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The Building of Green Camp’s Cob Pizza Oven

At Bamboo U we are experimenting and building with more than just bamboo. We believe in using all kinds of natural materials. As a part of the development of  Green Camp’s new location, we built a Cob pizza oven in May 2020. We wanted to build something that can crank out lots and lots of pizzas for hungry campers. We started designing the oven and bamboo roof structure following the existing conditions of the site. We made a scaled model of the oven with the roof structure.  This helped us to visualize the structure and when building with bamboo, the model acts as the blueprint for the whole construction process.

We started by making a dry stone foundation, compacting stone debris, and gravel. Then, we raised the oven base structure using random rubble masonry. Each piece of stone needs to be selected following its shape and put into place to lock in tightly with the other stone elements. To help secure the base we filled in the gaps between the stones with mud mortar.

From left to right, the model of the oven and bamboo structure, the making of the dry foundation, the making of the oven base, compacting the layers of the oven base

Pizza ovens are designed to retain heat and radiate it over an even cooking area. To retain the maximum heat you need good insulating layers underneath the base, that is why the middle volume of the oven’s base is filled with the soil that has been compacted using wooden rammers and also a layer of reused empty glass bottles. Glass bottles are made of silica which is the primary constituent of sand, which has a high resistance to heat and can help with insulation. These glass bottles are covered by sand and leveled to the wall height.

The base of the cooking area in the oven needs to be a flat surface where pizzas are placed to be baked. This top layer is made of tightly placed flat burnt bricks. Since some of our bricks were uneven, we sealed the gaps using a lime paste.

From left to right, Glass bottles layer, placing the flat burn brick layer, the finished brick layer with lime to seal any gaps

To control the flow of heat we designed the dome in proportion to the diameter of the base. The center height of the dome is 75% of the base diameter and the height of the door opening is 63% of the dome height, so that smoke can easily escape from the top edge of the door opening. 

Once the base was ready, we made the formwork for the oven dome. We made our template using bamboo sticks known here as ‘lidis’ and placed them on the top of the oven’s base. The formwork is built using sand similarly to how we build a sandcastle. We built a solid sand dome, which is later removed once the dome is built and secure.  

During this process, we were also busy making cob so that once the sand dome was built, we could start building the dome immediately. Cob is a mixture of clay, sand, straw, and water, mixed manually with our hands and feet. To have a good quality of Cob it is suggestable to have 20-30% clay and 70-80% sand depending on the soil available locally.

In building an oven dome we need to have a thermal mass layer and a thermal insulation layer to hold good heat for baking. So the first layer (thermal mass) of cob mixture consists only of clay, sand, and water. Whereas the second layer (thermal insulation) of cob mixture consists of clay, sand, straw, and water. The straw acts as an insulation material. We also added bamboo lidis between the two layers which acts as fibers and helps to avoid any structural cracks.

Once the oven was built, we cut the door opening and pulled out the sand, molded the opening with cob, and made a beautiful teak wood door for the oven. This door shape is achieved by using the catenary arch technique. The catenary arch is a type of architectural arch that follows an inverted catenary curve. Catenary arches are strong because they redirect the vertical force of gravity into compression forces pressing along the arch’s curve. This has been employed in buildings since ancient times.

Top from left to right, the sand done used for formwork, barefoot architect mixing the mud, catenary arch template, molding the door. Bottom from left to right, finishing of the cob work, part of the building team.

When you are building with material like mud it is better to avoid extra dampness and for durable natural buildings, the basic rule is to have a good foundation and a good roof. Here in Bali, we witness a lot of heavy rains in the monsoon season. The bamboo structure is designed to protect the oven. The bamboo structure is a post and beam system with a beautiful curvilinear roof, in harmony with the oven shape and the site surroundings. For the oven structure, we have made foundations for the structure and added beautiful river stones over the foundations to avoid the bamboo columns having direct contact to the ground.

We selected Bambu Duri (Bambusa Blumeana) poles of different diameters for columns, beams, and cross bracings. Bambu Jakarta (Thyrsostachys Siamensis) for the ridge beam and arch cross-bracing between the front columns of the structure. Bambu Tali Hitam (Gigantochloa Atroviolacea) for the trusses and rafters. And untreated Bambu Tali (Gigantochloa Apus) for the roofing. If you want to learn more about the bamboo we use please check out The 7 bamboo species we use the most.

We started building the structure on the ground, we put the beams in place, and interlocked the four beam poles by half-cut bamboo sections at the corners of the poles at the point where the columns support the beam frame. To rest the beam frame on columns we made ‘double fish mouth’ joinery. 

To achieve curvilinear roof shape we added a lidi bundle ring beam over the frame according to the design. Then we created a curved ridge beam using the Rub-Rub technique, which involves incrementally cutting slices in the bamboo so it can bend. The bamboo lidis are added over it as a structural reinforcement. Then we started adding the truss members and rafters. A truss is the web of triangle framework that supports and distributes the load of the roof.

Top from left to right, bamboo column sits over a Riverstone, joinery of the beams, making cuts in the bamboo for the rub-rub technique, covering the rub-rub arch with lidis. Bottom left to right, double fish mouth, fish mouth, the making of the roof structure, the roof structure with the rafters

Once the roof structure was built, we lifted it and rested it over the bamboo columns and trimmed the rafters wherever necessary because the oven is closely surrounded by trees. We added the cross bracings in the sides to make our structure stable. In the front, we added an arch for aesthetics, which also acts as cross-bracing.

Once the roof structure was fixed we started the first layer of the roof with repurposed Zinc metal sheets. We also made a vent at the top to allow hot air to escape. To cover the zinc we used bamboo poles split in half with the nodes removed and cut to the required sizes. The half bamboo poles were then placed in an interlocking way to form the external roof layer.

Top from left to right, roof vent, the process of the interlocking half bamboo poles Bottom pictures of the finished cob oven and bamboo structure

It took a lot of hard work to build such a beautiful and detailed small structure. However, the flexibility of bamboo and mud gave us the ability to make a lot of onsite decisions to achieve what we needed within our timeline. If you would like to learn more about Bamboo structures and natural building techniques, Come join us in the next Bamboo U 11-Day Design and Build Immersion Course!

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