Wood Fired Oven Building Principles
Due to the popularity of Wood Fired Ovens, there is a huge volume information out there on building them. Some of that information is good, some of it’s not so good.
Below are some basic principles to follow if you’re thinking of designing and building your own Wood Fired Oven. These are based on sound engineering principles and years of experience, if you’re planning on build a Wood Fired Oven of your own design, this is a great place to start! It’s not a step-by-step set of plans to follow, rather we are providing you with all of the fundamental rules to follow to build a quality oven. The information below can be applied to igloo style ovens as well as half-barrel shaped ovens.
You can see in the Cross Section image above that the entire body of the oven is cocooned in high temperature insulation, both underneath the oven floor and over the dome. While many of the oven plans available online focus on insulating the dome of the oven, a lot of them ignore the need to insulate underneath the floor of the oven, and suggest using a thick layer of sand or similar. The problem with this approach is that the heat in the floor of the oven will be passed straight through into your concrete hearth slab. This presents a host of problems;
- You’re heating reinforced concrete which is not designed to handle high temperatures.
- The hearth slab will be exposed to air which will absorb this heat and cool your oven.
- You’ll have increased your thermal mass in the oven floor by the weight of your hearth!
All this adds up to an oven floor that can take 4+ hours to heat up, which would be OK if not for the fact that the floor will rapidly cool due to the air absorbing all the heat out of it. This means that the top of your pizza will start to burn long before you can get the base properly cooked.
Oh, and then there’s the problem of the reinforcing steel in your concrete expanding with the heat and cracking your hearth slab, allowing water in which will rust your reinforcing and eventually cause the slab to fail…
So what’s the solution you ask?
We recommend you use Calcium Silicate (CalSil) Board to insulate between the concrete hearth and the oven floor. This material is the perfect engineering solution to the problem; high compressive strength to take the weight of the oven, easy to cut, exceptionally low thermal conductivity and is rated to over 1000 degrees C. With 50mm of CalSil board underneath your oven, the floor is well insulated and will hold its temperature for hours.
Oven Dome Insulation
To insulate the oven dome we recommend two separate layers; 50mm of Ceramic Fibre Blanket insulation followed by 50mm of Perlite Render. The Ceramic Fibre Blanket is an incredible high temperature insulation material, rated to over 1000 degrees C. With two 25mm layers of blanket over the oven dome, the temperature on the outside of the blanket is brought down to less than 100 degrees C, even if your internal temperature is over 800 degrees C.
The following 50mm of LiteFill Perlite Render brings the temperature down even further; regardless of how hot your oven is, and how long it’s been burning for, the outside of the Perlite Render will only be mildly warm to the touch.
Why not Vermiculite? Vermiculite particles are relatively large and flaky, Perlite particles are smaller and less easily crushed. Both Perlite and Vermiculite are usually used in hydroponics as they hold water, which is not good for making concrete. Because they absorb water, you’ll have to use quite a lot of water to wet out the mixture, which dilutes the strength of the cured mixture. This is why you’ll find a lot of people complaining that their ‘Vermicrete’ is weak and crumbly, even after a week of curing. A local company here in Victoria make a product called ‘LiteFill Perlite’; the Perlite is treated, each particle is coated to prevent it from absorbing water. This makes it terrible for hydroponics, but brilliant for making lightweight, insulating concrete!
Even using the LiteFill Perlite, this render layer is porous due to small voids in the structure of the render itself. You’ll need to apply a waterproof layer over this render, however you should wait until the oven is fully cured before you do so. Acrylic texture coatings work well, as they are almost 100% waterproof, and they usually have a latex content which makes them slightly elastic, so it will bridge small cracks.
What is Thermal Mass I hear you ask? In simple terms, it is dense material used to store heat. The more Thermal Mass you have in your oven, the more energy it can store as heat. This is an important principle to understand for Wood Fired Ovens, because you’re not cooking from the heat of the fire. In a brick oven, you’re actually cooking using the residual heat that is stored in the oven’s thermal mass. To put it another way, once the thermal mass in the oven has heated up you can let the fire go out, shovel out every last coal, and it’ll still be roasting hot inside! That’s your Thermal Mass at work.
