foundation depth confusion

[silvfox]

Re: foundation depth confusion

In cold climates with significant frost depth the ground can heave several inches especialy if you go into winter with saturated soil. Your floating slab will raise but will be straight up if site prep was good and slab reinforced. Your oven will never feel any stress. If you are against another structure such as the back of your house you have more to worry about as the temperature until slab and thus the frost heave will not be consistant. If that is your situation a foundation below the frost line becomes important. The slab should be pinned to existing structure and pinned to foundation with rebar. The frost will not be able to pull the foundation up from four feet of ground. Again, post footings will save you much excavating and concrete.
A post hole digger will get the job done if you don't hit rock. Don't forget to call diggers hot line to mark utilities.

[ronwass]

Re: foundation depth confusion

OK, my respected forum-mate from Minnesota, I now see what exactly I don't understand about the answers I am getting.

You say that, "the frost will not be able to pull the foundation up from four feet of ground" I assume you mean the footings. The foundation is the slab on grade.

But HOW does the footing prevent the slab on grade from frost moving. Footings aren't anchorages like for a suspension bridge. They are meant to AVOID frost completely because the ground they are on is not affected by frost. The slab on grade on top of footings will push up and if it is tied strongly enough to the footing will either pull the footing up with it, or crack in the middle because the edges are held down by the footing.

This isn't a problem with a house because the whole structure is built directly on the footings. The slab in the basement of a house floats between the footings. The Pompeii design has the structure directly on the slab NOT the footings, so would be affected.

PS: I totally get what you are saying about having just a slab which will rise and fall fairly levelly, which won't be much of a problem. My conjecture is that footings don't prevent this, and may make it worse. The only SLIGHT improvement to building on a slab would be to build ONLY on footings, no slab.

[johne]

Re: foundation depth confusion

This is what I would do, I live in PA not too far from you. Install 4" stone try to put corrugated drain pipe a foot in from edge of stone and allow to slope away from slab, maybe do this in two places if you can. Install plastic and re bar then pour 5" or more concrete. In my opinion having a slab pinned into footings will still crack as slab will want to move and footings will try to hold it and cause cracking. A single slab will move some but if you can keep it as dry as possible underneath it will be minimal.

[Faith In Virginia]

Re: foundation depth confusion

Ronwass, From what i have seen I think you are missing what a frost heave is and how that affects footers foundations and the like.

Frost heave and frost damage is caused by freezing moisture/water in the soil. As water freezes it expands. If it does not have the room to expand then it takes the path of least resistance which is usually up.

So the frost line is determined by the depth at which the ground will freeze solid.

So by placing your footers below the frost line you are getting below the depth in which moisture in the soil will freeze and expand... so no lifting.

Now with a continuous footer and a foundation wall on top you are preventing any moisture from getting inside of this structure. so the footers are below the frost line and if you back filled the center with soil ...it should be dry because of the outer walls...so if the soil in that area does freeze there is no moisture to expand and push things.

If your concerned about frost heaving you could always do a mono-slab/monolithic slab this is where the weight of the load is distributed throughout the entire slab and the outer perimeter drops below the frost line (but not a big as wide or thick as a footer.) Google (monolithic slab) the edges that drop low also prevent moisture from getting under the slab portion thereby preventing any frost heaving.

hope that helps

[ronwass]

Re: foundation depth confusion

I totally understand the first four paragraphs of your post, Faith.

Fifth paragraph seems to imply that there is only moisture movement horizontally not vertically. Moisture can certainly come up from below the slab to the middle and freeze, can't it? It's not that there is no moisture below that depth, it is that it doesn't freeze at that depth. Moisture does go up and down, especially with temperature changes.

Googled monolithic slab. Seems that that only works in the Northeast if there is ambient heat coming down from the structure from a house. Wouldn't apply for a WFO.

At the risk of sounding like a complete dope, I'm posting two more pictures and asking the question as simply as I can make it:

In the Pompeii oven design, where the legs of the stand are 4 inches in from the edges of the slab,

How does A (footing below frost line) prevent B (soil frost movement on floating slab on grade)?

My answer is: It doesn't.

