Building an Outdoor Oven

Test your skills by building an outdoor oven. This easy-to-construct, DIY clay oven fires up quickly and stays hot for days.

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by William Rubel
This easy-to-build clay oven is based on ancient technology. Not only does it heat up fast and hold its heat, it's also a beautiful architectural statement.

Test your skills by building an outdoor oven. This easy-to-construct, DIY clay oven fires up quickly and stays hot for days. Cook dinner at night and bake bread in the morning with its stored heat. This is the first of a two-part article on DIY backyard ovens. Part I discusses building an outdoor oven, and Part II will discuss how to cook with your oven.

The technology of the domed oven is ancient. In one of the world’s oldest excavated Neolithic villages, every house had a domed mud oven. That was 9,000 years ago! The technology hasn’t changed. Any refractory dome that has a hole cut into it (so you can heat the oven with an interior fire) will cook pizza and other foods when the fire is still burning. However, the ultimate power of the domed oven is that the dome functions like a battery, absorbing heat while it’s being fired. So, after the fire has died down, you can sweep out the embers and bake bread and casseroles with the stored heat radiating into the oven from the dome. When the oven is nearly cool, you can dry wild mushrooms, herbs, and fruit inside.

The oven I suggest you build is made of powdered clay — fireclay — and sand. It heats quickly; you can start baking dinner in 15 to 20 minutes while the fire is still burning inside the oven and achieve a full firing after about an hour and a half. While the shell is thin compared with brick, it’s so well-insulated under the floor and over the dome that it can be used for days on a single firing. In summer, after making a pizza dinner, if I close the door and put insulation in front of it, my oven will almost always be at least 400 degrees Fahrenheit in the morning — still hot enough for me to bake bread that afternoon. And, its temperature will still be higher than 200 degrees by the following morning.

Ovens are beautiful architectural and sculptural statements. I built my oven on a table made of the trunk and branches of a backyard plum tree. I was inspired by ovens I saw in Italy that were built on platforms made of chestnut trees. This is my aesthetic; you’ll want your oven to reflect your own aesthetic. A friend made his oven base out of pallets, setting his oven at an angle on the tabletop. A wooden base can be finished with stucco or siding, or built with dovetail joints and contrasting woods. It’s all up to you. You don’t need the structural strength of a masonry base, but if your aesthetic tends that way, then build a stone base.

Choosing a Site

Whether you build the oven I suggest or a different design, I encourage you to prioritize location. To choose your site, imagine cooking in the oven and serving your family and guests. An oven that isn’t used is just an outdoor ornament! My first ovens were too far from the kitchen — I used them half a dozen times a year. My current oven is 20 feet from the kitchen. It’s as easy to set a table by the oven as it is to set the table in my dining room, and if I forget salt or olive oil, it’s just a few steps into the kitchen to get what I need.

Building a Clay Oven

I recommend an oven 36 inches in diameter built on a sturdy wooden table, with a thick layer of insulating material between the oven floor and the wooden tabletop. The oven itself will be a dome made with a mix of 3 parts sand and 1 part fireclay that will be 2-1/2 inches thick on the sides and 4 inches thick on the top. You’ll build the dome over a pile of damp sand. You’ll then insulate it with two to four layers of ceramic insulation alternating with aluminum foil, and finish it with a coat of stucco, a wetter mix of fireclay and sand, or straw saturated with wet clay slurry and painted with whitewash. Protect it from the rain with a tarp or a simple roof, and you’ll have a beautiful oven that will last a lifetime.

1. Build a table base. The simplest raised oven is one built on a wooden table. Tables are easy and quick to build, can be made from scrap wood, and can have whichever exterior finish — siding to stucco — that suits your aesthetic. I suggest the height of the oven floor be 38 to 44 inches. This is taller than standard kitchen counter height, which will enable you to look into the oven without stooping. Taking into account the brick floor, the sand the bricks are set on, and the insulation, the legs should be 29 to 35 inches high. Make the base strong enough that you feel comfortable jumping or dancing on the tabletop. A ceramic base is structurally unnecessary, but you’ll find many handsome designs online if that’s your aesthetic choice. The optimal tabletop dimension for an oven that’s 36 inches in diameter is 4-1/2 feet.

