Concrete Construction Principles
The first step in the construction of any wood fired oven is to build a stand to support it. There are a variety of ways to achieve this, but the easiest and most economical method is to use a combination of reinforced concrete and bricks or concrete blocks. This is the method we will be covering below.
You should check with your local council for any restrictions on the location of your wood fired oven. Some councils consider wood fired ovens as barbeques and don’t place any restrictions on them, but you would be wise to contact them yourself to check the requirements for your area.
Check the ground conditions at your preferred site, you need solid footings to build your oven on; the total weight including the oven, stand and foundations can quite easily reach 4 tons! Avoid areas with poor drainage or sandy soil (there are of course ways around these problems, but avoid this if you can).
Planning & Dimensions
The most important step is to plan your build well! The old saying ‘fail to plan, plan to fail’ is apt in this situation – you need to decide the style, shape and size of the oven you’re going to build before you start building your stand.
If you’re thinking of building an igloo style oven, have a look at the dimensions of our various PreCut Oven Kits. Even if you decide to build to your own design, the layout diagrams will still give you a good indication of the stand size you will need. For a custom oven size, use the tips below.
For a brick oven, the wall thickness is usually as follows;
Half Brick: 15mm
Ceramic Fibre Blanket Insulation: 50mm
Perlite Insulating Render: 50mm
Total Wall Thickness: 15mm (430mm for both walls combined))
As you can see above, the walls of a brick oven are quite thick! You need to allow for this when you design the hearth, so that your stand is big enough to take your oven.
As a general guide for an igloo style oven use the dimensions below;
Stand Width: Oven ID + 430mm (walls) + 100mm (50mm clearance to sides)
Stand Depth: Oven ID + 430mm (walls) + 350mm (allowance for vent & landing area)
Example: You plan on building an igloo oven with an internal diameter (ID) of 1000mm
Stand Width = 1000 + 430 + 100mm = 1530mm
Stand Depth = 1000 + 430mm + 350mm = 1780mm
Please note that you could build the stand slightly smaller. If you’re low on space, you could do away with the 50mm clearance to the edges, and reduce the landing area at the front. If you did this, you could bring the stand size for the oven above down to 1430mm wide by 1630mm deep. We don’t recommend this as it will make building your oven a little bit harder, but it’s up to you!
This is the size of the suspended slab – your foundation slab should be 100mm larger in depth and width so the walls are not landing right on the edges of your foundation.
To begin, excavate the layer of topsoil until you hit clay. If you have perfectly flat, solid, well drained, hard clay you can skip the next step and pour your concrete slab directly on top of this base. If however you’re like most of us and your clay base isn’t perfect, you’ll need a sub-base layer.
To ensure your oven is well supported you’ll need a layer of compacted crushed rock. With your foundation dug, spread out a 50mm thick layer of Class 4 Crushed Rock. To compact the crushed rock, wet it down slightly and pack it thoroughly. To pack it down you can use a ‘whacker plate’ that you can get from most hire shops, or alternatively you could use a heavy crowbar with a square piece of wood, about 150mm square taped to the end. Pack the crushed rock layer down well.
To support your oven stand you need a reinforced concrete foundation, which should be no less than 112mm thick. Why so thick I hear you ask? It’s all about ‘concrete cover’ – steel reinforcing requires a minimum of 40mm of concrete between it and the air/ground to protect it from corrosion. See the diagram below for a better understanding.
To prevent your foundation cracking from the stress of supporting the walls and oven above, you’ll need some fairly heavy reinforcing steel in the areas directly under the walls of the stand. We recommend two 16mm reo bars 200mm apart directly under the walls of your stand, as shown below. The bars need to be suspended 40mm above the base using plastic/concrete spacers.
The center of the slab isn’t under any direct load, however it’s still under a lot of stress and needs reinforcing. Instead of heavy bars, you can use steel reo mesh here (7mm bar diameter, 200mm mesh spacing will be sufficient).
Set up formwork to pour your slab; effectively you just need to create a wooden box to pour the concrete into. You can use plywood, framing timber, treated pine, just about timber at all! The timber just needs to be straight, because you’ll be using the edges as a ‘screed rail’, to get the wet concrete flat and even. Make your timber box, and drive stakes into the ground against it. Screw or nail the sides of this box to the stakes to set the depth of the foundation, making sure it’s level. Keep the stakes below the top edges of the formwork so they don’t get in the way when you screed it flat.
