Make a pizza oven
A timely repeat of a popular Shed project
The aroma of wood smoke mingled with that of garlic, toasted cheese, tomatoes, and basil … a glass of wine and the company of good friends. What could possibly be better?
When we originally published an article on how to make your own pizza oven, Shed readers enthusiastically took up the challenge. Several years later I wrote an update of that feature sharing the tweaks I’d made to the design and building process to make it easier after learning a few lessons building many pizza ovens for others. I have used some alternative materials that became available but the overall shape and design of the actual oven have not changed, just the way it is put together.
HERE’S THE PERFECT SUMMER PROJECT — BACK BY POPULAR DEMAND — FOR SHEDDIES KEEN TO SAVOUR PIZZAS COOKED IN THEIR VERY OWN DIY OUTDOOR OVEN
We’ve had so many requests for the article that updated the original design that we decided to publish it again for all those keen pizza-loving sheddies. One recurring question we get is whether you can make the oven larger or smaller. The answer is: yes, it can be made larger, but no, it should not be made smaller. You can scale the oven up or down but the flames stay the same size. The original oven is optimal for a good, efficient flame pattern to consume all the fuel and produce minimal waste. A smaller oven would tend to choke and burn inefficiently.
A larger oven increases the number of flames, so to speak, and the fire will burn efficiently up to a point. But if it’s too big you would rarely light it and that defeats the purpose of the exercise. This oven can cook food for more than 20 people easily yet still be convenient for only one without consuming vast amounts of fuel.
Foundations
The oven rests on a foundation that has to be calculated for the weight it bears. The footings that form this are excavations in the surface soil filled with concrete. The oven weighs about two tonnes and the footprint of the blocks is approximately 1m2. That equates to 2kg per cm2. If the footings are extended 100mm out all around, you can double the footprint and reduce the bearing to 1kg per cm2. The footings need to be only 100mm deep on average soil. If you have very friable volcanic soil, then extend the footings either down or out to reduce the load even further.
The concrete is a standard mix of 8:1 (eight parts builders’ mix to one part Portland cement). The less water you use to arrive at a final mix, the stronger the concrete will be.
Once you pour the foundations and float them off, leave them for an hour or so and then lay the first course of blocks dry. Merely set them in position and then insert reinforcing rod (rebar) in the corner blocks and the end of the middle row. Make sure that the rebar is pushed well into the foundation concrete.
Support base
The support base is in the form of an ‘H’, a change from my original design. The H gives the oven extra support in the middle and also creates two wood storage areas. The dimensions of this base are 1400mm square.
This will allow for the oven dome, and sufficient room for the arch at the front. The blocks used for the base are standard 20 series measuring 390x190x190mm, and 190x190x190mm. Lay these dry and then backfill them with concrete. Although this is not the purist’s way of laying blocks, for our purposes it is both quicker and more accurate if your blocklaying skills are not perfect.
In all, five courses of blocks are laid. Fill them with concrete and any broken rubble you have. Make sure the concrete is well rodded around the rebar. Stop the concrete filling about 50mm short of the top of the blocks, as this will enable the slab to key-in when it is poured next.
Slab
Once the block base has set, it is time to box up for the slab. Standard fence palings are good for boxing, though some timber yards sell boxing grade cheaply.
Cut two pieces of 7.5mm ply to fit in the spaces between the blocks and prop them up from underneath with short lengths of timber. Then cut two palings to the exact length of the slab and two longer pieces to overlap the short ones to allow for nailing.
Strap the whole lot together with a tie-down winched up tightly. Cut wedges and jam them between the tiedown and paling if any bowing occurs.
Checking with a level, hammer up the boxing so that it is 100mm higher than the top blocks. This will produce a slab 20mm deeper than the one in my original build article and will add immensely to the mass, thereby improving heat retention.
