How to Build a Cob Pizza Oven
Building a Cob Pizza Oven—Step by Step
My wife and I were taught how to build a cob pizza oven at a Permaculture (Permanent Agriculture) workshop. We had previously read articles about cob ovens in Grass Roots and Earth Garden magazines and were anxious to give it a go. Armed with hands-on experience we went home and built our first cob pizza oven. (In my previous article "How to Become Self-Sufficient" I mention this briefly.)
We have since built a second cob pizza oven at another property we moved to and also one for friends. You can cook not just pizzas in these ovens, but amazing roast meats, and even bake loaves of bread. If you live in the country and have an abundant supply of wood, build a cob pizza oven to cook in all year round, saving on your electricity and gas bills!
Cob Pizza Oven Big Enough for Pizzas
If you are building a cob oven in your garden, make sure you create it big enough to accommodate at least one pizza at a time. Why? Because pizza is such a popular food. And making pizzas is quick and easy. Guests can choose their own toppings.
Commercially built pizza ovens can cost up to $1500. Yet materials to build our cob pizza ovens cost us virtually nothing apart from labour, because most of the material was on hand. Your pizza oven can be as lavish or as simple as you want to make it. The choice is yours depending on your time and budget. Just be sure to make the interior of your cob oven (and the door) large enough to accommodate pizza trays and any other pots you might want to use for cooking.
I believe in sharing knowledge. So below is a step-by-step guide to build a cob pizza oven for yourself.
Definition of ‘Cob’
Cob, cobb or clom (in Wales) is a natural building material made from subsoil, water, some kind of fibrous organic material (typically straw), and sometimes lime.
Where you live will determine the contents of your subsoil. I’ll give you a test to check if your local subsoil is naturally ready to make a cob oven. If not, it can easily be modified by adding sand or clay.
- Cob is fireproof.
- Cob is resistant to seismic activity.
- Most importantly, building with cob is inexpensive.
Straw is added as an ingredient to help retain the shape and structural integrity of the shape you create. It wouldn’t matter if you’re building a cob house or a cob pizza oven, you’d still add straw. (Without straw, you’d be building a mud brick or rammed earth house.)
Lime is often added when building a house or other structure exposed to the elements. When building a cob pizza oven, don’t use lime. Just provide shelter for your oven; a much healthier alternative than cooking food near lime!
Materials and Tools to Build a Cob Pizza Oven
- Clay soil (alternatively red termite mound/nest)
- Straw – preferably barley (approx. ½ bale)
- Glass jar (vertical-sided)
- Tin can – e.g. spaghetti or soup tin
- Large sheet of plastic or tarpaulin
- Extra plastic to place over sand mould (black rubbish bags etc.) Alternatively, wet newspaper can be used.
- Water supply
- Stones or bricks etc. for base (if required)
- Fire bricks or pavers (at least 40) for floor of oven
- Coarse river sand:
1. for setting pavers or bricks for floor
2. For mixing with clay if more than 15% clay content in soil
- Brickies loam: sand mould for oven (3-4 wheelbarrows full)
- Boots and gloves (if preferred, especially if using lime)
- Chalk, string, nail
- Music – CDs, radio, drums etc. (to listen to as you work.)
- Thin sheet of metal, and tin snips
- Rivets or screws
- Jigsaw, power drill
Remember, you want your door to be big enough for pizza trays. Factor that in when getting your wood.
- Bucket, ice cream containers – for mixing
- Some clay mix (minus straw)
- Coloured ochres if wanted
- Linseed oil (approx. 1 cup)
- Horse, donkey or cow manure – fresh if possible (a few handfuls)
- Builders (slaked) lime (1 cup)
- Decorations if wanted e.g. shells, stones, mosaic tiles
Some of the above materials/tools may not be required depending on the individual situation. For instance, we always build a door and have an air vent at the top, but I have seen certain people recommend that you need neither of these. I think it’s a matter of personal preference and trial and error.
We built cob pizza ovens using crushed termite mound (these are readily available not far from our home), sand, straw, lots of water, and cow manure and linseed oil to help bind it together. It was a good day’s physical work and we were delighted with the finished product.
If you are using red termite mound it is not necessary to conduct the following Clay Soil Test. Just break up the termite mound using a sledge-hammer or pick. Then crush as finely as you can using the same tools and a shovel while adding a little water to soften and moisten.
Clay Soil Test
Test the clay content in your soil by:
- Wetting a handful of the clay soil and do the ‘sausage test”. Roll it into the shape of a sausage, and if it retains its shape, this is a good indication that the clay soil is suitable for the cob mix. If not, continue with the following:
- Conduct the ‘shake test’. Half fill a vertical-sided glass jar with the clay soil mix, then, add water until ¾ full. Allow to stand for 24 hours or until all lumps are completely dissolved, then shake the jar vigorously before allowing the mixture to settle. The contents should then settle into at least three layers: sand at the bottom, followed by silt, clay, and the water on top. * this may take about 10 minutes.
