Foundation under the oven

[McLane]

Quick question: We are going to build our 5x7 oven inside of our building on top of an existing concrete floor. Floor depth is variable under the entire building, anywhere from 4-7". Seeing that this oven has the potential to weigh an enormous amount, I am worried about the integrity of the floor. We have commercial construction equipment available to us, and it would not be that big of a deal to use a concrete saw to cut out the outline of the perimeter of the oven. I was thinking that we could lay a proper foundation below the footprint, so that there would be little worry about any integrity issues.

What say you folks?

[brokencookie]

A foundation is cheap insurance. I think I would rather have taken the time to make sure all of my hard work was not going to sink down and end up making lopsided pizzas due to the tilting oven.

I think the recommended thickness for a slab is 5 inches of re-inforced concrete. If you have any questions about the thickness of the slab or if the original builder actually put in rebar... build a foundation.

[asudavew]

Originally Posted by brokencookie

A foundation is cheap insurance. I think I would rather have taken the time to make sure all of my hard work was not going to sink down and end up making lopsided pizzas due to the tilting oven.

I think the recommended thickness for a slab is 5 inches of re-inforced concrete. If you have any questions about the thickness of the slab or if the original builder actually put in rebar... build a foundation.

I have to agree.
I used an existing 3.5 inch slab.
Which I think settled under the weight. The result: a small crack in my hearth slab, that aligns with a bigger crack in the very back of my dome... the crack then runs up the top of the dome.. .... then all the way to my arch.. The arch crack resulted in a brick falling out.

so .. I would pour a 5 inch slab, and maybe even some footings.

Good luck.

[gjbingham]

JC,
What slab depth do you recommend for a barrel vault oven. I'm thinking if you're going to go through cutting out the old concrete, and you're living in Pennsylvania, go down to 7 or 8 inches.
G.

[McLane]

Thanks for the advice guys.

I didn't give any great detail above, so I apologize. This oven is being constructed inside a building that was originally built as a 59,000 sq ft Ames department store. The oven will be built on top of the existing poured concrete floor. I'm a bit concerned that the floor as it sits, will not be able to withstand the weight of the oven. Perhaps it will, but I don't like to take chances in situation like this if I can prevent a future catastrophic "event" if I go heavy duty now. I just don't know if I can trust a floor that may only be 4" thick. Am I being paranoid, or should I cut concrete and put a suitable foundation under this thing?

[gjbingham]

I'd send a private message to Canuck Jim and ask him. He's done a bunch of ovens.

I just looked through The Bread Builders. I think you read it too. Not much specific information on the slab topic. Your local building department can tell you the minimum slab depth based on the freeze depths of the soils in your area. Without worrying about frost heave, four to five inches should be adequate depth. Since you're nowhere near Miami, I'd shoot for deeper.

[Xabia Jim]

You could do a test cut to see how thick the concrete is, what the reinforcing is and what it's up against (sand, rock,??).

A core drill or a concrete saw would work.

How old is the building....are the floors in good shape now or cracked? if you have 7 inches of concrete on a good foundation....isn't that a lot?

Where is Joe Engineer when we need him?

[CanuckJim]

McLane,

Everybody's on the right track here. I'd definitely do a core sample or a test cut to see just what you've got. In your location, it's likely that the substrate is sandy, so the drainage will be good. You will be constructing inside a building, so frost heave should not be an issue, but if you plan on doing this soon, be certain you do not see any ice crystals in the soil you uncover. If you do, you'll have to go deeper than what's recommended below. How deep will depend on code in your area; here it's four feet.

I'm thinking that a supermarket slab, usually done quickly and to a minimum thickness, probably is not sufficient. If you find that the test shows the slab is thin or there is no or insufficient reinforcement, I'd cut out the area where you want to put the oven in a square or rectangle (depends on oven shape, probably rectangular) about three feet wide on all four sides (this width will give you lots of wiggle room when it comes time to lay 8" block for the stand). Tap the forms and rod the mud to make sure it's well settled. Then, I'd excavate the trenches to a depth of around a foot, install 1/2" rebar, plus mesh, to sit midway in the footings you'll pour. This might be a bit of overkill, but I'm known for that. Thing is, it's insurance from the very beginning that doesn't really cost that much in the overall scheme of things. Bagged Sakrete (easy to estimate cubic footage) would work just fine for this, or you could mix your own concrete.

You really don't have to excavate the entire rectangle, just the areas where the footings will be, although you can if you want. You'll only be storing firewood on the remaining old slab. But you could skim coat the remaiining slab with a couple inches of concrete if it worries you or the remaining slab is not level.

One more thing: have a wander through the building to see if there are any stress cracks anywhere in the slab. Alarm bells if there are.

Don't forget: level, level, level.

Jim

[Mojoe]

I went 5 1/2" thick with #5 rebar and about a 4500psi mix. I only went 2" deep of compacted crushed stone on my sub base, got tired of hacking roots. Poured last September and I have not seen any cracks yet but I'm sure I will as almost all concrete cracks eventually...its the huge, gaping fissures you don't want to see.

[McLane]

Update: The poured floor averages about 5" depth, with wire mesh embedded in the middle. We are going to cut the floor and pour a footer and cover that with an 8" pad.

Thanks for the help! I'll start a new thread when we encounter the next issue.