In the other design, where the legs of the stand are directly built up from the footings, and which is closer to a house structure, and there is no slab, my answer is,

Slab B doesn't exist so nothing to prevent.

Refutations?

Maybe I should have called this thread, Oven stand foundation, alternate possibly better design.

[Faith In Virginia]

Re: foundation depth confusion

One more try. you are correct there will still be some moisture in the center under the slab but not enough to cause heaves. (that is unless you built it on top of a spring)

There is nothing wrong with your #2 picture. There are many ovens here built on piers. Good idea...have at it. Just be sure that the pier footers are large enough to support the total weight of the oven and are below the frost line. If your going to build your piers out of block I would use two block per level then core fill them ( be sure to rotate every level) That will be steady as a rock and last a long time.

Have fun with your build!!!
Faith

[Derkp]

Re: foundation depth confusion

What about earthquakes? Just kidding. Our oven is on 6" slab on 6-8 inches of 1-1/2 gravel, block stand with 8" slab on top with oven and structure on top of that. We have used ours for about 3 + years and no movement, up down or sideways, until the earthquake comes. Mesh, rebar and core filling throughout. Do what is adequate for your area with a little overbuilding just to be safe.

That's my 3 1/2 cents worth.

Good luck !

[ronwass]

Re: foundation depth confusion

Ok i was emailed another post that also agreed with me somewhat but it seems to have been deleted from he thread. Anyway thank you all so much for all the thoughtful comments. I am probably going to do a slab with some extra drainage underneath and use no footings. Thank you thank you.
Ron

[AaronTheGeek]

Re: foundation depth confusion

You probably don't need another opinion but.......If you don't want any heaving, then place 2" foam insulation under the slab and at least 3' out around the parameter. It's a little expensive at about $22 a sheet from HD, but we pour a lot of floating slabs like this in Wisconsin.

~Aaron

[Faith In Virginia]

Re: foundation depth confusion

Aaron, I'm familiar with doing slabs that way, I have lots of family in Vermont. What would you do with the overhanging insulation around a high traffic area like a WFO? Pavers?

[ronwass]

Re: foundation depth confusion

Thanks Aaron. What does the insulation do? Does it compress to prevent the slab above from rising?

Also, do you place it between the slab and the gravel layer?

[AaronTheGeek]

Re: foundation depth confusion

Aaron, I'm familiar with doing slabs that way, I have lots of family in Vermont. What would you do with the overhanging insulation around a high traffic area like a WFO? Pavers?

We first cut the insulation to fit inside the forms and place the pieces around the outside after the forms are removed. We usually dig down around the edge, and slope the it down from the bottom of the slab and just backfill over it. How much we slope it depends on the situation, but for a wfo you would only need about 6" of coverage. With a 6" slab your about there anyhow providing you backfill close to the top of the slab.

Thanks Aaron. What does the insulation do? Does it compress to prevent the slab above from rising?

The insulation keeps the ground underneath from freezing, and therefore also from heaving. They say ground will freeze at roughly a 45 degree angle inwards from the edge of the insulation, hence the need to extend it beyond the edge of the slab.

Also, do you place it between the slab and the gravel layer?

Yes, the concrete is poured directly on top of the insulation.

Hope that helps,
~Aaron

[silvfox]

Re: foundation depth confusion

AaronTheGeek is spot on regarding the frost protection of foam insulation. Just make sure to get extruded polystyrene (closed cell and water proof) and not expanded polystyrene (open cell and water permiable) i.e. white bead board. And the 2# density has better compressive resistance than the 1-1/2# board. In the Dow brand the 2# is blue and 1-1/2# is gray.

[AaronTheGeek]

Re: foundation depth confusion

AaronTheGeek is spot on regarding the frost protection of foam insulation. Just make sure to get extruded polystyrene (closed cell and water proof) and not expanded polystyrene (open cell and water permiable) i.e. white bead board. And the 2# density has better compressive resistance than the 1-1/2# board. In the Dow brand the 2# is blue and 1-1/2# is gray.

Good points silvfox. We use Owens Corning brand. The FOAMULAR 250 (25psi) is what we use most of the time. The FOAMULAR F-150 (15psi) is what HD has around here, and I don't see why it wouldn't work for a WFO.

~Aaron