This is the second part of my “Building my Wood-Fired Brick Oven” post series, which is all about laying the foundation slab. The first post was about digging the hole. I’m including descriptions of the materials, and links to the exact products (or best matches if discontinued) which those in the UK can use to do what I did without too much guesswork. I’m not one for uncertainty.
Given that what I was about to build weighs in the region of 5,000 lb, and took a couple of years to complete (in my case) running a small business and bringing up young kids left me 2-3 hours some weekends, I thought pretty long and hard about the project before a started to lay the foundation. I wasn’t going to be able to afford the time for re-runs if it all went pear shaped.
In order to create a draining and even surface on which to lay my slab, on which in effect it can float, I used around 10 bags of pea shingle from Wickes. I raked this into place using a stiff rake. The ground in our yard can get quite damp in the Winter months, and it does go below freezing during the middle of Winter where we are – and the climate might well be important to the type of concrete you choose.
I free poured the pea shingle into my square wooden form, described in Part 1. This hard gravel isn’t going to get crushed later under the compression from the masonry that’s going to get build on top of it, and it is easy to level. I bought about 12 units of “Pea Shingle Major Bags” which cost me about £2.40 from Wickes. The closest product is from Wickes is now 10mm gravel, and now costs just £1.79. Bonus – it’s got cheaper.
I then laid a 6-8 inch deep concrete foundation slab, on another two weekends, in two layers. You should think about what this layer of concrete has to do: it has to take quite an enormous weight, but it is sitting on a layer of shingle which supports it, which in turn lays on the ground.
I decided against including rebar mesh inside my concrete slab, but if I had, I would have used 6mm rebar mesh cut to size with a small cheap angle grinder (155m), however, I later used rebar mesh to support a concrete table on which I built the oven – I’ll cover that later. Because my slab won’t encounter temperatures significantly below freezing for very long periods, I didn’t think reinforced steel bar (“rebar”) mesh was necessary. I might think different if I was in Canada, New England or lived further North in Europe.
Another consideration was whether to lay polythene under the concrete foundation as a vapor barrier. I decided against this, as the shingle would stop water from my slab being sucked into the soil – so it would cure well in any case. As I planned only to use the slab as a foundation for the oven table, and to store wood – a vapor barrier would be overkill.
The first couple of inches of concrete slab was from around 8 x 25kg bags of Blue Circle Slablayer (20kg bags, £5.49 per bag), which is normally used to provide a basis for laying paving stones. Technically, this was an error – though it is stronger than top soil.
My further layer was concrete, made from premix dry multipurpose concrete from Wickes. I think around 12 bags sufficed in my case. Use an online calculator to calculate how much you need, and think about getting a delivery – I had to replace the suspension on my car after overusing it to transport building materials! I mixed my concrete up in a damaged Chad Valley Blue Sand and Water Pit, basically a paddling pool. Later in the project, when I was mixing up my own concrete I used a 1.5m square MDF sheet which was waste from a building site (with permission), this proved very durable and decent to mix on – and much easier than the paddling pool! I used a value shovel for most hand-mixing, but switched later in the project when the shovel’s weight (of caked on concrete) grew too large.
Wickes don’t seem to sell their own brand multipurpose concrete anymore, probably a good thing as it was packaged in very low quality paper bags which ripped when you handled them. The current equivalent is probably Hanson Instant Concrete Maxi 20kg. These currently cost £5.99, and you want more not less.
Each time I laid a layer, I let the concrete cure underneath a well pegged out tarpaulin. These are up to £10 for a relatively large tarp (3.6 x 5.5m) from Amazon. Don’t splurge and get a good quality tarp, you will probably badly ruin your tarp during in your build. A major challenge was transport of all these materials… my advice is to calculate what you need, add 25% – then book a massive delivery to your house.
Half the challenge was – in my opinion – calculating the mass of each substance needed – and scheduling a delivery, and then failing to control my enthusiasm for the project by taking the car to Wickes to get stuff early so I could get ahead in what little time I had. As a consequence the suspension on the car was replaced at a later service. Your standard family car will not like a 500kg addition to the boot – which can produce interesting steering effects.
If you happen to live in the Home Counties, AWBS Landscaping Oxford supplied the hydrated lime used later in the project (£11.08), which was one of the most difficult to obtain materials.
Later in the project, I opted to mix my own concrete. This was partly because of cost, but mostly so I could determine the mixture of the components, and therefore control the strength. Finally, once you are pouring the slab – it is time to order hollow concrete block and get mortar components or premix.
In Part 3, I will write about laying the concrete block on which I poured the concrete table.