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One of the first projects we wanted to tackle after our passive solar house was finished was building an outdoor cob oven. I’d built one by the seat of my pants two decades earlier when I was living in Pennsylvania and recently heard that that oven is still turning out homebaked bread and pizza for its new owners.
So, I wanted to do it again here at my forever home with Jeremy, my forever human, to enjoy homemade cob-oven-baked goodies for ourselves, family, and friends for the rest of our days.
An ancient building material composed of clay subsoil, sand, and straw in varying proportions, cob can last hundreds and even thousands of years. This sustainable material has been used all over the globe, including for building those quaint thatched-roof cottages you see in Thomas Kinkaid paintings.
A fire- and earthquake-resistant material, cob has also been used to build modern homes, benches, garden walls, and — you guessed it — ovens.
Building a cob oven is a great DIY project because it’s relatively easy and forgiving in a non-technical way. It's also inexpensive, eco-friendly, and, once finished, fun to use (cob oven baking parties are fun, too). You can also get creative and customize the shape and size of your oven to suit your needs, preferences, and imagination. One thing's for sure, as you'll see from our photos, a cob oven doesn't have to look picture-perfect to work well and last a very long time.
In this guide to building your own backyard cob oven, I’ll share the step-by-step basics we followed, along with some additional resources if you want to delve deeper into the details of making, building, and cooking with cob. I’ll also share some info on how to make your cob creation last for decades and then some to come.
An Overview: Steps to Building a Cob Oven
To give you a general idea of what you’re getting into with this project, here are the basic steps and links to jump ahead to each one below:
Choose the right location for your oven, considering convenience, safety, drainage, and shelter.
Prepare the site by clearing the area, leveling the ground, and laying down a layer of sand or gravel.
Figure out the size of your oven, including the outer perimeter of the dome and the doorway, so you know how big to build your base.
Build the base for your oven, which you can do in a variety of ways, and fill the inside with gravel, rocks, or other materials. Add an optional insulating layer of glass bottles within the base and cap off the base with sand.
Lay fire bricks on the sand bed to form the floor of the oven, and draw a circle on the floor to mark the diameter of the oven cavity.
Build an archway to form the door of the oven.
Build a dome form (a mold) on top of the circle to create that oven cavity. One way to do this is by using wet sand and packing it tightly, then covering it with plastic.
Cover the dome and doorway arch with a thermal layer composed of clay and sand approximately three to four inches thick. If desired, add a flue (chimney) to your oven.
Add an insulating cob layer about six inches thick to retain the heat inside the oven.
Cure and finish the cob. Remove the dome form via the doorway, build a small fire to cure the interior, let the cob walls fully dry, and then cover the insulation layer with lime plaster to protect the cob from the weather and add strength.
Cob Oven Tools and Materials List
You may find you need or want to try other tools and materials as you go, but here's a list of the basics to have on hand:
Clay subsoil — Soil that's 15–25% clay is great. We used leftover soil from the excavation of our foundation
Sand — Course sand with varying particle sizes is best for cob-making. Concrete sand is a good option if you'll need to buy it, while masonry sand is a bit too fine.
Straw — You'll want a straw that's still pliable, not too dry and brittle. We used pine straw because it's plentiful around here and free. Brown fallen needles that are still fresh enough to have some bend to them are great for cob.
A level
Gravel for beneath the oven base
Base materials, such as stones, bricks, or blocks, and mortar if applicable — We used ICF block leftovers from our house construction
Mortar-mixing tools and a trowel
Base fill material — Depending on how you decide to build a base for your oven, this could be rock, gravel, cinder, sand, etc
Glass bottles for an optional insulation layer beneath the oven
Buckets
Water for mixing cob
Measuring tape or yardstick
A beam compass or other method of measuring and drawing a circle, like a string with a pin and chalk
Fire bricks — How many depends on the size of the oven you decide to build.
Scrap wood or other materials for making a doorway form
Mold-making materials for the dome — This could be (wet) sand, newspaper, plastic, etc., depending on how you want to make the form.