Any gaps between the formwork and the blocks can be patched with a couple of layers of damp newspaper torn into strips. You simply want to stop concrete dribbling through. Pour in a mix
of concrete to a depth of 50mm and then lay in rebar/mesh/chicken wire or similar. Ensure that this remains in the bottom third. The object is to avoid exposed metal and rusting. This also overcomes the problem of concrete cracking when under stress. Concrete is very strong under compression but poor under expansion so the rebar compensates for this. Pour in the remaining 50mm depth of concrete. Trowel the whole lot off, checking for level. Cover with damp sacks until it is cured, over about two days.
Brick base
Cover the slab with kitchen foil and spread a sprinkling of sand over the surface (about 5mm thick). On this, lay whole bricks on their flat faces. They are not mortared; simply butted up to each other. Adjust the sand underneath each one if any discrepancy in level occurs. The bricks are held by haunching or a supporting concrete line around the edge, using a weak sand/cement mix (8:1 of sand to cement).
This combination of poured slab and bricks is the heat sink that will maintain the cooking temperature in your oven long after the fire has gone out. On top of these bricks, we are shortly going to lay the cooking hearth using smooth, 25mm thick firebricks known as ‘pizza hearth tiles’.
Dome
The dome is built up entirely of half bricks, which are the most manageable for this size of oven. Half bricks supply sufficient mass. The bricks are cut in half with a bolster and hammer. This is easier than you imagine — a matter of confidence more than skill or number of bricks cut, so do a few to begin with and then more as work progresses.
To find the circumference of the dome, mark the centre of the slab by drawing diagonals from corner to corner. Then chalk a circle with a radius of 420mm. The circle prescribes the inside of the oven. The first course of half bricks is laid around this circumference, which is broken with a gap of 530mm (see diagram) at the front for the door.
Lay the first course flat dry, then poke just cement/mud between the bricks once they are in place. The mud/mortar mix is around half or 50 per cent clay/ subsoil with sand and cement added. Aim for a sand content of approximately 20–40 per cent and 10 per cent Portland cement.
Mix it in a wheelbarrow, as a concrete mixer will simply ball up the mass, which will go round without mixing. The sticky mud holds everything
together while the mixture dries, unlike sand/cement strength. You will need about 100 mortar that is friable until it dries. The mud is also capable of flexing with the intermittent hot and cold cycles your oven will go through.
Door opening
The door opening is 530mm wide. The early French settlers in Montreal built domed mud ovens for baking bread and over a period worked out the ideal proportions.
Recent research has established that most ovens had a door arch that was 63 per cent of the internal dome height. This allowed for an efficient flow path for the flames to consume all the fuel and smoke. Therefore, if our oven is 840mm diameter and we build a perfect dome, our internal dome height will be 420mm, the radius of the cooking circle raised vertically. Calculate 63 per cent of this, which is 265mm. Thus, the arch is 265mm high, which is the radius of a circle with a diameter of 530mm. So our door width is 530mm. Place a brick on each side to mark this.
Inner arch
The inner door arch is built next, using as a form two plywood semicircles of 265mm radius with a block of 100mm scrap in between them.
Lay a few strips of newspaper over this form and lay half bricks, with cut edge facing inwards, in the shape of the arch, packing them with a little mud mix. Bond them together properly later. The newspaper will enable you to remove the form easily. It’s a good idea to make a handle on the front piece of ply for this.
Cooking hearth
The cooking hearth is the next part to go in and this is best made using 25mm thick pizza hearth tiles that measure 280x242x25mm. You will need eight of them, and with the use of a cheap angle grinder and diamond blade they can be used to make your hearth fit snugly, as shown in the diagram.
By putting the hearth in after laying the first course of dome bricks, if you crack or damage a hearth tile you can easily remove and replace it.
They are simply laid on a thin bed (5mm) of a sand/cement mix at a 10:1 ratio. The dome holds them in place. After they are laid, they are brushed
with the same mix to fill in the minute gaps. It is beneficial to have a slightly loose edge for when they expand. Cover with a layer of newspaper or plastic to keep them clean.
Start the next course of bricks at the arch end and angle it slightly to create the dome. Use a quarter template of 420mm radius to guide you in this operation. Continue these courses from alternate ends until gravity begins to take over and it is impossible to keep the bricks in place.