With Thermal Mass the more you have, the longer your oven will take to reach a certain temperature, and will need more timber to burn. However, once hot, and oven with more thermal mass will stay hotter for longer. Think of Thermal Mass as a battery – the bigger the battery, the longer it takes to charge, but it’s storing more energy than a smaller battery.
There are two primary materials that we recommend to use for the Thermal Mass in your oven; fire brick or refractory castable. So why these materials? Why not pressed solid bricks, or cobb?
The Problem with Pressed Solid Bricks
We get asked regularly why it’s necessary to use firebricks to build a wood fired oven, why can’t you use standard pressed solid bricks instead? Pressed solid bricks are fired to over 1200 degrees C when they’re manufactured, so you CAN take them back up to high temperatures again – what they can’t handle is Thermal Shock. Thermal Shock is what materials experience when they go through a rapid change in temperature. In a Wood Fired Oven, the internal surface temperature of your dome and floor can go from ambient (let’s say 25 degrees C) to 500 degrees C and higher in less than an hour! That’s a very quick change in temperature. This Thermal Shock causes microscopic fractures in pressed solid bricks, and these cracks will grow each time the oven is fired, until eventually the bricks start to spall and fall apart. This isn’t an overnight process, it takes several years depending on how often the oven is fired, but it will happen eventually.
That being said, if you simply cannot afford the additional cost of buying firebricks, then we say go for it. Build your oven with pressed solids, the oldest ones you can find preferably (they were made with higher quality clay back in the early to mid 1900’s). This oven will eventually fall apart, but you’ll get a whole lot of delicious food and fun out of it in the meantime!
The Issues with Cobb for Wood Fired Ovens
Cobb is a mixture of clay, sand, water and a natural fibre such as straw, which is hugely popular in sustainable building and is an incredible material. Please don’t get me wrong, I think Cobb is an excellent material – we are going to build an internal rocket-stove with cobb for the Thermal Mass, but using it for a Wood Fired Oven has some issues.
Firstly, cobb/adobe clay is not waterproof, even if you fire your oven for weeks (you just can’t get to the temperatures needed to turn the clay into a ceramic in that environment). Most people build their ovens outdoors, and cover the oven with a layer of render for waterproofing and aesthetics. It doesn’t matter how carefully you apply this render, you will get some hairline cracks in it over time which will let moisture in. If enough water gets in, the cobb will soften and might start to degrade.
Second, it’s quite difficult to find the right clay – you need a clay with a high alumina content to resist the thermal shock. We have had customers build cobb ovens and even after carefully curing the oven, they’ve had large cracks appear on the first big firing.
Just to be clear, while these are issues it doesn’t mean they can’t be overcome! There are ways of making your oven completely waterproof, such as building a roof over it, and there are people out there who specialise in cobb and could point you in the right direction for the clay you need. As an engineer I have an inherently conservative approach – I want these ovens to be standing in 30 years time, not falling apart after a few years and having to be rebuilt.
You’ll notice in the Cross Section view that directly above the Calcium Silicate board layer there is a 25mm layer called ‘Sub-Floor Refractory Castable’. This is a very dense material, almost identical to the fire bricks, which we use to increase the thermal mass in the oven floor. Without this layer you would only have 50mm of dense fire brick tile as thermal mass for your floor – this is just enough for an oven with an ID of 100cm, but you’re much better off having more thermal mass than less. The amount of thermal mass in your oven floor depends on two main factors: the size of your oven, and whether it’s for domestic or commercial use. The larger your oven, the more thermal mass you’ll want in your floor. For commercial ovens, having plenty of thermal mass in the floor is critical.
If you’re building to your own design, you could use our 75mm thick straight firebricks for the floor surface, rather than the firebrick tiles. This removes the need for the sub-floor castable layer. The only drawback to this is that you’ll have a lot more joints in your floor, which gives you more edges to catch your pizza peel on! Also, every joint represents a small amount of thermal resistance – the less joints you have in your floor the easier it is for heat to be conducted through it.
A common question for the floor is whether to build the walls on top of the floor, or around it. The benefit to building the walls of the oven around the floor bricks is that you can remove all of the floor bricks in the future, if they need replacing. If you intend to use the oven for commercial purposes, you may need to replace the tiles after a few years. However, if your oven is for domestic use it’s unlikely you’ll ever need to replace the floor bricks.