Tarp(s)
Bricks and mortar for an arched doorway
A fork or other implement to score the cob (even a stick will do)
Step 1: Choose a Location for Your Cob Oven
Unless you build your oven on a cart, it is where it will forever be once you build it. So choosing a site is worth some thought.
Here are some things to consider when choosing a permanent location for your cob oven:
Convenience and attractiveness: You want to choose a spot that’s easy to access and also fits well with your landscape—not to mention that cob ovens look really cool, so you’ll want yours to be seen.
Safety: Avoid areas that are prone to flooding, and if possible, choose a location that’s protected from strong winds. You may also need to check for any buried plumbing or electrical wiring that could interfere with your build. It’s best to surround your oven with non-flammable materials like sand, gravel, or stone in case of any sparks (although the risk of that with a cob oven is minimal).
Shelter: Choose a spot where you can protect your oven from rain and snow, as moisture can damage the cob and reduce its lifespan. If you don’t already have a shelter, you may want to build a lean-to or gazebo type of structure for the oven.
If you do want to build your cob oven in a mobile way, here's a great video of such a project.
Step 2: Prepare the Site
Depending on the location you’ve chosen, this might not take much.
Choose or create a flat and level spot for your oven base. You can use a spirit level to check the evenness of the ground.
Clear any branches or other potentially flammable items and, of course, anything that will be in the way of your oven and any structure you may build over and around it.
Step 3: Figure Out the Size of Your Cob Oven
Cob ovens can be built in a range of sizes, so you’ll need to decide what you want to fit in yours, including through the doorway. Also, think about how much you want to be able to bake at one time: one pizza or two; one loaf of bread or casserole or four; a dozen cookies or three dozen. You get the idea.
The smallest cob oven interior size that I’ve heard of has a 16” floor diameter. Any smaller, and you probably wouldn’t have good heating or a large enough cooking surface.
Though some rules of cob-making can be broken, here are some general rules of thumb for sizing cob ovens:
Use a cob oven ratio: The interior ceiling height should be around 75% of the interior oven floor’s diameter, and the door height should be about 63% of the ceiling height. For example, if you want to build an oven with a finished interior floor diameter of 36 inches, the ideal ceiling height would be 27 inches, and the door opening would be 17 inches tall. Why, you ask? These proportions help ensure the oven heats evenly and maintains its temperature effectively, which is essential for cooking and baking. Building with this ratio means the oven will require less firewood to reach baking temperatures and still release heat over a long duration. The door height ratio optimizes the balance between heat retention and proper airflow. If the door opening is too large, too much heat will escape. On the other hand, if the door opening is too small, the combustion gases won't flow properly.
Decide how big you want your cooking space to be: The interior diameter of the dome should be at least 36 inches to accommodate a 24-inch oven chamber. The oven chamber is smaller than the dome's interior diameter to create a more efficient airflow to the wood fire inside. The hot coals are pushed to the sides of the chamber, creating a space in the center for the food to be baked. This allows the heat to circulate around the food, cooking it evenly.
Add inches for cob oven walls: Factor in the thickness of cob oven walls, which can vary. The first layer is often referred to as the “thermal layer” or “dome” and is usually made with a mix of just clay and sand but no straw. A thickness of four inches is common for the thermal layer, but some people go with a thinner three-inch dome for faster heating. The outer cob layer is the insulating layer, which includes the straw and is usually a minimum of four inches thick. Six inches is even better, so at its maximum, your cob oven could be as thick as 10 inches or more.
Step 4: Build the Base
As with the size of your cob oven, you have plenty of options when it comes to how you build your base. Many cob ovens you see online are built on stone bases, some mortared and others dry-stacked.
In our case, we had leftover materials from building our home, so we used ICF block to build a square base and then filled that with a mix of concrete rubble, also leftover from construction, rocks, cinder, and then sand at the top. We added a layer of glass bottles for insulation on the sand layer you see in this photo and then covered that with sand, too.