You’re ready for concrete! Take our advice – use your local mini-mix rather than mixing your own concrete. We’ve been there and done that, any money saved on materials is COMPLETELY wiped out by all the extra effort it takes to shovel cement, sand and gravel. Plus the cost of hiring a cement mixer. Did we already mention the effort?
When you make the right decision... and order your concrete from a mini-mix supplier, you will want to order the following;
25MPa Concrete - Strength of the concrete
80mm Slump - How stiff, or ‘thick’ the mix is
14mm Stone - Maximum aggregate size
Volume - Easy to calculate from your slab area and thickness
Always order at least 10% more concrete than you have calculated, to allow for any wastage and just to be on the safe side! The mini-mix company will usually take back any excess concrete at the end of the pour for no extra cost.
Vibrate the concrete
When you pour the concrete it will be a very thick mix. If you just try to poke it between the reinforcing you’ll end up with voids and air pockets throughout your slab. If you vibrate the wet concrete as its being poured, it will flow around the reinforcing almost like water, filling formwork completely. The vibration also allows all of the air bubbles in the mix to rise to the surface, condensing the mix and making a stronger slab. Vibrate the concrete using a concrete vibrator, easy to find at any good equipment hire shop. This is important, so don’t be cheap and try to get away without it!
Once you have filled the formwork with concrete and vibrated it thoroughly you need to screed the slab flat – this is the easy bit! You need a straight edged piece of metal or timber (your screed), that’s at least 100mm wider than the slab you’re screeding. Work the screed from one edge of the slab to the other, sliding it back and forth to ‘screed’ the slab nice and flat. Don’t try to do any trowelling at this point, you’ll just make a mess!
Let the concrete take its initial set. This can take anywhere from one to several hours depending on the weather conditions (hot & windy = quick, cold = slow). The surface will be firm to the touch, when it gets to this stage you can use a steel trowel on the surface to give it a better finish if you feel the need. This isn’t entirely necessary, as this slab will be built over soon enough!
Cover & Cure
Once the slab has taken its initial set, and you have floated it with a steel trowel (if you feel the urge) you need to cover your fresh concrete with a wet sheet, and then a tarp or sheet of black plastic. This prevents the concrete from dehydrating too quickly, which causes drying shrinkage cracking.
Keep the sheet damp by soaking it with a hose once a day or as often as is necessary (on a 40 degrees C day you’ll want to soak it several times). Do this for 7 days.
The walls of your oven stand could be built with normal house bricks (if you intend to do this, build it two bricks thick) but we highly recommend using concrete besser blocks. They come in several sizes but the ones to use are the Series 200 Full Block, 190mm wide, 390mm long and 190mm high.
This is an important thing to note when you design your slab – design for the blocks! If you dimension the walls of your stand to use full and half blocks, you won’t have to cut them down.
If your foundation slab is flat and smooth, you can just dry stack the blocks with no mortar in the joints. However if your foundation slab is a little uneven, you’ll need to bed the first layer of besser blocks into a layer of mortar to get them flat and level. You don’t need to worry about mortaring them together because the next step will lock them together more strongly than any mortar possibly could!
Pouring the Cores
To lock the besser blocks together we pour concrete into some of the ‘cores’ (the hollow sections in the blocks). You don’t need to fill every single core, but it’s important to fill the cores on the corners and at the ends of the wall. In straight sections, every second core will do nicely. To avoid wasting concrete on cores you don’t want to fill, block the core about 100mm down with cardboard or similar. You still want to put a little bit of concrete in the top of these cores, just to seal them off.
Before you pour any concrete, soak the cores with water. This prevents the wet concrete from dehydrating and shrinking away from the blocks, and makes it easier to pour the concrete. Pour wet concrete into the cores, filling them completely. Take a piece of 12mm or 16mm reinforcing bar and push it into the wet concrete, wiggling it side to side until it hits the bottom, then get it centred in the core. This reo bar should protrude at least 50mm above the top of the blocks, so that it can be tied into the suspended slab.
