We live in the wet tropics of far north Qld, Australia, and it gets hot up here in the wet season.
Humidity can run at 90-100% for several months at a time, and the kitchen in the house is not a great place to be if your daily task involves cooking and baking.
The heat is inescapable, and the house is not air-conditioned, so the solution to the hot kitchen problem was to cook out in the pool area under a large roof.
We had a gas BBQ that was showing its age and was ready to fail, and it was about this time that we completed a Permaculture Design Certificate (PDC) and, during the course, had been introduced to the world of cob construction.
The idea dawned on us that we could build a cob oven out of termite nests.
The nests we used are prolific along the main highway past Mt Carbine heading north towards Cape York, and we knew that these nests stood up to fire because the country around this area burns off each season or so, and the nests hold up just fine.
If you are interested in clay from termite mounds, this article can shed some light on what the nests are made from.
We didn’t know it then, but the nests’ clay was great for making cob ovens similar to the type we designed and built.
This post is how we went about building it, untrained but mad keen. All we can say is thank God for youtube.
We had ideas on top of other ideas that pushed aside other ideas. We ended up so confused that we tossed out all designs that other people had used and took a different path. This is the design process we took.
- It needed to incorporate a rocket stove as the heating source.
- It needed to be large.
- It needed to be cheap to build.
- It needed to look good.
- The cob needed to be easy to repair.
The build should not be rushed. We discovered there are many design variables that need consideration and so to help, we suggest you start with “How do you design a cob oven?“. It details step by step the design process.
Why make an Oven with a Rocket Stove Base?
We had been inspired by the information shared within the PDC about some North African communities growing eucalyptus trees for firewood.
They coppiced the trees every few years to harvest firewood that grew the best size for their food cooking systems, including cob rocket stoves.
They are incredibly efficient in fuel use. You can harness the heat as it winds its way up galleries within cob walls of buildings to internally heat them passively to eventually exit via an opening, sometimes tens of feet away from the fire.
It works the same as a traditional wood-fired cob oven in that it traps the heat within the wall and slowly releases it over many hours.
This came from a small rocket stove used for cooking food, with custom-grown firewood as the fuel. Very practical and has no cost to run after the build.
So we designed this low-tech process into the base and then proceeded to build upwards.
The idea of a rocket stove is that the heat generated from the fire is drawn along a horizontal tubular chamber for a certain distance. This chamber turns upwards to form a chimney that when the fire reaches a certain temp, the chimney draws so hard that the fire roars like a rocket.
This method is one of two ways to get heat into the oven. The base thickness is important because it should retain as much heat as possible. We go into this in detail in this post titled “how thick does a cob oven base need to be?” as it is quite a critical factor in building these ovens.
The specs we used for the horizontal tube to the vertical chimney are 1 to 3, or the chimney is three times longer (As in tall) than the horizontal section.
This creates a good draw and gets the heat into the oven. The top of the cob chimney vents into the base of the oven, and we have placed a BBQ plate over the top of the vent with a gap for the heat to move around the plate edges.
It has worked this way for a few years without an issue. The cob the chimney is made with is holding up really well, and needs no maintenance.
An interesting thing has happened with the cob fire-pit section. The cob mix we used to make this has turned into terracotta and is as hard as concrete. It glows red when the fire is going.
Building it Big Enough For a Crowd.
The construction method we used is masonry blocks for the four legs and a half sheet of compressed fiber-cement as the platform for the actual oven. The cob for the oven base sits on this sheet.
These legs were eventually smeared with a layer of cob to blend with the rest of the oven, and need no maintenance.
The internal dimensions of the finished oven are 18in high (460mm) x 27in wide (685mm) x 34in deep (860mm). It is large enough to cook several things simultaneously, like a couple of roasts with veg + cakes + bread.
It has been used for all sorts of things.
Low Cost and DIY.
The main cost for this build was in the masonry blocks and the cement sheeting. Everything else with the build was scratch and scrounge.
