I am building a pizza oven and kitchen area in my garden. I am trying to work out how to lay a 2m*2m concrete slab.

My calculation for the weight (that the concrete slab needs to support) are

Dense bricks: 70 * 18.2kg = 1,274kg
Concrete slabs: 13 * 15kg = 195kg
Pizza oven: 1 * 50kg = 50kg

Total: 1,519kg

I did some research to find out how much concrete (with reinforcement bars) I need but it may turn out the bigger issue is what the concrete will be laying on (how much weight a specific sized concrete slab can carry?)

How do I determine this? In my case, I want to pour the concrete slab over mud, which I would have dug out to size (with a timber frame as well). I was hoping to pour a layer of gravel down but I'm not questioning if that is going to be enough.

I believe our soil is quite chalky.

How do I prepare the ground before laying the concrete slab in the UK?

  • youtu.be/40uCoIgxPC0 Mar 15, 2021 at 17:40
  • Pouring a concrete slab over mud (dirt mixed with water) strikes me as a Very Bad Idea™. As the water dries out, the dirt will settle, but probably won't settle evenly and you'll end up cracking your brand new slab.
    – FreeMan
    Mar 15, 2021 at 18:14
  • You might mean clay soil? If you mean literal mud, that's generally bad for numerous reasons, especially if your climate ever has freezing weather.
    – Ecnerwal
    Mar 15, 2021 at 19:10
  • I mean do I just put down a dpm and then some hardcore rubble on top? Mar 15, 2021 at 19:23

1 Answer 1


You definitely shouldn't pour concrete over mud. Also, I guess your local can get quite cold during the winter, thus you need to dig the soil, at least, down to the frost depth to avoid differential settlement due to heave. The soil is considered "competent" if it can support the weight above without significant settlement. You can get the information from the local building code, a engineer, or an experienced contractor. Below is the suggested details.

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  • Where I live the frost line is about 3', we don't put anywhere near that amount of base. Mar 15, 2021 at 22:08
  • I understand. It seems an overkill for such trivial matter, if you don't mid the oven to tilt in the following spring. But you must clear out the mud, and replace the excavation with clean, sound backfill.
    – r13
    Mar 15, 2021 at 22:40

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