Concrete to Use in the Cores
Let’s do the math: for the stand below we need to completely fill 15 cores. The void in each brick needs almost 3L of concrete, so with a column of 4 blocks you’ll need 12L of concrete for a one full ‘core’. Multiply by 15, plus 10% for sealing the top of the empty cores and you need almost 200L of concrete!
If you have a cement mixer then you could buy cement, sand and aggregate and make your own mix which would be a fairly cost effective way to go, probably less than $120.
Basic Concrete Mix Ratio for filling Cores – Parts by Volume
- Part Cement
- Parts Sand
- Parts Aggregate (7mm, 14mm or 20mm stone)
If you have a cement mixer but would prefer to use the 20kg Premix Bags that you can buy from your local hardware store you’ll be paying a bit more. A single 20kg bag will make around 8.5L of concrete, so you would need 24 bags to make 200L. At the average price of $6.50 per bag you’ll be paying $156 if you follow this method, plus hire of a cement mixer if you don’t have one!
The other option you have is to use your local mini-mix. We suggest you order a 20MPa mix, 100mm slump, 14mm stone. The minimum order is generally 0.4m3, and you’ll probably only pay around $170 for it. With all that extra concrete you can fill all of the cores, and you know the concrete will be mixed to perfection, delivered to your door. I’ve built quite a few of these ovens now, and I’ve tried all of the methods above. I now use the local mini-mix every time I build an oven. Period. The cost savings of mixing your own concrete (bagged or otherwise) simply aren’t worth the sheer amount of hard labour you’ll have to put in!
Firstly you will need to ‘box up’ the underside of your suspended slab. You’ll need plywood (17mm formply works well, but other ply can be used providing you stick to 17mm or thicker), framing pine (90x45mm is cheap and easy to use), a hammer drill and a 6mm masonry drillbit.
Start by attaching pieces of timber to the inside face of your blockwork, 25mm down from the top of the blocks; hold the pine in position and drill straight through into the block behind it (preferably a block with the core filled, for strength). You can insert plastic plugs into the holes and use screws to attach the timber to the blocks, or you can simply use two big nails hammered in together (they squeeze into the hole and lock).
With timbers attached along each inside face, cut the plywood to fit the hole you’re trying to fill. Drop the plywood on top of these supporting timbers and screw or nail it down. Fill any gaps between the ply and the besser blocks with silicone.
If you have a wide span between your walls, you would be wise to put an additional piece of framing pine on the underside of the plywood with a leg running down to the floor, to stop the plywood bending under the weight of the wet concrete.
Suspended Slab Reinforcing
Before you put up the sides of your formwork, take the opportunity to set up the steel reinforcement for the slab. As an engineer, I like to be 100% sure that whatever I build is going to stand the test of time. For this reason, I recommend using 16mm reo bar spaced at 150mm centres, in both directions. That’s a lot of reo to be sure, but you are going to be putting a 1200kg oven on it so you want it to be good and solid!
Remember, the reinforcing needs to have 40mm of concrete ‘cover’ between it and the edge of the slab. Use plastic or concrete bar chairs to support the first layer of reo (one direction) above your plywood base. Cut the bars to length so they stop 40mm short of either end. Use the first layer to support the second layer of reo, and tie the bars together using annealed tie wire.
(Tip – buy the reinforcing from somewhere like ARC, Smorgons or OneSteel – they’ll cut the bars to length, and they stock the bar chairs and tie wire that you’ll need).
Formwork for Sides
Time to form up the sides of the slab – you’ll need four strips of plywood cut to around 200mm wide, and 90x45mm framing pine.. Start by cutting two strips of plywood to the exact length of the sides of the slab. Cut pieces of framing pine to the same length, and screw the framing pine to the back of the plywood strip, about 20mm down from the top edge. Use plenty of screws! The plywood is the forming face, while the framing pine is there to give strength and stiffness to the plywood, to stop it bowing out under the pressure of the concrete.
Take this assembly and attach it to the side of the blockwork using either the screw or nail method described earlier, so that the top edge of the plywood strip is 115mm above the edge of the block wall. Put in one screw, then check the top edge of the plywood with a spirit level. Providing your formwork is dead level, put a screw/pair of nails in every 300mm or so.