2.  Insulate the tabletop. It’s important to stop the heat radiating from the oven floor to the base. There are multiple ways to insulate the oven floor from the tabletop. Here’s my way. First, use 1-by-6s to make a frame the dimensions of the table. Mix perlite or vermiculite with Portland cement in a 6-to-1 ratio to a depth of 4 inches, and top with a compacted, leveled layer of fine sand. An insulation layer is required even if your oven base is made of concrete or stone.

3. Lay a brick floor. Common red brick makes a reliable floor for a home oven. Lay the bricks on their broad sides on the compacted, level sand. The exact number of bricks you use will depend on the dimension of your tabletop (see SlideShow).

4. Chalk a circle. Mark a chalk circle 36 inches in diameter centered on the brick floor but set toward the back so that when the oven is complete there will be a small brick shelf extending roughly a foot in front of the oven. (See SlideShow.) This small shelf will provide a place to rest pots, pizza, or the bread peel.

5. Shape the sand dome. Cut a small stick to 16 inches. Shovel sand into the middle of the chalk circle and insert the stick. Proceed to shovel fifteen 5-gallon buckets of damp, fine sand within the chalk circle until you reach the top of the marker stick. (See SlideShow.) Think sandcastle. Use your hands as your primary building tool. Compress and shape the mound as you go. Important: The walls should be as vertical as possible up to 10 inches and never inclined more than 25 degrees. When you think you’re done, smooth the sand all around and take stock. Check for symmetry. If, upon review, the walls need to be more vertical, slap handfuls of wet sand against the sides. A short 2-by-4 may be helpful in compressing and shaping the sand pile.

6. Make a newspaper layer. After you’ve completed the sand form, cover it with newspaper and spray it with water so the newspaper lays flat. This layer will keep sand from sticking to the clay dome.

7. Sculpt the clay dome. The basic mix I use is 3-to-1 fine sand to powdered fireclay. Other mixes are possible. If you’re building when the weather is cold and damp, add 1/2 part Portland cement. The cement will make the dome set up, even when it’s too cold out for the clay to dry on its own.

Dome thickness directly influences both how quickly the oven can be brought up to temperature and how much heat the dome can store. I’ve found that a mix of 40 gallons of sand (eight 5-gallon buckets) and 13.5 gallons of powdered fireclay (a little over two and two thirds 5-gallon buckets) produces an oven that meets my needs. Don’t use less material.

I advise working in small batches. I work with 6 gallons of fine sand and 2 gallons of powdered fireclay per mix.

Whether you mix inside a wheelbarrow or on a tarp, measure out the sand first, followed by the fireclay. If you’re adding Portland cement, add it last. Fully mix the dry ingredients before adding water. The amount of water you’ll add will depend on how damp the sand is.

Before making production batches, make several small test batches. Mix 3 cups of sand with 1 cup of fireclay. Add a little water and mix. Aim for the driest mix that will hold together. Test your mix by forming the clay into a ball. Drop the ball from shoulder height. If it splats, there’s too much water; if it crumbles, not enough. If it mostly holds its shape, you’ve got it just right. When you’re sure you have the feel for it after making test batches, then proceed to the main mix.

Apply the clay mixture handful by handful over the sand form. It doesn’t matter where you start. As you add clay, forcefully strike or beat the clay with a 2-by-4 to compress and smooth the oven exterior. Beating the oven with open hands, like a drum, is also effective. Make the dome smooth and beautiful.

To test for thickness, insert thin sticks marked at 2-1/2 inches to get the thickness consistent on the sides, and use a stick marked at 4 inches for the very top of the oven. As heat rises, the extra mass on the top will translate into better oven performance. When you think you’re done, test the dome for thickness and adjust.

What can go wrong? Our instinct is always to add too much water! If your fireclay mix is too wet, it will slump — clay will flow from the top of the sand mound to the base, possibly even leaving a gap. If you get a batch too wet and haven’t used it yet, then add a ratio of 3-to-1 sand and powdered fireclay to dry it out. If you’ve used the wet batch and see it’s slumping, don’t panic. Cut away clay that has flowed to the base and put it back where it flowed from. Be patient. Repeat as needed.

Other options: You can dig your own clay. You can also buy three sacks of refractory concrete. Follow the instructions on the sacks. Return the third sack if you don’t use it. Refractory concrete is foolproof and will last a lifetime, but it’s not a green building material and is expensive.

8. Cut the oven door. Cut the door as soon as you finish forming the clay dome. When the oven’s in use, cold air will enter through the bottom of the door, and hot air will be expelled out the top. The door’s height must be 63 percent of the height of the interior dome. For a 16-inch dome, the door must be 10 inches high. Make the width sufficient to slip in the oven peel and a favorite frying pan; 14 inches is a comfortable width. Also, if you have a favorite bean pot or small stewpot, be sure it will slip through the door with its lid on. Make minor adjustments as needed.