Oven Dome – Fire Bricks
If you’re building your own pizza oven, the most common method for building the dome is to use fire bricks, held together with a refractory mortar mixture. There are a few key points to note if you plan to follow this method.
- Keep the joints on the inside face of the oven as tight as you possibly can – brick on brick if possible. You can have a large wedge of mortar showing on the outside face of the brickwork, but the joints on the inside face need to be as tight as possible.
- The joints mentioned above need to be filled with a refractory mortar mixture; providing the internal face of the joint is tight (less than 5mm). If the joint showing on the inside face of the oven is greater than 5mm, use refractory castable to fill the joint instead.
- Soak the fire bricks in water before laying them; fire bricks are very porous, and if you try to lay them dry the mortar will dehydrate as soon as it touches the brick. This makes it virtually impossible to achieve tight brick joints. It also makes the joints very weak; the mortar debonds from the brick as it shrinks from the rapid dehydration.
- There are two main ‘types’ of fire brick – Insulating fire bricks and Dense fire bricks. Insulating fire bricks are easy to pick; they’re much lighter than dense fire bricks and are commonly an off-white colour. These are NOT the bricks to build your oven dome with, as they won’t store any significant amount of heat – they’re designed to insulate!
- Dense fire bricks are classified by their Alumina content, as the Alumina content increases, so does the maximum temperature that the brick can handle. However, they also become more conductive, and less tolerant to thermal shock. The optimal Alumina content for fire bricks in brick ovens is between 35% and 45%, these bricks are rated to over 1300 degrees C and are highly resistant to thermal shock. They’re also affordable!
- You can use straight bricks or arch bricks to build the dome – the benefit of using arch bricks is the gap between each course of bricks is minimal. This means you won’t need wedges to hold the bricks on the right angle after they’ve been laid, as that small wedge of mortar won’t sag under the weight of the brick.
- To keep the brick joints tight, you’ll need to cut and shape the bricks to suit each row. For the first few courses of the dome you can use half bricks, but once you get beyond those courses you will notice the vertical joints opening up at the base, forming an open triangle of mortar. To fix this, you will need to taper the sides of the brick to get rid of this gap. This is where the real work comes in! Cutting the bricks into halves is relatively easy, tapering the sides of the bricks is somewhat harder. The inside edge of the brick is the only part that touches its neighbour, so you can use an angle grinder to taper the sides of each half brick as needed.
- When you get to the upper sections of the dome, you’ll find that using half bricks makes the curve of the row too angular, to ‘chunky’, and you start getting significant open sections in the horizontal joints. To avoid this, you’ll need to start cutting your bricks much smaller, while still keeping the sides tapered. There are plenty of great examples to follow on the Forno Bravo Forum.
- For the dome keystone, you have two options: either you delicately craft the perfect keystone out of firebrick (if you feel a burning need to do so) OR you can pour the keystone using refractory castable. Castable is basically high temp concrete, and sets just as hard as the brick itself. Because you’re pouring it as a wet mixture, it will fill the void completely, making it the perfect keystone.
Oven Dome - Castable
If you don’t want to use fire bricks to build your dome, your other option is to use refractory castable. Essentially, refractory castable is high temperature concrete, and once set it has very similar material properties to fire brick. The points below describe the method to use;
- You can make a mould for your dome using a few methods, but the easiest is to use sand. Basically, you want to make a mound of sand in the exact shape of the inside of your oven dome. To get the curve just right, draw the curve of your dome onto a piece of timber, and cut it out with a jigsaw. Use this template to shape the mound of sand; take your time with this as it’ll be permanent!
Tip – don’t build the entire mound out of sand, use polystyrene boxes or similar to make the bulk of the dome, and fill the remainder with damp brickies sand, or you’ll need lot of sand.
- Once you are happy with the shape, you need to cover the sand mound with a membrane to prevent the sand sticking to the wet castable. Clingwrap works very well (avoid creases as much as possible) and wet newspaper will also work.
- To get the best possible results from refractory castable, treat it in a similar way to concrete. To minimise cracking in a large cast part like the dome of an oven the castable needs to be reinforced, but ordinary steel reinforcing can’t be used for a number or reasons. Instead we recommend you mix stainless steel fibres through the dry castable, using a 4% fibres/castable ratio (by weight). These stainless steel ‘needles’ will spread through the mixture and act as reinforcing, increasing the tensile and shear strength of the cured castable.
