All things intended to last a long time should have some design aspect integrated into the building of that thing. Cob ovens are no different.
To design a cob oven, a list of desired attributes should be made. Then research should be undertaken to ascertain the most beneficial method or process to deliver the desired outcome.
A rocket stove cob oven is a versatile and energy-efficient outdoor cooking appliance. It combines the functionality of a rocket stove, which efficiently burns wood and produces intense heat, with the traditional cob oven design. This hybrid oven utilizes the rocket stove’s combustion principles to heat the cob oven chamber, allowing for efficient and even baking. It offers a sustainable and cost-effective solution for outdoor cooking while minimizing fuel consumption. This is the design we built.
Why starting with a plan matters.
A cob oven is typically a large, heavy item that will be almost impossible to relocate if the oven is built in a poor location or the oven is dysfunctional through either inadequate or a complete lack of design planning.
Any unplanned process is fraught with performance risk, so investing time and effort into a project before work commences is sound thinking.
Where do you start with Designing a Cob Oven.
To begin the design process, you would do well to list what you would like the cob oven to do for you. You could also list what the oven should look like aesthetically. A bit of time looking through the images on the web will give some great ideas.
A cob oven can be used for many things and referring to the potential uses will help to steer your design parameters towards a good outcome. There are two types of cob ovens to consider at this stage, and they are discussed in detail in this article titled “Cob oven vs rocket stove oven, what best suits you?“
The following list is things you might consider, and each item is expanded on further into the article below.
- Does the intended oven site flood in heavy rain.
- Expected weather exposure of the oven at the intended location.
- The aesthetics of the oven.
- The materials to be used in the construction.
- The height of the oven.
- The required internal size of the oven.
- The ovens internal base construction.
- Wall thickness of the oven.
- Clay component.
- What grass straw to use.
- What fire wood is available.
- Closed chamber or rocket stove fed oven.
1. Are cob ovens weather proof?
Because cob ovens are generally made of clay, straw, and sand, they are not weatherproof completely.
They will deteriorate slowly as rain and wind gradually dissolve and remove the clay component from the outer layers. This exposes the larger particles of sand, which eventually fall off and leads to further exposing more clay.
A possible solution to the weathering process is to mix clay and either linseed oil or pure tung oil (if available), make a spreadable paste called render, and smear this mixture over the external surface and let it dry. It will take many weeks to dry completely.
The location of your oven should be above any low ground to avoid water damage to the lower base sections.
It may seem logical, but the number of things built in dry times that end up surrounded by water in wet times is huge. The surface water can play havoc with a mud oven built at ground level if the water drainage is not considered and the site is poor.
Even surface water can cause issues so look at the site location carefully.
2. Do cob ovens need to be under a roof?
It is preferable to either build the oven under a roof, or build the roof over the oven eventually.
The life span of the oven will be significantly extended when it is protected from the elements. We have noticed a significant difference in the cob surface of our oven after seven years of life.
The rear of the oven is exposed to the elements at times while the rest of the oven is protected. The face that is exposed is the only bit that will need some touch-ups in a year or two.
This oven has no render over the surface, so it is a good test case for the termite cob mix and its weathering ability over time.
3. Cob ovens can look stunning.
Many of the Cob Ovens designs that have been built over the years across the world are simply beautiful to look at.
It is even more special when you consider that the oven is a structural element in that location and serves to cook meals for many people at times. It is form meeting function.
Consider the visual aspects just as much as the usefulness requirements. If the oven is less than attractive, it can be a long life looking at something wrong.
4. What materials are needed to make a Cob Oven?
Materials for cob can include the following items
- Clay, or clay substitute like termite mound.
- Thermal additives like Scoria.
- Grass stalks like hay, straw, coconut husk.
- Rocks for a foundation.
- Fire bricks, or pizza stones.
- Hydrated lime for a render
- Linseed oil.
- Empty bottles.
