Concrete foundation on a building site
Concrete foundation pouring

Is it better to have a compressible void former?


In almost all cases where a foundation void is necessary, a compressible void former will save you money in labour costs, muck-away costs, and transportation costs. It’ll also usually result in a more robust foundation slab.

Let’s look at why.

Compressible void formers absorb ground movement effectively

Traditionally, minor ground movement was counteracted by building on top of footing foundations. The foundation on which the building was constructed was therefore built with a void necessarily included in the construction. But the weight and versatility required of many modern buildings can only be supported by a rock solid foundation slab.

The problem then comes when ground movement pushes against the foundation slab. We outline how foundation slabs and voids work in this blog but, traditionally, the slab or slab and non-compressible void has to withstand any ground movement. A compressible void former, on the other hand, creates a true void that can absorb and account for ground movement – putting no pressure on the slab itself.

Compressible void formers allow for thinner foundation slab

Now, most compressible void formers made from polystyrene are designed to be compressible up to a point: they’re designed to break apart under a certain amount of stress. This allows for a thinner slab than you’d otherwise need without a void, but then you’re not only left with a bunch of non-biodegradable plastic in the ground, you’re still having to account for the upward pressure. 

Our void formers can have the compressive strength effectively ‘turned off’ with water when the foundation slab has set. A key benefit, then, is that with essentially zero pressure on the slab itself, you don’t need to make a really thick foundation slab to counteract the upward pressure of any ground swell.

So why is this a benefit? Surely bigger is always better?

Well, the crucial thing here is that you’re able to make a slimmer slab with the same end strength. This has a knock-on effect of being more environmentally friendly as well as saving significant costs. 

You might not expect this to be an environmental issue, but optimising the use of concrete (in this case, using less for the same result) saves a significant amount of emissions that would otherwise be created through needing to produce more concrete.

Thinner slabs make for significant cost savings

Another key benefit is that a thinner slab means less ground that needs to be excavated. This itself saves money in a variety of ways. All the labour and muck-away costs involved with excavating are reduced, as well as the material cost of the concrete. Your project time will also be reduced because of the shorter amount of time required for the slab to set.

All this, in short, adds up to significant savings and shorter project runtimes.  

Go with an environmentally friendly void former

Our Clayboard compressible void formers are made with a honeycomb paper core which make transport & installation a breeze. The honeycomb shape gives them a surprising amount of strength which lets them hold up the slab while it sets, and the paper biodegrades when water is added.

If you need help on your foundations, let’s talk.

Eco property house with stilts
What sort of permanence is intended?

A concrete foundation is an undoubtedly permanent thing – while it’s technically possible to dig out the concrete, doing so is so much work that in practice it means that it’s like to stay there forever. As such, planning officers tend to have much tighter controls about when and where a concrete foundation is permitted. Not only from a permanence perspective, but because replacing the soil with impermeable concrete dramatically affects how much rainwater is absorbed by the ground.

If the eco property in question is intended to be ecologically friendly and as permanent as possible, then a foundation slab (with a suitable void) is a great option if it’s approved by the local planning authority. If a concrete slab isn’t approved, however, all is not lost. You can still make a sturdy foundation with well-built and well-designed footings. Most old buildings relied on footings and are still going strong!

Failing that you can actually build above-ground footings. One of the most beautiful ways is to create a part-concrete footing with a large rock on top, and then scribe posts onto that rock.

This process, completely without concrete or modern tools, is quite calmingly presented in this video:

Eco property living roof
Is the property going to be earth-integrated?

In climates that necessitate more protection from weather, integrating buildings into the ground helps to maintain a regular temperature. A common way to do this while still letting light in is to either build a house into a hill, or to bank soil around it to create an artificial hill once it’s been built.

Both of these methods, when properly built, create a great deal of weatherproofing, but require the structure to hold up a lot more weight than usual above-ground houses. In these cases, a solid foundation slab with a good void is almost always your best bet.

Isn’t concrete not eco friendly?

The production of concrete is responsible for a significant amount of emissions, but you can argue that the production of most things causes emissions. The important thing is to calculate whether those emissions will be offset by the longevity of a structure. 

A concrete structure might cause some emissions at the point of manufacture, but (based on some ancient Roman concrete structures) it’ll also be inert and in place for, theoretically, thousands of years.

Need to talk about foundation slabs?

The best way to pour a foundation slab is over a compressible void former. And if you’re building an eco property, it makes sense to choose a void former that’s eco friendly. Our Clayboard is just the thing!

Let’s talk about how to make your eco house a forever home.

25 February 2022

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