A cob oven base should be at least the thickness of the walls but will perform far better with 1.5 times the thickness of the oven walls.
The base will perform well if it is 8 inches thick(200mm) or slightly less if thermal aggregate is added to the cob mix.
This can include fire bricks over cob or an all-cob base and is the actual internal base of the oven where food cooking takes place.
Many ovens are supported on a built structure of stone or similar, which is discussed in more detail below. For clarity, this is not considered a part of the oven for this article.
Why is a thick base better?
A thick cob base for your oven is superior because of the larger mass and, therefore, greater potential heat storage.
The cob oven operates as a thermal battery that is charged by fire. The more mass there is within the oven structure, the more heat it can potentially hold or store.
I say potentially because different battery chemistries used in vehicles and solar power systems behave very differently; different cob mixtures and thicknesses also have their characteristics.
Any thickness can be enhanced with structural additives like scoria that can significantly increase the thermal capacity of the cob mix. We used this material in our cob oven build.
We wrote about the material in “can scoria be used in a cob oven mix?” You might be interested in exploring it if it interests you.
The best cob oven mixture is one that can withstand intense heat for prolonged periods and can store that heat for later use. The ovens with thicker bases should outperform thinner ones, all things being equal.
There is a technique of thermally isolating the base of the oven from the base support structure to prevent the heat from continuously soaking away from the cob oven where you want the heat to remain.
Empty glass bottles have been used for this step. The bottles are laid into the cob base and become air pockets. We suggest having 4 inches of cob over these bottles to make up the oven base.
Thickness is not just relegated to the base, though; this post “how thick does a cob oven wall need to be?” goes into detail about the thoughts behind thermal mass and cob oven wall thickness.
What is the best Cob Oven base?
There is no best type. However, some good alternatives to cob oven base construction are fire bricks over a cob base mix, pizza stones over a cob mix, and just a simple cob floor.
The pizza stone oven bases are often suited to the smaller ovens and allow the stones to be removed with the food on them, assuming the front opening is large enough for them.
Glass bottles have also been used within the actual cob mix below fire bricks, and straight cob has also been used. Each method has its merits and its downsides. Most issues revolve around item and material availability, so every cob oven ends up as a custom design.
Is a Cob Oven better on the ground or a raised base?
A raised oven is better to access the oven opening from a reasonable height. A ground-level oven can be awkward to access but potentially be thermally superior.
The choice of oven height is mostly a personal design criterion more than anything else. A ground base oven potentially has massive thermal banking capacity after many uses.
You will understand what is being said here if you have felt the ground beneath a campfire well after the fire has been put out. The heat really lingers.
Many uses of the oven will completely dry out the soil below and can convert the natural clays within the ground into a low-quality form of terra cotta.
This will help protect the oven from rising damp from the ground and will only happen after many uses.
What materials are best for a raised Cob Cven support base?
Stacked stone laid with cob mixture as a natural cement is good for a support base, as are concrete bricks/blocks.
This creates the platform to support the oven base, as mentioned in the first section of this article. The clay component can be dug up from your location, or alternatively, the termite mound can be used instead of clay.
We used termite mounds in constructing our oven and cannot be happier with the performance.
Stone or rock base foundations are often used below the cob oven base in colder climatesto stabilize the ground in heavy snow country.
We used masonry (cinder) blocks stacked four high as legs and used a compressed fiber-cement sheet as a platform to then create the straight cob.
Our oven is heated by a rocket stove chimney that delivers super-heated air into the oven from below the oven floor. If you are unsure what this looks like, “building a wood-fired termite cob pizza oven” explains everything.
Will a raised Cob Oven lose heat faster?
A well-designed cob oven will not lose heat externally at any significant rate, either above ground or ground level.
The idea of building the cob oven is to have enough thermal mass to maintain a hot-enough cooking environment for up to a day after the fire has been removed. It should not make any difference to this ability if the design incorporates good materials.
Is a Cob Oven more expensive to build above ground?
The cost to build a cob oven on a raised platform will generally cost more than a ground based oven.
This comes down to whether fire bricks are used and the method of building the support base for the platform. The most cost-effective oven is ground-based. As noted above, the base thickness is massive in a ground oven.
The materials you use in your oven base should be considered well before starting the oven build.
We spent many hours scouring the internet, getting ideas and information to help with our final choices. To help others interested in oven design, we put a more extensive article together that goes into the design steps from the beginning. The report is “how do you design a cob oven?“
It came down to finding and using the most thermally efficient materials we could find locally and using thickness as effectively as possible within the simple design specifics we had set.
To help with more research, there are several articles on the Cob Oven topic on our site, and you can find them all in one place by using the search icon at the top of the page and entering “cob oven.” We hope this helps.
Article by Tim Blanch for TheTropicalHomestead.com. He is a qualified Permaculture designer.