While most of the listed items are mentioned in this post, there is little on the use of fire bricks because this is a subject on its own. We have a post titled “Does a cob oven need to use fire bricks?” We recommend reading it for more detailed explanations. Straw is required in some form, and some people may be confused about what straw is the best. Can hay be used? We go into this in-depth in this article titled “What straw is best for a cob oven mixture, will hay do?“
5. How high should a Cob Oven be?
This depends on your design and the intended uses for the oven.
The end height is the result of the desired oven floor height for cooking, added to the required door opening size that has clearance for cooking trays and similar, the desired inner oven chamber dimensions, and finally, adding the roof thickness of the cob.
This last component lets you get creative and go as tall as you wish. Minimum cob wall thickness should be maintained for effective oven operation.
6. What size cob oven do you need?
The oven chamber should be large enough to accept a good sized fire, if the oven is to be an internally fired design.
The ovens internal chamber height should be based on the hearth radius being equal to the chamber internal height. Variations of 5-10% will be ok with the tendency towards a slightly lower roof top, more-so that a taller dome top.
The table below gives a range of guidelines to consider when designing chamber diameter vs height.
|24 inch (600mm)
|21.5 inch (550mm)
|25 inch (635mm)
|30 inch (760mm)
|27 inch (685mm)
|36 inch (910mm)
|32.5 inch (825mm)
|37.8 inch (950mm)
|42 inch (1065mm)
|38 inch (965mm)
|44 inch (1115mm)
Taller internal domes offer no measurable benefit toward heat retention of the oven and will use more wood to achieve the required amount of stored heat.
The oven’s internal dimensions are variable according to your planned usage and the cooking equipment you have available.
The chamber can be too large because the wood required to heat a large oven can become costly if the wood is purchased.
The oven can also be too small because the size of the fire cannot be large enough to heat the oven effectively.
7. How thick should a cob oven base be to work well?
The base of a cob oven should be more than 8 inches thick(200mm) if using straight cob.
Thermal additives like scoria can be added to the cob mix as aggregate similar to a concrete blend, which can lower the base thickness if desired.
Six inches should be a minimum in this case. Thermal isolation should be considered if building on a support base that may parasitically draw heat away from the oven.
Glass bottles can be used for thermal isolation; anything that will form closed air pockets within the oven base and not melt or burn can be considered.
For more detailed information on base thickness, this post is recommended. “how thick does a cob oven base need to be?“
8. Should the Cob Oven walls be thick?
Because we depend on the cob walls to hold heat, a minimum wall thickness should be 4inches (100mm), but this is a hard minimum. Thicker walls are more efficient.
The oven walls and roof are where all the overhead heat is radiated from as you cook, so designing the optimum thickness is wise. To help with your research, we have a post dedicated to wall thickness here. “how thick does a cob oven wall need to be?“
9. Can termite mound be used in a Cob Oven build?
Termite mound (nest) can be used in a cob mixture as a great clay replacement.
It is very effective and arguably even easier to use than natural clays that require screening for larger stones and rubble.
Termites only build with tiny granules of soil/clay mixed with saliva-like substances to glue the medium together. Termite nests are also fireproof. We put an article together about termite nest cob mixes titled “Can termite mound be used in a cob oven mix?” for the curious to explore.
10. What sort of grass fiber should be used in a cob oven?
Any natural grass can be used as long as it has stalk like structure to give strength in tension.
Straw stalks can come from many grasses, and include hay, straw, barley grass, and others.
We used coconut husk in our oven build, which has worked well. We also used local hay for sections as well.
11. What sized wood will a Cob Oven work best with?
The best wood for a Cob Oven is dense hardwood that is dry and cut to an appropriate length for your oven.
If you need a fast fire, use smaller logs/split sections. This will limit the heat that can be pushed into the cob wall.
Use bulkier logs once the fire is well lit for a slower fire and a hotter oven with a longer cooking timeframe. Practice will let you know what size is appropriate for your oven.
