This is the third part of my “Building my Wood-Fired Brick Oven” post series, which is all about laying the blockwork structure on which the “table” will be supported. It is on this concrete table you will build the oven hearth, the bricks on which you will cook. The first post was about digging the hole for the foundation, and the second post was about
laying the foundation slab.
A wood-fired oven is usually built on a fairly sturdy support. I think the most conveient way to do this is to create a 3-sided structure out of concrete blocks, and to either lay a piece of made to measure concrete on top, or pour a concrete table into formwork on top. By concrete block I mean aggregate concrete blocks. The blocks I used weigh around 50lbs each, so you need orange rubber builders gloves to avoid shredding your hands.
When laying block there are two main ways to go: you can either lay them like brick, using mortar in between them, or you can lay them on top of each other – and pour concrete into the holes to form concrete cores. The latter method can be made stronger by inserting lengths of rebar prior to pouring in the concrete. Remember – concrete sets by a chemical reaction, not the same as drying – so it doesn’t matter some concrete will never see the light of day. In my project I used mortar, and learned to lay block by watching youtube videos, and used spirit levels. I wouldn’t recommened doing that, as if you are laying block for the first time, your mortar thicknesses will vary by more than the inter-block variation, so you will introduce error. Better slap some mortar down where you are building, and lay the blocks in place – then mix and pour a bunch of concrete into the cores – quicker, but more expensive for you and the environment. Before I started, I tried to get an idea of how it would look by laying the bricks out. About one if four blocks is going to be a “splitter” with readymade blocks with asplitting line built in. You can see one on the right in the picture at the top of this page. To split block you need a 3-4lb lump hammer and a brick bolster, and you tap it hard vertically on either side of the readymade split until it breaks. Just make sure to do it on a hard surface, like the foundation. I split most of mine on an old sleeper. You can see the nearly finished structure on the right. About 6 more blocks to lay. The beauty of the 3 sided structure, is that your wood-storage structure is immediately below the oven. The fully finished structure is pictured below, ignore the steel mesh I was just storing them there. But they are cut to size for the next part, which is to reinforce a concrete plinth which will be poured onto formwork above the concrete block support structure.
I did not take photographs of the structure of the conrete plinth, but I will write down what I did. Firstly I screwed battening onto the inside of my structure, just below the level of the top of the blocks, and dropped a cut-to-size piece of MDF, filling in the gaps with offcuts and this and that. This provides the surface onto which the concrete will be poured. Then I created a frame out of 6×1 inch reclaimed planks, which I attached to the outside of the structure, effectively forming a lip beyond which the concrete could not leak down the sides. I did two main pours, firstly a 2 inch concrete pour to form a base structure which would hold a much heavier layer. This contains a piece of steel mesh. A few days later I lay three lintles across the structure, one at the front, one in the middle and one at the back. Between the lintels I lay pieces of steel mesh, and a further steel mesh layers across the top. I used a strong concrete mix for this “table” because it would have a big job to do later on. Holding up a dome made of hundreds of half-bricks. I used C35Pmix, being a mix of 1 part portland cement, 1 gravel and 2 of sharp sand. I used Wickes Sharp Sand (£2.30 a bag) and Mastercrete Original Cement (£6/bag) both of which weigh 25kg per bag. The table weighs about 800kg and contains upto 12 bags of aggregate, 12 bags of sand and 6 bags of concrete. I used reinforced lintels which is further over-engineering, just make sure you get the size, as you don’t want to be (attempting) to cut these. Once poured, I laid a tarp over my structure, and wrapped it around with bungee rope. Once set – it is probably true that you could survive in the structure if a tank was dropped on it – but then – that gives comfort because you’ll be building a bloody great fire in a structure on it, and it wouldn’t be very nice if it collapsed. For better or worse, these things will survive all of us.