?
- What would you expect to see in the structure if the layer were not doing its job?
- Does anything else in the build-up beneath this floor depend on where this layer sits?
- Are there ground or landscaping changes around the building we should avoid making later?
Blind spots
Commonly overlooked points
- The product is specified to fail in a controlled way, which is the opposite of how most buried materials are chosen.
- An insulation board is not a void former, even when it looks similar and fits the same gap.
- A former intended to break down and a permanently compressible layer solve different problems and are not alternatives for one another.
- Once the concrete is cast, the layer is beyond inspection for the life of the building.
- Damage from site traffic before the pour is easy to cause and hard to see afterwards.
- Changes to trees, drainage and landscaping after completion can alter the ground conditions the design assumed.
What this page does not do
- This page does not state what pressure behaviour any product offers or what any element requires; that is an engineering determination for a qualified professional.
- Whether a void is needed at all, and how deep, follows from a site-specific geotechnical assessment that this reference cannot substitute for.
- Nothing here should be read as saying that a product prevents ground movement or protects a structure from heave.
- Build Design Hub does not supply, test, approve or endorse void formers or any supplier of them.
Related entries
Common comparisons
Entries met at the same decision but which behave differently, so the comparison is about the difference rather than a swap.
Related entries
Often used alongside
Entries commonly encountered together with this one. Co-occurrence only — never a claim that any combination is compatible or suitable.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
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