12. What type of Cob Oven is better, closed-chamber or rocket-stove fed?
No one oven is better than the other. Each design offers different benefits and comes with its own set of drawbacks.
The design criteria will be guided by the following.
- intended oven uses
- Available hardwood
- Expected cooking times
- Required temperature ranges
Intended oven uses.
The closed chamber cob oven is appropriate if you intend to have extended cooking sessions for large groups.
If you wish to cook for smaller groups with a shorter time between fire lighting and the start of cooking, then a rocket-fed oven could be for you.
The amount of hardwood available can be a deciding factor if you do not have access to an abundant supply nearby. Urban dwellers will have rising wood costs as time moves forward. The price is already climbing rapidly.
A rocket fed oven will typically use approximately half the wood of a closed chamber oven, but will not have the extended cooking times that the closed chamber has.
The closed chamber oven will use twice the wood of the rocket-fed oven but can have longer cooking times because more heat is pushed into the walls from the internal fire.
The differing factor between the two Cob Oven types is that the closed chamber oven can harness the radiant heat from the fire. In contrast, the rocket-fed oven relies on thermal heat being introduced outside the oven chamber, usually from below.
The available heat decides the cooking times of all ovens. The section above touches on why the two designs differ and how this difference allows for cooking styles to be fine-tuned in each.
The closed chamber oven has a slow temperature decline, allowing planned cooking events to take place consecutively.
This means the depreciating temperatures in the oven can be matched to the heat requirements of a food item. The temperature is continuously declining once the fire is removed.
There is no cooking time limit to the rocket-fed oven. You can cook food as long as you are prepared to stay with the fire and tend it.
Once the rocket stove flame has been shut down, the oven then converts to a version of the closed chamber oven and can be used to dehydrate food as the thermal heat bank dissipates into the chamber.
This is assuming the oven has operated long enough for the walls to absorb plenty of heat before the rocket section is shut down.
The rocket-fed oven allows for a temperature range to be held indefinitely simply by adjusting the burning fuel amount.
The rocket-fed oven relies primarily on the fire burning while the food is cooking. By adjusting the flames, the temps can be either raised or lowered. It is an adjustable cooking method.
The closed chamber oven is very different in temp range.
Because the fire is lit internally and no cooking takes place while this happens, the temperature falls in a linear fashion when the fire is finally removed and cooking begins.
The chef has to gauge the cooking of any particular food with the temperature as it drops through the temperature range required for that food.
The speed of the linear temperature fall is decided by the effort to make the cob oven walls and base. A good design makes for a far slower temperature drop over time, allowing extended cooking times.
This also can be measured in wood used / food cooked.
It is reason enough to make your oven as thermally efficient as can be done, using the materials available to you.
What is the best Cob mixture or recipe to make an Oven?
An effective base cob recipe is equal parts clay and straw and two times as much sand. This can vary considerably if the clay has soil particles, so the sand should be adjusted down slightly if the clay is gritty.
|base cob mix
|Thermally enhanced mix
|Thermal termite mix
The termite mix has a lot of natural fire resistance built in and resists cracking better than many clays.
Some oven builders have trouble with the standard clay component because it can be challenging to gauge the clay behavior when not blended with the rest of the cob ingredients. We suggest trials of several blends to see what is sound when it is dry. It should not crumble at all.
To read more of why adding scoria is so beneficial, this post titled “can scoria be used in a cob mix?” should help you decide on your mix recipe.
There are many aspects to consider when in the design phase. If every angle is covered and decided upon before construction commences, you are well on the way to having a fantastic piece of equipment that will last for decades.
If you are interested, we have quite a few posts on cob oven design components on site, and they can be found all in one place by using the search icon at the top of the page and entering “cob oven.”
Article by Tim Blanch for TheTropicalHomestead.com. He is a qualified Permaculture designer.