Depending on how big (or small) an oven you’ve decided on, you’ll want to build a base that’s at least the same size, including the added thickness of the cob walls. Since our base is rectangular, we made sure to build it large enough that the diameter of the finished cob oven and the doorway would fit entirely within the outer edges.
Here are the basics of building a base for your cob oven:
Mark the area: Measure out and mark the ground with the desired diameter (or dimensions, if it’s not a circle) of your oven base.
Cut out the base footprint: With a shovel or spade, remove any grass, weeds, or rocks from the area. Dig about six inches deep to create a foundation for your oven base. Fill the hole with gravel and tamp it down firmly. This will provide drainage and stability for your oven base.
Build the base walls: Lay rocks, bricks, or blocks—whatever you’re using to build your base—on top of the gravel.
Fill in the base: Fill the center of your base with your choice of stone, rubble, cinder, etc. You can also top it off with a layer of glass bottles covered in sand for added insulation.
Cap the base with a layer of insulation: This can be the sand from the step above, or you can add a thermal cap on top like we did. We made a cob mix just as we later would for the oven’s insulating layer. Some people use perlite, a lightweight volcanic rock with excellent insulating properties. It can be mixed with cement or clay to make perlite concrete.
Smooth and level the top of the base: However you cap off your base, you'll want it to be smooth and level because you'll lay out your oven floor on it.
Step 5: Lay out the Floor of the Cob Oven
Now that your base is ready and capped off, it's time to create the cooking surface and doorway floor. You'll want a single smooth surface without a lip so you can slide your pizza peel, bread pans, skillets, or other cooking vessels right in through the doorway.
Add fire brick: Start by laying out fire bricks on the base of the oven. Make sure the bricks are level and tightly packed together.
Mark out the oven footprint: Once the fire bricks are laid out, measure and mark the footprint of the oven dome on the bricks. This should be a circular shape with a diameter that matches the interior floor measurement you figured out in Step 3 when sizing your oven. You can do this with the string and pencil (or chalk) method, use a cardboard template, a tape measure beam compass... however you want to draw your circle. Just be sure to leave enough room out front for your arched doorway, should you wish to build one (some people mold a doorway with the cob).
Step 6: Build an Arched Doorway
Every oven needs a door, right? Traditionally, cob oven doorways are arched. While some people choose to mold a doorway with the cob itself as they build the oven, others (like us) go the brick archway route.
Create a form: First, build a form for the arched oven doorway. This can be made from wood or other sturdy materials. Some people use a bucket cut in half to make an arch. Regardless of how you make a form, remember that the doorway height should be about 63% of the interior ceiling height, and the opening's width should ideally be about 50% of the dome's diameter. If you make the form from wood, you can cut two arched pieces to the same shape and size as the opening you want. Connect those pieces to form a double arch so that the depth of your form matches the length of the bricks you'll use for your arched doorway. Check out the photo of a simple arched doorway form in this "The Year of Mud" post.
Position the form: Once the form is ready, place it on top of the fire brick base at the front edge of the circle you drew to mark out the dome cavity.
Build a brick arch: And now it's time for arch-building using bricks and mortar. The bricks should be cut and arranged in such a way that they form a stable arch over the doorway form. We made our own adobe bricks, much the same way the cob for the oven is made with clay, sand, and pine straw, which we mixed in a bucket with a paint mixer and then dried in a scrapwood form. We made our archway bricks the same size as our fire brick, about 9x14".
Remove the form: Once the mortar has dried and the arch is stable, carefully remove the form from underneath the arch.
Step 7: Make the Dome Form or Mold
Now that you have a fire brick oven floor and arched doorway, it's time to create a mold for the dome over which you'll build your cob oven. Once the mold is removed, you'll have your oven cavity.
While there's no one right way to make the dome form, one of the best and most common methods is to use wet sand.