Support
Now you have to create some form of support to hold the bricks up until they have dried. In the original article, we used a framework of lath and covered it with sacking and sand. More recently we used a Swiss ball. A Swiss friend who built an excellent oven locally has no idea why it is so called, so any suggestions are most welcome.
On a platform of bricks and pieces of plywood (for easy removal through the door) sitting on the cooking hearth, place the ball, semi-inflate it, and leave the nozzle facing the front. Pack the gap between the ball and the bricks of the oven dome with old towels, sacking, or whatever.
Cover the exposed crown of the ball with dampened sand to create a rounded dome. When you are satisfied with the shape, carry on laying bricks, which will be supported by the sandcovered ball.
Continue until you have about a 200mm diameter hole remaining. Fill this hole with refractory concrete (see Refractories on pages 94–96). This concrete is in a dry, ready-mixed form so you simply add water. Use only enough water to dampen it (almost too dry) and ram the mix in firmly with a length of dowel or similar. If you make it sloppy, it will form a weak structure. You can bulk this out with broken brick if you like, but it is important that the mix is well tamped together for strength.
Dome
When you have rammed the dome cap, turn to the rest of the dome. Cover it with a layer of mud mortar about 50mm thick. Then over the arch bricks, lay another mix of refractory concrete, like a collar to key them all together.
When the dome is dry, it is time to remove the sand. Carefully locate the Swiss-ball nozzle and deflate the ball. Then wiggle out the brick and ply support and the sand mass should fall down. Remove the sand and brush out the oven.
After emptying the dome, mortar the joints on the face of the arch with a slurry made from refractory concrete sieved to reduce the number of big lumps in it. Now you can light a small fire — small being the operative word — to dry out your mass. It doesn’t have to be bone-dry but simply have the majority of water removed. It also allows you to see if the proportions are correct. They are correct if the smoke is circulating within the fire and then spilling out under the arch.
Outer arch and flue
The outer door arch uses whole bricks and is wider than the inner arch. This allows you to fit in the door to bake or roast, by raking out the embers and closing the oven. For the outer arch, you will need to make another plywood former of 325mm radius. Build with whole bricks up on either side for five courses and then continue with half bricks, positioning the cut edge facing upwards this time. This allows for the placement of a flue transition piece, which will have to be made from stainless steel.
The flue redirects the small amount of smoke away from your face and is not an integral part of the burning process, so the size does not matter. You could get away without a flue at all if you didn’t mind the odd puff of smoke in your face. A stainless-steel flue needs to convert from 250x100mm to 115mm diameter (the standard pot-belly flue) or whatever fits your available flue.
As a universal practical solution, surround the flue with small pieces of rubble held in place by chicken wire and it is ready for you to apply a base layer of render. This loose outer casing accommodates any expansion or contraction of the stainless steel.
A suitably sized field tile surrounding the stainless-steel flue will serve as a chimney pot.
Insulation
You need to insulate the oven before rendering it with its final coat. Cover the whole oven with cooking foil prior to adding the insulation. The best insulation for this type of structure is vermiculite, readily available from horticultural supply companies. It is bulky and usually comes in coarse, medium, and fine grades. Get the coarse grade. It looks and feels like puffed wheat made from polystyrene.
To cover the oven in this is a challenge and we have arrived at a true sheddie solution. Cut the legs off about 10 pairs of old pantyhose and fill these legs with the vermiculite. Tie a knot in the end and use these sausages of vermiculite to coil around your oven. You can hold them in place with short lengths of wire or sharpened chopsticks until they are covered. Now form a cage of chicken wire, pressing it firmly to the shape. This will be the key for your render.
Rendering
‘Rendering’ is an overcoat for your oven that needs to breathe and shed water.
Traditional lime/cement/sand render is, in my opinion, the best material for this job.
Over my many years of oven making, I have received no complaints about LOS (leaky-oven syndrome). The process involves a base layer and final topcoat layer.