The cob was cost-free, and the plywood used in making the wall bricks came from offcuts from other projects. There are sixteen masonry blocks for the legs and a 3ftx3ft (900 x 900mm) sheet of 3/4in (19mm) cement for the base platform.
The internal cooking bases for pizza’s we use are round pizza baking stones. They work great with this oven.
You don’t have to make bricks if you prefer not to. We made and used them because the main cooking area of ours is like a short tunnel. The conventional method is to use a sand dome and layer the cob outside of it.
It should be attractive.
Beauty is in the eye of the beholder, so it is said; and while we like the shape and size of this cob design, no doubt there will be people who prefer a different style, and that’s fine with us.
The typical earthy natural shapes that some designs have may not suit some homes and this is a factor outside of the scope of this article. We know that our oven suits our lifestyle and yard, and that is all that matters to us.
This oven ticks all the design boxes we initially set out, and we admit we are somewhat biased. We like the natural curves that cob allows, and the grainy texture of the oven walls.
We absolutely love the rocket stove component. It is brilliant for saving wood.
It may look like some nest that a bird might make, but when it gets going, you soon understand why they are called “rocket” stoves.
The door we use is a simple panel of untreated pine studs left over from something else. It was intended to be temporary, and that was six years ago.
It is close to its expiry date because of the heat involved. It has gaps between the studs that let the smoke out at times, so it now looks the part.
Making it with Low Maintenance Termite Cob.
This is probably the best attribute of the whole building process.
Because we happened to stumble onto a good recipe for cob by using the termite nest as the clay component and adding straw and scoria to the mix, we now have a tough, low-maintenance design that will be with us for decades.
The straw component we used was two types of plant fiber. Hay was used in parts, and coconut husk in others. If you need more info on what is suitable to use, we have an article titled “What straw is best for a cob oven mixture?” for you to investigate.
Our oven is mostly protected from rain, and the walls are holding up well without cracks or decay. The internal walls are sound but very smoky. I imagine all wood-fired ovens are this way.
It gives a good flavor to the food, especially pizzas.
One thing to note if you build one; the first few fires have trouble getting to any serious temperature.
The moisture in the cob keeps it down. Once the cob is fully cured and dry, the oven can maintain 180C for hours with a bucket of sticks. The oven has reached 240C without effort. It uses a bit more wood at that temperature, and you must be around to feed it.
So there you have all of our why’s for building a wood-fired cob oven. Now, let’s discuss building it in easy to follow steps below.
What materials are needed to create a Rocket Stove Pizza Oven?
The materials listed below are separated into required and good to have but not necessarily required. This list is based on our design and is close to the specs as it stands now. Building it requires some simple skills and a little sweat equity.
Examples of material substitution.
We could have used clay sourced from local farms yet chose to use termite nest. We could have used a conventional cob recipe yet chose to add scoria to the mix. These are just two examples of required materials being exchanged for others. This is dependent on availability and personal choice while maintaining the integrity of the cob.
Required Materials for the Dome build.
- Your DESIGN BRIEF.
- Clean clay.
- Straw or hay.
Establishing a design brief.
This is where mistakes can be caught before they become reality. Research every aspect of the build if possible and keep details of what you want included. A place to start with build design choice is this article on Cob Oven Design. The design process can be broken into steps like the simple list below. It should include the following
- Expected uses
- Available firewood supply
Where is the best location for a cob oven?
The best location is a site that is flat, flood-proof, easily accessible, and undercover if possible. We placed our build on pavers within a pool enclosure. It is mostly undercover with the rear external face exposed to harsh weather. This helps protect the cob from dissolving during our intense wet seasons.
What size is best?
Size is determined by what you intend to cook and the available space where you prefer the build to rest. We have supplied two average oven sizes below in table format that can be a guide. Every build becomes a custom design so work this out before starting.
Does the shape of the dome matter?
The shape is important for air movement if the oven is a conventional style where the fire is lit internally. The rocket stove style has more latitude for chamber shape, generally speaking. The beauty of cob is how easy it is to work with and shape as you please.
What finish is best for clay cob?
