I have a 28" frostline.. Do I need to build my base foundation to this level?

The frost line, also called frost depth or freezing depth, is most commonly the depth that the groundwater in soil is expected to freeze. The frost depth depends on the climatic conditions of an area, the heat transfer properties of the soil and adjacent materials, and on nearby heat sources. You have to have freezing weather for a while to have a deep frost line. - wikipedia.com

If you have a 28" frost line, you will need to build your base slab 28" below grade (“Grade” means “Ground Level”). If your frost line is at 28", you slab will need to be about 36" thick.

As for the need to building a base foundation… that is going to be your call. Several city / states require any load bearing structure be built on a frost line slab. Others city / states have no written law regarding frost line construction, so a standard 6" slab on 4" of gravel is fine.


Hi - First post. We’re building our 28” Cupola in Michigan, where the frost line is 42”. The previous post talks about a 28” frost line and needing to dig out and then pour a 36” foundation. It later says or you could use 4” of stone and pour a 6” slab on top of that.

My question - Which is it? I certainly want a good result, but don’t understand such differing options here. Any advice?


Welcome to the BrickWood forums, Larry!

The problem that BrickWood addresses in this post is the variety of building codes and ordinances, or lack thereof.

It took me a while to sort this out too. The goal is to have a base for your slab that won’t be affected by heaving ground due to frost. The second image, with plans for four footings sunk below frost line and then tied up into your slab, is the one you want. My advice would be to do that whether or not code requires it.

You would do the full slab below the frost line for an inhabited structure. Your base shouldn’t be considered as that by your local authority (if any). The footings will keep your base and oven stable through the coldest winter for your region.

If you end up being required to pour the entire slab to below the frost line, you are better off renting a small excavator and having concrete poured. My town exempts “recreational” structures from requirements like that.

Hope this helps clarify.

Thanks, Matt. Very helpful. So I would only need to dig four post holes say 42+4-5 inches deep for the sonotubes, as indicated in the diagram, following the diagram instructions to fill with concrete, add rebar, etc.? (There are no building code requirements in our instance.)

Thanks for any additional insights/tips you may have!


Hi Larry,

If you go with the pliers that is a great decision. Especially in cold climates. You want to go 8 inches below your frost line. It is also a good measure for a couple of inches of base rock for drainage.

That’s what I did. I live in the Chicagoland area and our frost line is about 38 inches. Because I had a big machine to dig the holes, I went 52 inches deep. I used cheap river rock in the pliers and tapped it down. From there 2 inches of rock for the base slab and 6 inches of concrete all connected with rebar. Poured it all at the same time. With rebar sticking up from the slab to except the cinder blocks.

Over kill? Absolutely! But all that material is cheap. And that’s a good solid foundation and insurance for your investment. Brickwood oven plans that I followed are awesome. Your oven can literally be a bomb shelter.

Happy building!


Thanks, Pete. Great suggestions!

I have to expose my ignorance on your mention of “pliers” - how does that fit in here?



My apologies. Pillars is the correct spelling. Darn auto spell.

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That’s it. And everything @423tommy said. Pay attention in the supplementary instructions to tying the rebar in to your slab. That’s what will make the slab itself completely anchored and totally frost proof.

(Also keep in mind that these maps were all tuned to more “normal” winters than the ones we get now. You still want to adhere to their recommendations because of occasional oddball seasons. Same theory as the 100 year flood plain.)

Thanks, Pete! Looking forward to the challenge!!

Question about sonotube position and rebar for slab blocks.

I’m building the Mattone Barile Grand in a climate with 60" frost depth. Digging footers for sonotubes approx 70" deep and placing sonotubes 8" from corners per instructions above, and tieing them into slab when poured.

Accompanying Mattone Barile Grand instructions - step 30 - say to position rebar 10" from corners of poured slab - but this will be in the same spot at the sonotubes which have already cured? Therefore, instead, put a third piece of rebar in each sonotube before it cures per step 30? Thanks!

Hi and welcome to the BrickWood forum.

Great question about sequence. I’m not sure whether you’re saying that you’ve already poured the footings into the Sonotubes, or you’re still planning. Hoping that you’re still planning.

Your Sonotubes will be level with the crushed base rock, and buried once you pour your slab. The 24 inch lengths of rebar in step 30 are going into the slab itself; they might contact the footing and that is actually a preferred outcome just in the sense that you want a “positive stop” for the rebar.

The instructions for frost-country footings are an auxiliary thing, not part of the main plan. You technically could sink the rebar into the footings, but you’d need longer lengths and it is not necessary.

I’d go with step 30 (and don’t skip the rebar covers in step 31!) as written. You need that 18 inches of rebar to anchor the concrete blocks properly, and it’s a simpler way to secure the base.

Good luck with your build, and I hope you can find time to update us periodically.

Still in planning stages . . . seeing this now in instructions - fill with gravel to height of sonotube, then proceed.

In oven instructions, I’m to dig 8" deep, then fill the void 3" - 4" with crushed gravel. Given that sonotubes are to sit at level of crushed gravel - if this isn’t easy to measure because of obstacles ourside of the hole, I can assume sonotube should stick out of ground 3" - 4". Therefore slab will be 8" - (3" or 4") = 4" or 5" thick?

4 posts were split to a new topic: If I’ve built a steel frame instead of concrete base do I need to install a pad?