Traditional plastering uses three layers: key coat, scratch coat, and final coat. But we already have a key coat in the form of the wire over our insulation, so can get away with two coats. The base layer is made up of fine or plasterer’s sand and cement in a 6:1 mix. Spread it by hand onto the wire to about 40mm thick. Try to let it dry slowly by covering it with tarpaulins or plastic sheeting so that it sweats. Smooth with a damp sponge after it has dried for an hour or two.
The next day, spray the surface to dampen it and apply a finishing coat of 4:1:1 (four parts silica sand to one part white cement to one part lime paste). Lime paste is made up of builder’s lime soaked in water for as long as possible — weeks is better than days. In this process, daily stirring disperses the small particles evenly so they do not burst out as little white spots in the render at a later stage.
Spread on the finishing coat with a sponge. Wear gloves, as this will play havoc with your delicate sheddie hands. I rinse my hands in a bucket of water with a cup of malt vinegar in it whenever I feel the lime getting to my skin.
If your final colour is not white, then you may get away with Portland cement instead of white cement (third of the price). Apply tile work or inlaid stones at this stage. The interior dome of the oven remains the same in all cases, but the exterior can vary according to taste.
The Door
We made our door from 50mm thick macrocarpa, cut to the shape of the inner arch with a 20mm overlap.
Pin a length of heat-seal fibreglass rope from a wood-stove retailer around the inner perimeter of the door and fill the space with vermiculite. Cut a sheet of galvanized-iron sheet to size with enough extra to bend under the bottom edge and nail it in position with galvanized clouts. We also screwed it to the door with screws and screw cups. Our door had a wrought-iron handle but any garden shed–type handle will look good. The door was well-oiled with tung oil.
The sill
Terracotta tiles cemented at the entrance to form a sill make a nice finishing touch, as well as providing a level plane and smooth transition for the pizzas going in and out of the oven. You could use a glazed tile if you prefer. Tell your tile supplier what you want them for, as some tiles do not take the heat so well.
Firing your oven
Allow the oven to dry for a day or two before you light a real fire in it. The first firing will remove most of the remaining moisture. As an outside brick construction, your oven is bound to take up moisture, especially in the humid regions of New Zealand. If you plan to use the oven after a long spell of inactivity, use a small firing the day before to dry things out.
Light a small fire in the middle of the oven from small manuka twigs, dry cabbage-tree leaves, bracken, or any
finely split wood. Gradually feed this with thin sticks, maintaining a hungry fire rather than one with too much fuel.
As the fire builds up, feed in more wood. Pieces under 50mm diameter are best and pine or macrocarpa make the best fuel with high calorific value, giving up their heat quicker than other woods. We are looking for quick burning rather than long, slow burning. The old ovens in the Middle Ages were, after all, fired with bundles or faggots made of furze (gorse).
Reaching an efficient temperature should take about two hours before you put the first pizza in. Push the embers to the back and sides of the oven with a ‘rooker’, a form of rake, and occasionally feed them with small sticks. The temperature of your oven should now be in the region of 400°C. The brickwork will absorb a lot of this heat and slowly release it back as usable cooking heat at around 200°C–280°C.
You may notice a slight smell of drying masonry in early firings. Any hairline cracks that develop under heat in the new outside covering can be repaired with the lime cement coating. I strongly advise getting an infra-red digital thermometer. These make control of the oven temperature child’s play and they are reasonably cheap.
Cooking pizza
The first pizza you place in the oven is usually a test one and can cook in as little as 45 seconds. Place the pizza in the oven using a wooden peel (paddle) that has been sprinkled with semolina to act like ball bearings and enable the pizza to slide off easily. But use a stainless-steel peel to turn the pizzas around and remove them from the oven, as the wooden one would burn. Occasionally, to remove melted cheese, etc., you will need to brush the floor of the oven with a brass barbecue brush. As the temperature drops, the pizzas will take a tad longer to cook.
Well … happy building. You will be joining the ranks of the truly converted and changing your lifestyle when you embark on this really enjoyable and achievable exercise.
Buon appetito …