9. Remove the sand. Remove the sand with a hoe after the dome has firmed up — which could take hours or up to one or two days, depending on weather. If you added cement to the mix, then the oven will usually set up in a matter of hours.

10. Fire the oven. At this point, you can start using your oven. You don’t have to wait until it’s insulated. Start with a very small fire that can burn all day to gently drive off the water. When you no longer see steam rising from the dome, you can start increasing the heat. Using your judgment, increase the heat until the oven is fully fired. An oven is fully fired when the hot face, the inner dome, turns from black to white, and the uninsulated exterior of your oven is hot to the touch.

The thinner the wood, the hotter the fire will burn. The drier the wood, the hotter the fire. Best practice is to dry the wood for the next firing by placing it in the oven when you sweep it clean of embers after you’ve finished cooking food.

To reduce smoke, always build the fire close to the oven mouth. After the fire is burning well, you can push it back farther into the oven. Add fuel as needed. After about an hour, you should have the oven so hot that you’ll see a cloud of flame filling the top third of the oven. After the walls turn white, you can push embers and burning wood to the side. Sweep a section of floor clean of ash with a damp cloth mop. You’ll then be ready to cook pizza and flatbreads. If you keep the fire a little tamer, then you can cook just about anything next to the fire. I often start cooking 15 to 20 minutes after starting the fire.

The oven dome is likely to get cracks in it. In my experience, they won’t be structural and therefore won’t matter. Fill the cracks with a 1-to-1 mix of sand and fireclay.

11. Insulate the dome. You can’t over-insulate. Fully fired, the temperature of the outside of the oven should only be a few degrees higher than the outside air. I recommend two to four layers of ceramic fiber insulation, with each layer covered by a sheet of aluminum foil. Ceramic fiber insulation, often used for kilns, is usually sold in 25-foot lengths in 24-inch widths and 1-inch thickness. Wear rubber gloves, and cut insulation with a sharp knife. Skimping on insulation will increase the rate at which the oven cools. With two layers of ceramic insulation, the oven will be approximately 350 degrees the morning after cooking dinner. With additional layers, you can increase that to as much as 425 degrees. An 8-to-1 layer of perlite or vermiculite concrete on the outside of the dome can reduce the amount of ceramic fiber insulation required. You can test the insulating layer as you build it by firing the oven and measuring the rate of heat loss.

12. Add the final layers. When you have the thickness of insulation that works for you, then cover the dome with a layer of aluminum foil followed by a layer of chicken wire. Finish the oven with stucco, lime plaster, a spreadable mixture of the fireclay mix used for the dome, or straw softened in a clay slip (pictured in the SlideShow). For the latter, in a wheelbarrow, mix water and powdered fireclay to the consistency of thick cream. Add clumps of straw. Stir with a hoe. After 20 to 30 minutes, the straw will have absorbed the clay water and will be soft. Cover the oven with it. Apply it with your hands and smooth it to create a satisfying form. Cutting the straw up into shorter lengths will create a smoother surface.

Finishing Touches

Paint coat. Whitewash (builders’ lime mixed with water) is a traditional paint for the outer coat and is what I use. Whitewash hardens over time. It’s water-resistant, but not waterproof. Wear gloves when you’re working with whitewash.

Oven door. You can improvise a door using a cookie sheet to close the oven when there’s still fire inside. After the embers are swept out, you can use a wooden door.

That said, the oven door is the weak link in the chain of oven insulation. I use a door made of a piece of ceramic insulation wrapped in several layers of aluminum foil.

Weather protection. If the oven will be rained on, then build it under a shed roof or cover it with a tarp when you’re expecting precipitation.

External insulation. To improve heat retention during windy or cold weather, cover the dome with a tarp after the embers have been swept out. If you find your oven is under-insulated, especially in below-freezing temperatures, you can sew a crude quilt with a tarp and home insulation to further improve performance.

William Rubel is a traditional foodways expert who writes about bread and cooks many evening meals in his outdoor oven. Thanks to Devon Pearse for testing out the instructions and to him and Kiko Denzer for reviewing the article. Find Rubel’s Bread: A Global History, the Magic of Fire and Denzer’s Build Your Own Earth Oven online. 

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