- You would be wise to use a cement mixer to mix the castable, particularly since you’ll be needing quite a bit of it. If you don’t have one, we suggest you hire one for the afternoon of the pour. Add enough water to make a very thick, solid mix. You should be able to take a ball of this mixture, throw it up and catch it without it running through your fingers.
- Apply the wet castable mixture to your form, building it up to the thickness you’re after (for a domestic oven, we recommend around 60mm). You can either trowel the mixture on, or use your hands (wear gloves – those stainless fibres are sharp!). As much as possible, vibrate the castable mixture as you apply it to get the bubbles out. Once you have covered the entire form, cover the castable with a damp sheet or cloth, to stop the castable drying out too quickly (which causes drying shrinkage cracking).
- Give the castable at least 7 days to properly cure, then you can dig out the sand and boxes that made up your dome formwork. Give the dome a good clean from the inside to remove any traces of clingwrap or sneaky bits of sand that got stuck to it, then voila, your dome is complete!
Entry and Vent Arch Design
There are a few factors you should consider when designing the Entry Arch for your oven – this is the opening that penetrates the dome.
- Width: Make sure the opening is wide enough to take the trays you plan on using, but remember that the bigger your oven mouth is, the more heat you’ll lose out of it. As a rule of thumb, your Entry Arch Opening should be around half the internal diameter of the oven.
To give you an idea: for our D950 oven the Entry Arch has a width of 450mm. For the D105 oven, the Entry Arch is 500mm wide, and is the same even for our D130 oven.
- Height: The internal height of the Entry Arch should be 63% of the internal height of your oven chamber. The ‘63%’ figure was determined through a study of ovens in Quebec, and it works. The reduction in the height of the dome as it nears the entry causes a local venturi effect, accelerating the flow of hot gasses out of the oven mouth. This acceleration in the gas flow causes a corresponding acceleration of fresh air into the oven chamber, giving your oven excellent combustion.
The Vent Arch is the brickwork at the very front of the oven; why do we need a second arch you might ask? Well, if you make the Vent Arch slightly wider than your Entry Arch, you’ll have a rebate, or ‘lip’ that an oven door can seal up against. It also gives you an area to create a flue chamber, or ‘Flue Gallery’ as we call it.
- Width: Make your Vent Arch roughly the same shape as your Entry Arch, but make it around 40 to 50mm wider. This will give you rebate mentioned earlier, which you can use to seal off the main chamber of the oven with a door.
- Height: Try to keep the height of the Vent Arch around the same height as the Entry Arch, or even slightly lower. This encourages the flue gasses and smoke to travel up your flue, rather than out of the oven mouth and into your face!
You oven needs a flue to draw the smoke and hot gasses from the chamber, and a stainless steel flue is the most popular method used here in Australia. They come in a range of sizes, with double and triple skins available for penetrating unlined and lined ceilings, bends and a range of caps to suit your area. If you’re planning to run your flue through a ceiling of any kind you should engage a qualified plumber, or risk voiding your home insurance!
While using stainless steel flue is simple, creating a flue gallery for your oven is a little tricky. The Flue gallery is an opening in Vent Arch that directs smoke into your flue. There are several things to consider when designing your flue gallery;
- Make the flue opening (throat) in the Vent Arch as wide as possible – a narrow throat will allow smoke to flow around it, out the front of the oven. If possible, make the throat the full width of your Vent arch.
- The depth of the throat should be between 80 and 160mm – if the throat is narrower than this you may find it won’t draw as hard as you would like.
- If you’re planning to use a stainless steel flue, you’ll need to funnel the throat down to the flues diameter, and come up with a way of holding the flue in place. Important - steel expands when it heats up. Here in lies a problem: You need to hold the flue tightly, so that it won’t fall over in the wind. At the same time, if you don’t allow for the expansion of the flue as it heats up, you’ll get some impressive cracks in your brickwork! To overcome this problem, create an expansion joint around the stainless steel flue, around 10mm wide, and fill the joint with Ceramic Fibre Rope (the kind you see around Wood Heater doors).