This will give you a rough idea of the percentages of the different elements. 10 – 15% clay with a high percentage of sand and a little silt is usually good.
- Make test ‘bricks’. Allow the mix to soak for at least 24 hours then shape into the size and shape of house bricks. Leave the "bricks" to dry to test for cracking and crumbling. If the bricks fall apart you may have to bring in either additional sand or additional clay and adjust the mixture to suit.
Constructing the Cob Pizza Oven
There are a number of options for building a base for your home-made cob pizza oven. Depending on your preference you can:
- Build it directly on the ground.
- Place it on an existing terrace.
- Build a base (stone, brick, cement block, cob etc) as high as required. All the cob ovens we have built have been on a base about waist height constructed from cement blocks.
- Prepare base (as required). If using my preferred method of a raised base, construct legs out of cement blocks, place a wooden pallet on top (the perfect size which allows a small shelf at the front and sides), followed by a sheet of corrugated iron, colorbond, or zincalume.
- Then place fire bricks or pavers on top of this and cover with a layer of cob mixture. It is a good idea to place a wooden frame around the pallet to hold the bricks/mixture in place.
- If constructing the base on the ground, put down a sand base and then lay bricks or pavers in the standard way (any pattern), tap in place, and level. Hold in place with cob mixture or border.
Mixing the Cob
- When the right mix is obtained (see Clay Soil Test above), or termite mound is crushed to as fine as possible, shovel into a cement mixer (if available) and add water (and sand if required) as mixing.
- If no cement mixer is available, shovel the clay mix (and sand if required) into a mound in the middle of a sheet of plastic or tarpaulin. Make a well in the centre and add water. Leave for at least 24 hours to allow the mix to become soft and pliable. Then mix with feet (good to do with a group and music accompaniment) until the mixture feels ‘plastic’ (wetter mix is preferable). Shake straw liberally over the mix and trample in, checking to see if evenly mixed. Test by making a ball in your hand. If the right consistency it shouldn’t pull apart easily.
Making a Cob Oven: 10 Steps
- Hammer a nail into the centre of where your oven will be. Cut the string between 1m to 1.05m (average) then tie one end of the string to the nail and the other to a piece of chalk. Draw a circle on the base bricks to mark the outside diameter. Walls need to be 12 – 15cm thick, so adjust the length of the string and draw a second circle marking the inner diameter.
- Using damp sand (brickies loam) build a mould for the oven. Keep sides as vertical as possible. A guide is – height is slightly more than the internal radius. Cover the mould with plastic (trimmed so just level with base).
- Build the oven by pressing handfuls of cob mix onto the base, using fingers to push layers together. Don’t pat or press down too hard or bulging will occur. The object is to knit the straw between layers to help hold them together.
- Check that the cob is an even thickness all around, except for where the doorway is to go. Mark the doorway and make this section about half the width of the wall. The door should be approx. 60% of the internal height of the oven, and wide enough to insert a pizza tray and your hands at either side. The door itself can be made before or after the oven. Make sure the inner door is metal (or it will burn) and riveted or screwed to a wooden outer door with handle.
- If leaving a vent or flue at the top of the oven you can use a jar or tin can as a mould. You also need to build or find a “plug” to seal it when needed.
- Mix the render – a mix of clay soil, manure, linseed oil and/or lime and water. It should be the consistency of thick paint. Begin to render the oven while the cob is still damp or the render will crack. If it is beginning to dry out, dampen with water before applying the render. Use your hands or a paintbrush, but I found hands better. You will need to do three or four thin layers to also reduce cracking.
- More linseed oil can be applied as a separate sealant after rendering if required.
- After about three days (when completely dry), cut out the doorway where marked (make sure to leave a lip for the door to sit against).Then remove the sand mould and plastic from inside.
- You need to make a shelter to cover and protect the cob oven from the weather or it will eventually dry out and crack from the heat or be eroded away by rain. It is up to you what type of shelter you erect. I have taken a photo showing the way we shelter one of our home-made cob pizza ovens.
- Allow oven to dry completely (about a week) before cooking for the first time. Pile wood towards the back of the oven and make a fire. Place the plug in the flue.
Enjoy Your Real Home Cooking in Your Cob Pizza Oven
Now that you have built your very own cob pizza oven, your cooking will only be limited by your imagination. Get to know your cob oven. Experiment. The possibilities are endless. As well as pizza, we bake bread, biscuits, curries, casseroles, soups, and roasts. When your cob oven cools down later in the night you can even use it to dry foods and herbs etc.
The amount of fire time (before putting food in your cob oven) equals the approximate cooking time. For instance, a pizza has a cooking time of about 15 minutes. So you let the wood burn for 15 minutes and die down to coals before inserting the pizza. 15 minutes later, remove the cooked pizza.
An average sized roast only takes around an hour to cook. Much faster than a traditional electric or gas oven. Make sure you continue to rotate the food so it doesn’t burn on the side closest to the fire/coals.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
© 2016 John Hansen