What is Cob?

Cob is an ancient building material that has been used for centuries worldwide. It is a natural and sustainable material made by mixing earth, sand, straw, and water. Cob has many advantages over other building materials; It is cheaper, more eco-friendly, and has excellent insulation properties.

This blog post will discuss how we constructed a cob and lime plaster pizza oven from scratch. Cob ovens are increasingly popular in the UK and Europe and for a good reason! They are easy to construct, very efficient and easy to use. In addition, building an oven from cob can save you a lot of money compared to making it with conventional brick and mortar.

 

The Project

A couple of months ago, I was approached by a lifelong friend Carel Bester to build him a pizza oven, and after some research, I suggested a cob as a building material. Our original design was quite ambitious, with the entire base of the oven, consisting of a countertop, oven and a small planter to be built entirely out of cob. With a maximum budget of R7 000, this was regarded as the best and cheapest material for the project, and to further bring down the cost, we decided to build it ourselves.

Building with cob has become a bit of an oddity nowadays. As a heritage construction technology, few people can be regarded as masters of this form of construction. For the past three years, I have selectively experimented with small samples of bio-based construction materials to test their strength and weathering qualities. Surprisingly, I found cob quite robust, and as an architect and potter, I knew I could control the material.

 

Cob has a low carbon footprint as it uses local and natural materials. It also has excellent thermal insulation properties, allowing the oven to cook food faster and more efficiently.

At our client’s request, we added a Braai to the design, which extended the entire square meterage of the project to 12 sqm. I have experimented with cob before but never on this scale, so I saw it as a way to experiment and learn what can be done with cob.

 

The Build

Start date: 18 March 2023

Location: Sasolburg, Freestate

Material: Cob and lime plaster

End date: 16 April 2023

 

The first weekend: The Frame and starting the base

Taking inspiration from wattle and daube construction, I decided to design it around a light timber frame that would help keep the structure rigid and guide the construction as we continue the build. Carel sourced poplar cuttings from a local outcrop, and our first step was to assemble the frame and put it in place on-site.

The frame in place

Then we started the laborious task of building the structure layer by layer. At the end of our first weekend, we were almost at the height of the braai. Carel continued construction during the week with Paulus to get the rest of the base up to the same level.

End of the first weekend

Later that week

 

The second weekend: Base Continue

Finally, we realized we needed some help. Carel arranged for Paulus and his friend Andries to assist us with building and mixing cob, respectively. With their assistance, we completed the braai counter and started the arches for the oven and countertop. Building Arches seems like an easy process, but cob quickly becomes top-heavy when still wet, so take it slow and be patient, allowing your arches to dry a bit before continuing building.

Good Progress – End of our second weekend

 

The third weekend: Counter height at last!

Once again assisted by Paulus, we finally completed the body to counter height. The design called for a sand and glass bottle bowl below the oven to help retain heat for longer, so we left the bowl empty to dry for the following week.

Countertop at last

 

The fourth weekend: The oven

On arrival, we quickly filled the bowl below the oven with sand and glass bottles before laying the brick base for the oven and the slate tiles we chose to line the bottom of the braai area. Then the construction of the dome began. Once again, we used a timber frame to help support the cob as we built the dome to a thickness of about 15cm.

Superstructure finally completed

 

The fifth weekend: Plastering

The last weekend finally arrived. The body was finished in lime and sand plaster, while the countertop was finished with a thin layer of pure lime mortar. Concerned about using synthetic oxides in the mix, I ordered pure pigment from a supplier but underestimated the necessary quantities, leaving us with too little pigment to finish the counter in a dark black colour and ending up with a light grey instead.

The end result

 

So how much did this cost?

Material breakdown

QT     Material                                   Price per unit        Sub Total

60      Bags of soil                      @      R40/bag     =       R2 400

10      Bags of builders sand    @      R40/bag     =       R400

2        Bags of plaster sand      @      R40/bag     =       R80

1        Bale of Hay                      @      R20/bale     =       R20

1        Bag of Lime                     @      R235/bag   =       R235

10      Pigment                           @      R50/vile      =       R500

1        Bag of plaster fiber        @      R70/bag     =       R70

1        Ball of cotton twine        @      R20/ball      =       R20

 

Additional Cost

6        Labor                              @      R350/ day   =       R1 500

Grand Total          =       R5 225

Please Note: This total only indicates the material costs when constructing it yourself and excludes the additional labour of myself and Carel over the five-week build. If we include all other expenses, the installation cost will be closer to R15 000/R20 000, which is still less than half of what a conventional brick oven would cost.

So what’s next?

The final step is for the cob and plaster to dry properly over the next couple of weeks before Carel can enjoy the results of our labour. Once dry, he can start firing the oven. Small amounts of firewood should be added to the oven, gradually building a larger fire. It is essential to allow the heat to circulate the oven walls to get an even internal temperature of between 350- 400°C.

Once the fire burns down to coals, brush the ashes aside and put your pizzas in the oven. It can take 60-90 seconds for the pizzas to cook, so keep an eye on them, turning them every 30 seconds.

Conclusion

Constructing a cob and lime plaster pizza oven was a fun process. Cob is an amazingly robust material and easy to work with, even if you have no tools since all you need are your hands. Cob ovens have many benefits compared to brick or steel ovens; better insulation and heat retention, cheaper and eco-friendly as an added bonus. Inexpensive and abundant, building with soil can save you money while producing delicious food. If you liked this project and want to build a cob oven of your own, contact us today for an obligation-free design and construction quote.