Building sustainable buildings with cob

The key to building the sustainable homes of the future might not lie in innovative new materials, but in going back to natural materials that have been used for centuries

The key to building the sustainable homes of the future might not lie in innovative new materials, but in going back to natural materials that have been used for centuries. One such material that’s growing in popularity is cob - clay mixed with sand, straw, and water. 

Cob and clay are two natural building materials that is often “mixed” up. They differ a lot in terms of their composition and application. Clay is a fine-grained soil primarily composed of alumina, silica and water. Cob is a mixture of clay, sand and straw (more about this later on). Cob is typically used as a building material rather than as mortar. Clay or clay plaster is on the other hand, often used as a mortar.

Cob can be strong like concrete, and used to build warm and energy-efficient homes. It’s incredibly sustainable, with the potential to reduce the hugely damaging carbon footprint of the construction industry, and concrete in particular. 

Cob construction has been used for centuries in various parts of the world, particularly in areas where the necessary materials are readily available. Here are information you need to know about this not-so-new, environmentally friendly material.

Sustainable, recyclable ingredients

Clay is a natural, renewable material that’s found underground all over the world. It’s often dug up to make tunnels, and sent to landfill - but instead, we can turn this wasted resource into a strong and effective building material. Since clay is found everywhere, it eliminates the emissions from transporting materials around the globe on trucks and ships. 

After the raw materials are mined, cob is mixed by adding straw and sand to the clay to reinforce its strength. Anything that isn’t needed or doesn’t turn out to be high enough quality can be turned back into raw materials to be reused again, and the material can also be easily recycled at its end-of-life. Remains of a mix of traditional concrete is often carefully packaged and disposed in a dumpster, but with cob you can put it back into the garden. A minimal amount of energy and water is used to make cob, so the process has a very small carbon footprint.

Building high-quality green buildings with cob

While sustainable building materials are important in themselves, it’s also crucial that they’re used to build houses that will provide comfortable homes for generations and run efficiently. A house made with cob uses a fraction of the energy of a house made with concrete or bricks, so these buildings will be green throughout their lifetimes - and will save homeowners money, too. 

Cob is excellent at preserving heat and regulating temperature, so it will keep the building warm all year round with little need for central heating. The material acts like a sponge, absorbing heat when it’s hot and releasing it when the temperature cools down, and the straw in the cob mixture also adds to its insulating properties (cob can also absorb smoke).

Cob also provides natural fire resistance without the use of added chemicals; benefits air quality; and its breathability means it is resistant to mould, even in damp and cold weather. It’s also highly soundproof, so it protects residents from noise pollution.

Houses made from cob are incredibly long lasting, with some standing strong for centuries. The material is stable and robust, so it won’t shrink, crack, or swell in weather conditions, and can even withstand natural disasters like earthquakes, fires, and floods. 

Driving down construction’s carbon footprint

The construction industry’s environmental impact is huge, and with climate change rapidly worsening, it needs to step up and make significant changes to how it operates. 

But we don’t have to choose between high quality buildings and sustainable ones - switching from traditional concrete to natural materials like cob could dramatically reduce the sector’s impact, and build warm, comfortable homes at the same time.

Mixing cob in a Forced Action Mixer

Cob is a natural, historic building material that is once again growing in popularity thanks to its low carbon footprint. It’s traditionally mixed by foot - but if you’d prefer a far less physically exhausting and time consuming method, cob can be mixed ten times faster and much more easily in a Forced Action Mixer. Building with cob is a hard physical task, so let machinery like a Forced Action Mixer do the hard work. Since cob is a heavy material, ergonomics of the machinery is important – advantages like a bucket stand, telescopic legs, wheelbarrow fit, easy removable mixing arms for cleaning and more are useful tools. Read our blog on ergonomics in a Forced Action Mixer. 

Again - cob is made from clay, water, sand, and straw, to reinforce its strength. The exact proportions of sand to clay that you will use will vary, as clay is found all over the world and has slightly different properties depending on where it’s located. However, a good starting point would be to use two parts sand to one part clay. The volume of mixed cob material needed must be fit to the size of Forced Action Mixer chosen. 

SoRoTo has the widest range of Forced Action Mixers, from 40 liter to 300 liter. The standard arms for a Forced Action Mixer will do the job, as there is no small stone or rocks in the mix. Should you have a special mix with stones larger than 8mm, we recommend changing to our rubber paddle mixing arms. 

Our Forced Action Mixers gear/motor combination has been chosen with focus for maximum effect, but with lowest possible amps used – so you can plug them to power in places with “minimum” electricity available. Our standard mixers has a speed of 30 rounds per minute, which is optimal fitted for mixing concrete, will also do the job of mixing cob. However since cob is a natural product only needed to get “massaged” into shape, our VARIO (additional asset) motor can decrease or increase the mixing time. The SoRoTo Dust Controller can reduce the dust from mixing without compromising the view to the mix.

Should you need mason buckets for plastering og cobbing, then the SoRoTo H-Buckets has great advantages to increase ergonomic and reducing numbers of buckets used due to the long lifetime. If doing smaller volumes of mixes, then our Electric Mixers can do the job – but we recommend a smaller Forced Action Mixer to spare yourself.

How to mix cob

Run the forced action mixer and add the clay, sand, and water. The machine will mix the ingredients rapidly, so this should only take a few minutes. If your material is too runny and doesn’t hold together, add more clay. 

Next, you’ll want to add the straw. However, because this will make the mix so tough, your forced action mixer might struggle if it isn’t powerful enough. If you have this problem, you can either make the mix with plenty of water, then take it out of the mixer and let it dry out a bit before you work with it - or make the mix without the straw, and add it in by mixing with the traditional method of stomping on it.

Ready to start building with cob? Check out SoRoTo’s range of Forced Action Mixers and find the perfect machine for the job.

Disclaimer: The following blog text contains guidelines regarding cob and how to mix it as communicated by the company SoRoTo. However, it is important to note that these guidelines are for informational purposes only and should not be considered as a substitute for the manufacturer's guidelines. 
SoRoTo recommends that users always refer to the manufacturer's instructions and guidelines for their specific product to ensure proper and safe usage. 
SoRoTo will not be held liable for any damages or injuries that may result from the misuse or improper application of their guidelines.