Cob oven foundation

I’m planning to build a cob oven in Montana. How deep do I need to make the concrete pad?  Does it need to be floating foundation/monolith style?

Hi Eileen, sorry this is a late reply, but if you are set on doing a concrete pad it does not need to be set deep at all. A floating pad for something as small as an oven would only need a concrete surface of 4" deep. I recommend digging down 4" as well and backfilling this with road base or gravel, then create a form with 2x4 lumber and pour your concrete pad.

Alternatively I would suggest not pouring a pad, but using larger rocks or urbanite from old cement walkways for the foundation. This works well and will save you money and labor. I still suggest doing the gravel base though. A small pad of concrete or stone, even if it heaves, will not matter much.

Are you planning a roof structure over this oven or just a water resistant finish?

Thanks, Daniel. That was what I had thought. I might do a cover. It would be best but not sure I have it in me this year 🤪

Since I wrote the initial post, I’ve started thinking about doing an earthen rocket oven instead. Do you have any experience with that?

No, I haven't done a rocket oven, only rocket mass heaters--but there are many here who will gladly help

I suggest starting at the following link if you haven't read it yet. Definitely the way to go.

https://permies.com/wiki/60370/Rocket-Powered-Oven-build-super

and here is the ebook link

https://us.permacultureprinciples.com/product/rocket-powered-oven/

Hi Eileen and Daniel- Thanks for the info. I'm also planning to build a rocket earthen oven. Mine will be more like a rocket tandoor oven, so the barrel would be set on the ground with the lid facing up and a circular brick surround would be built to hold in insulation. It's hard to describe but I'm planning to post more on that later.
For the alternative to a concrete pad, how deep would you dig? Would you put a layer of sand, then gravel, and then large rocks/rubble? How would you ensure that the surface is level? Just use cob or something to fill in the gaps and level things off?

I wouldn't dig too deep as it is such a small build that a little heave won't damage it, just make some good drainage underneath. I would dig down about a foot, fill it about 2/3 of the way with gravel, tamp it flat, then use big stones or urbanite to lift it six inches or more off the ground, then level it as you suggested with cob.

Thank you! That's very helpful.

Thanks for the advice. I was able to find some urbanite–big chunks of concrete–at a construction site near me. I fit them together roughly as best I could to cover off on the area of the oven.
I dug down about a foot and put a few bags of gravel, I'll have to get more this week. It was quick-crete all purpose gravel. The gravel chunks are smaller than I pictured. Do you think this will work if I continue filling about 2/3rds of the hole? Or is there a different type of gravel that works better?
The urbanite pieces are rather thick, so it will just be a single layer, and I planned on filling holes/gaps in with smaller pieces. And then I want to lay down a sort of level platform of cob to start building on.

Daniel, thanks again for your advice! It worked out great, I just laid down a layer of cob in the rough shape of my oven.