We didn't go that route. Instead, we made a very ugly and rather lumpy form using a mix of sandbags and random rags, then covered that makeshift dome with plastic, all of which were later removed via the doorway once the cob layers had cured. We went with this by-the-seat-our-pants method because we wanted to conserve our sand (leftover from construction) for making the cob. Also, we live in such a dry environment and wanted to conserve water at the time rather than wet a bunch of sand.
Using the more traditional option as our example, here are the very basic steps:
Make the mold: Create a dome of wet sand to fit the circle you've drawn on your fire brick. This dome should be built to the height you've calculated for the interior of your oven.
Cover the mold: Cover the wet sand dome with wet newspaper to prevent the first layer of your oven, the thermal layer, from binding with the sand mold. You can also use plastic to cover the mold, all of which you'll later remove through the doorway. The idea is that you want to add a layer between the mold and the first layer of cob, the thermal layer, so it doesn't stick together and you can remove the mold.
Step 8: Add a Thermal Layer
Now comes the real fun part: making mud and getting dirty, a/k/a making cob, to cover your dome mold and your arched doorway.
As far as the mix for this layer, I've seen it made both with and without straw. We went without, using just clay and sand (and water, of course). While the straw fiber acts like rebar in concrete to strengthen the material, the first layer of a cob oven, also known as the thermal layer, is designed to absorb and hold heat from the fire. This layer is typically made without straw for a couple of reasons:
1. To maximize thermal mass: The thermal layer is primarily for absorbing and storing heat. A mix of clay and sand, without the insulating straw, has a higher thermal mass, allowing it to store more heat.
2. To prevent cracking: Straw can create air pockets in the cob, which might lead to cracking in the thermal layer due to the high temperatures.
While one can go into a lot of detail when it comes to making cob, I'll give you the bare bones basics since that's what we did for our oven and the one I built two decades ago that's still standing. So, if you have good clay soil to begin with (soil that is roughly 15-25% clay content is great), you can do as we did and make cob with what you already have and not worry too much about making "the perfect mix."
For cob, the ratio of clay to sand can vary depending on the specific properties of the materials you have, but a common starting point is 1 part clay to 3 parts sand. We went with a 1:2 ratio of clay to sand because our subsoil already had a good amount of sand in it.
If you haven't already, it's a good idea to test your cob mix before using it for your oven. You can make some test blocks or rolls (aka lumps) with the cob and leave them to dry for a couple of days. Check their strength and adjust your mixture if necessary
Gather materials: Collect sand, clay subsoil, water, a tarp, and buckets. (I didn't include straw in this list because we didn't include it in our thermal layer mix.)
Sift the soil: Use a screen to sift the clay subsoil and remove rocks, pebbles, and other debris.
Measure materials: Place your dry ingredients (clay subsoil and sand) in the middle of your tarp in a pile. The ratio of sand to clay is typically 2:1, but this can vary depending on your local soil.
Dry mix the materials: Mix the dry materials together on the tarp. Grab two corners of the tarp, walk forward to the center of the mix, and fold the tarp in half.
Add water: Pile up your dry ingredients in the middle of your tarp and dig out a small crater. Add a little bit of water into the volcano crater.
Mix with your feet: Some people use other methods, like cement mixers, to mix cob, but we like to do it the more traditional, messier way: with feet. (Not to mention that it's a great excuse for a social gathering—a cob-stomping party!) Use your feet to push the outsides of the dry material into the center of the crater filled with water. Once you cover the water, start stomping on the pile.
Keep adding water as needed and mixing, then use your tarp to roll the cob into a "burrito" until you have the right consistency. The consistency you're going for is kind of like playdough. You don't want it so sticky that you can't get it off your hands but not so dry that it will crumble and won't cure right. One way to test your cob mix is to take a baseball-sized lump, make a ball, and drop it from shoulder height. If the ball flattens to less than two-thirds of its original height, it's too wet. If it cracks, it may not have either enough water or enough clay.