We have left our build as it dried and have not applied any coating at all to the outer layer. This will not change the way the oven operates, and coating it is dependent on your situation. You can use a lime-based paint to seal the cob if desired, or linseed oil with fine clay as a paste over the cob. The oxides used to color concrete can be used to tint the coating to your liking, and can also be added to the cob as you build the oven.
Design for your expected uses.
What we mean here is how you imagine the oven being used, and what food you want to cook in it. We wanted a large opening, with plenty of room internally without needing a lot of firewood. This then helped us decide other aspects of the design.
How much firewood will it use?
This is where a choice has to be made in your design brief. To help you choose what is best for you we have a dedicated article that details the differences in the two types of ovens. It describes how and why a conventional build works, and also explains the rocket stove concept.
Your build choice governs the wood required for a normal cooking run and that comes down to why you are considering the build in the first place. If it is intended to be an entertainment centerpiece and not for regular off-grid baking, the conventional style could be for you. Be aware these ovens do use a lot of wood.
If you want an oven that acts like an oven and can be up to temp quickly, the rocket stove build may suit. It will use far less wood but needs more attention to maintaining the fire as you bake. This style also requires specific wood sizes to operate at peak efficiency.
One notable disadvantage with the rocket stove style is it can make soot that builds up on pans and pie dishes as you bake. This is from the fire exhaust venting into the oven chamber. We have noticed that soft wood will create more smoke and soot than the proper hard wood, and the rocket-fed versions run brilliantly with the hardest wood.
Materials that can be a substitute or an additional ingredient.
- Termite nest or mound.
- Coconut husk fiber
Why use substitute materials?
Sometimes materials are available that can lower the cost of the build, or the material substitute is equal or better than the original ingredient. Below, we explain our thoughts on the three listed.
Termites build their mounds our of excavated earth mixed with moisture they excrete. This earth is a filtered form of clay, and they know what particles to choose while building their nests. We think it is safe to say that these nests are made with cob.
Observation proves that these nests can withstand fire, flood, rain, and sun. These are the attributes we look for when choosing our cob. We use the nest material as the sorting and sifting has been done for us. We have found it to be as good if not better than traditional cob.
Can Coconut husk fiber be used instead of straw?
When building with cob, you will come across places in the build where the straw or hay within the cob mixture has trouble bending and conforming to our desired shape. This is usually around a tight bend or curve that a DIY cob design often includes.
It is in these situations where coconut husk fiber shines. It is strong in tension and is pliable enough to bend in a very tight radius while maintaining integrity, and this opens up the design parameters. Curves are not going to create problems with air pockets in the cob or straw not conforming to shape and limiting the ability to be creative.
Why add Scoria to the cob recipe?
We added this material to the cob to harness the thermal properties that it has. We considered these qualities carefully before adding it to the cob, and have no regrets with incorporating it in. The reasons are covered in detail in this article here.
Can volcanic rocks like Pumice be used similar to scoria?
You can use pumice in the cob mix if it is available, however we decided not to use it as scoria is a much harder material and we think it will last longer.
How much clay is needed to build a cob oven?
This is where your design brief needs to be sorted out before you start building. Every building starts with a plan, and yours is no different. This design brief should have what you plan to use the oven for, and this helps guide what size it needs to be.
The amount of clay (cob) required for something similar to our build is in the table below. We have included separate tables for both imperial and metric users.
How much clay is in an oven dome?
We have included two tables below to give some idea of the clay required for building two different sized cob domes. Naturally, every build will be different as will the clay amounts, but you should be able to get an idea of quantities from the data below.