Build up the thermal layer in rows: Once you have a consistency you like, scoop some up and pat it into a long, round shape between 3 and 4 inches thick. Begin building up your thermal layer around your dome and arched doorway with these round "blobs." Once you've covered your form, it'll look a bit like a beehive. You can blend the rows as you build.
Add a cob doorstop: I'll stick this in here since this is when we added ours. Thinking ahead to making an oven door that won't fall in, you can build a lip or doorstop around the cob oven doorway. This can be done by adding an extra layer of cob around the doorway, creating a ridge for the door to rest against. You can also build a small ledge or doorstop outside the oven, just below the doorway, to keep the door from falling out.
Score the thermal layer and let dry: We recommend scoring this first layer of cob (we used a fork) to help the next layer (the insulating cob layer) adhere to it. Then let your thermal layer dry, which can take several days, a week, or more, depending on the weather and how much sun it gets.
Optional: Add a chimney
Sometimes cob ovens are topped with a chimney or flue, but this is completely up to you. Without one, what smoke the oven produces will flow out through the doorway, which is fine. Most cob ovens I've seen don't have a chimney at all, while some have chimneys made of cob, and others have just a hole at the top. We added a flue because we had an old one on hand, but you can certainly go without any type of chimney or vent.
Step 9: Add an Insulating Cob Layer
The second cob layer should include straw in the mix. Straw is a good insulator and helps to retain the heat within the oven. So, while straw is usually excluded from the thermal layer to maximize heat absorption and minimize cracking, it’s included as a binder in the insulation layer to add tensile strength and help hold in the heat.
Gather, sift, measure, mix: Repeat your cob-making process as you did for the thermal layer, first mixing the clay and sand with water.
Add straw: After achieving a good consistency with your sand, clay, and water mixture, add your straw of choice, and stomp and mix some more. Pine straw is a great (free) alternative. How much straw? Oh... kind of, sort of a lot... but not too much. Basically, enough that each handful of cob has some straw running through it but not so much that it's difficult to work with.
Build up the second layer in rows: As with the thermal layer, make round, brick-length shapes with the cob, this time with a thickness of four or more inches. Press them into your thermal layer as you build up this insulation layer.
Let the cob dry: As with the thermal layer, drying time will vary.
Step 10: Cure the Cob Oven and Finish
Congratulations! You've built a cob oven. Just a few more things to do...
Remove the dome form: Through the oven doorway, dig out the sand and, if you used it, the plastic from the inside of the oven (or pull out or burn out the form, depending on how you constructed it). If you used newspaper over a wet sand form, that will burn out.
Cure the interior: Heat up the oven with a small fire, gradually increasing the temperature to cure the cob and remove any remaining moisture. Want to know how hot your cob oven is? We love this little gadget. It's an inexpensive digital infrared thermometer. Point it at whatever you want to read the temperature of, and... there you go. We've tested ours by comparing it with other types of thermometers, and it's ready right about the same. It's also proven itself by how well our food bakes in the cob oven.
Add a protective finish: While it's best to build a roof over a cob oven — or build a cob oven under a roof — to protect it from rain and snow, adding a third and final layer is a good idea. We started using our oven before doing this, partly because of the wintery weather catching up with us, we'll be adding a lime plaster in the spring (and updating this post). 1. Lime Plaster: Lime plaster is a common choice. It’s made by mixing one part hydrated lime with three parts sand. Lime plaster is durable, weather-resistant, and can be applied smoothly for an aesthetically pleasing finish. Here's a really handy guide to making a lime plaster on cob.
This is our oven after applying a lime plaster without added color.
2. Earthen Plaster: Earthen plaster is another option. Similar to cob, it's typically made from clay soil, sand, a fiber material like straw, and sometimes includes a natural binder like wheat paste. Earthen plaster is breathable and has good thermal mass, but it’s not as weather-resistant as lime plaster. Learn more about making earthen plaster here.
3. Cob with High Straw Content: A layer of cob with a high straw content can also be used as a protective layer. The straw provides insulation and helps the cob to adhere to the underlying layers.
4. Fibers or Mesh for Reinforcement: Natural hair, high-quality polypropylene, or alkali-resistant glass fibers can be added to the plaster for extra strength. Mesh can also be used for further reinforcement. You can learn more about these options here: A Guide to Lime Rendering Cob Walls.
Add a Door, and Your Ready for Cob Oven Cooking
With the thermal and insulation layers of your cob oven complete and your oven cured, it’s time to add the final touch: the door. The door can be as simple as a thick piece of untreated hardwood (but pine works, too) cut to fit snugly within the arched doorway, with a handle added.
Before you cook, it's best to soak a wooden door for several hours so it’s fully saturated to prevent it from burning. Or you can insulate the door by attaching a layer of ceramic fiber blanket or other insulating material to the inside, or apply a heat-resistant, epoxy-based paint or varnish. A metal door is another option.
And now that your cob oven is complete, the culinary adventures can begin. The unique heat properties of a cob oven make it perfect for a wide range of dishes. From crispy pizzas and golden loaves of bread to succulent roasts and roasted vegetables to cakes, biscuits, and pies, the possibilities are many. And the more you cook with a single firing, the more efficient and sustainable your cob oven cooking will be. The cob oven isn’t just a cooking tool; it’s a gateway to a world of flavors, textures, and delightful culinary experiences in a very cool way.
We went with a lean-to-style gazebo kit to shelter our cob oven, with extra room for other outdoor cooking options.
More Cob and Cob Oven Resources
How to build your own cob oven — This is similar to how we built our oven with some slight variations. There are good photos at the end that help illustrate some of the steps and stages we didn't take pictures of.
Building With Cob: Soil Tests and Mixing Cob — This article shares more details about soil horizons and subsoil, the soil jar test, and the steps to making cob.
150 Best Cob Oven Ideas — Looking for a lot more inspiration and cob ovens that are a lot prettier and more slick-looking than ours? If so, check out this Pinterest page.
Building a Cob Oven FAQ
What is a cob oven, and why should I build one? A cob oven is an outdoor oven made from a mixture of clay subsoil, sand, and straw. It is eco-friendly, cost-effective, and provides a unique cooking experience. Cob ovens can last for decades and are great for baking bread, pizzas, and other dishes.
What materials do I need to build a cob oven? You will need clay subsoil, sand, straw, fire bricks, gravel, and materials for the base (such as stones, bricks, or blocks). Additional materials include a soil sifting screen, buckets, water, a tarp, and tools like a level, measuring tape, and a trowel.
How do I choose the right location for my cob oven? Select a location that is convenient, safe, and protected from the elements. Avoid areas prone to flooding or strong winds, and ensure the site has good drainage. You may also want to build a shelter to protect the oven from rain and snow.
What are the basic steps to building a cob oven? The steps include choosing a location, preparing the site, figuring out the size, building the base, laying out the floor, building an arched doorway, creating a dome form, adding a thermal layer, adding an insulating cob layer, curing the cob, and finishing with a protective layer and a door.
Why is it important to use specific proportions when building a cob oven? Proper proportions ensure the oven heats evenly and maintains its temperature effectively. The interior ceiling height should be around 75% of the oven floor’s diameter, and the door height should be about 63% of the ceiling height. This optimizes heat retention and airflow.
How do I ensure proper drainage and moisture control for my cob oven? Proper drainage and moisture control can be achieved by drilling drainage holes in the oven base, using gravel beneath the base, and ensuring the site is level. Additionally, regular maintenance and using a lime plaster or earthen plaster can protect the cob from moisture.
What are some tips for curing and maintaining a cob oven? Cure the oven by gradually heating it with small fires to remove moisture. Use a digital infrared thermometer to monitor the temperature. To maintain the oven, add a protective finish like lime plaster and build a shelter to protect it from the weather. Regularly check for cracks and repair them promptly to ensure longevity.
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