Metric clay requirement for the dome only.
|Base Radius (ext)||600mm||760mm|
|Base Circumference (ext)||3.77mm||4.775mm|
|Floor area (int + wall thickness)||1.13m2||1.81m2|
|Dome height (ext)||550mm||685mm|
|Dome surface area (ext)||2.08m2||3.29m2|
|Dome surface area (int)||1.42m2||2.44m2|
The table below is imperial calculations for the clay required for the oven dome only. The clay for the base is discussed further below.
|Base Radius (ext)||24 in||30 in|
|Base Circumference (ext)||150.8 in||188 in|
|Floor area (int + wall thickness)||1809.5 in2||2827 in2|
|Dome height (ext)||21.5 in||27 in|
|Dome surface area (ext)||3261 in2||5117 in2|
|Dome surface area (int)||2218 in2||3785 in2|
|Dome/wall thickness||4 in||4 in|
|Clay required***||6.28 ft3||10.23 ft3|
How much clay is in an oven base?
The amount of clay needed to form the base is variable. We built a base that is 100mm or 4 in thick. Why we chose this thickness is detailed in this article here.
The diameter of the base will determine the amount needed and is dependent on your design brief. Some examples of quantities are in the table below.
|Oven Base Diameter||Base Thickness||Clay quantity|
|900mm/3 ft||100mm/4 in||.06 m3 / 2.12 ft3|
|1200mm/4 ft||100mm/4 in||.17 m3 / 6 ft3|
|1500mm/5 ft||100mm/4 in||.32 m3 / 11.3 ft3|
The clay amount is certain to vary according to your design brief, so the data above should give a guide to begin with. The clay quantities above are indicative of the final cob mix after blending all ingredients and is the material you build the shell with. It includes the clay, straw, sand, water, and extras that you choose to include.
Does the base need a fire brick cooking surface?
If fire bricks are available and you wish to include them in the design, by all means, use them. We had none available when building so we used cob, and as our oven requires a blast plate to spread the heat as it vents into the chamber, we use other cooking methods instead of brick. A fire brick base is often used for cooking pizzas and we have a work-around with this by using the circular cooking stones.
What sand is best?
The sand that we recommend is clean river sand, or something similar to the sand you would place in a kid’s sand box in the yard. We found that the termite nest contained small grains of sand and stone, so we didn’t need to add any to our cob recipe.
How much water should be added to the mix?
You are looking to make a cob mixture that can be rolled into a ball where it will keep its shape without collapsing. The optimum moisture content for your cob mix is where two of these balls will stick together if placed side by side, again without collapsing. This allows each additional handful of cob to adhere to the previously placed cob as you are building.
This list is dictated by your design brief and the style of pizza oven you choose to go with. There are many variables to consider when building and we cannot cover all in this article, however, if you will spend some time looking at images online you will come to some idea of what is required.
We suggested plywood earlier, and this is for shaping the oven chamber if you decide on building something similar to ours. The plywood was cut to form a support structure that allowed us to lay cob and scoria bricks that we had pre-cast.
These cob and scoria bricks were cast in forms that were made from plywood, and worked well. Once these had cured we laid them in an arch with a thin layer of cob between each brick.
We then placed a standard cob mix over the entire brick shell to hold everything in place. After seven years of use, we are very pleased with the results with the cob holding up better than we could imagine.
We also used a sheet of compressed cement sheeting as the base platform to support the cob and it was designed to remain in place. This material will not burn, rot, or decay when used in this situation. It has performed as expected so we recommend using it if you are building the oven on a raised platform similar to ours.
The join where the cob rests on the sheet will form cracks over time, and is not something to be concerned with. This is from the cob expanding and the sheet staying static.
Do these ovens need a solid base?
It is not a requirement to have a solid base or cob plinth, however building one into your design can be aesthetically pleasing in many situations.
A solid cob base can be desirable if you are building for aesthetics and have a design that blends into your location nicely. Bases of many cob ovens are made with natural materials like stone or brick and look great.
We chose the rocket stove base, and this required access to the fire pit built into the burn chamber and chimney rise. This vents into the oven’s chamber and supplies the required heat to bake with.
The cob base is separated from any material that can soak heat away from the oven as it operates, making this a very efficient way of building. It also eliminates the need for the fire brick floor.
This article was put together to help the prospective oven builder discover what goes into making one, and the design thoughts we worked through as we worked on ours.
Article by Tim Blanch for TheTropicalHomestead.com. He is a qualified Permaculture designer.