Why dont I need to dig footer below frost line?

[suzie queue]

Re: Why dont I need to dig footer below frost line?

Hi Mooch, I just saw your post and I am sure you are done with your kitchen by now but here are my thoughts for someone else wondering about the same question:

I am in southeast Ohio so the weather is similar to Pennsylvania. I had a regional builder of metal buildings build my work shop. Since you built a kitchen for your oven, I am assuming this is an enclosed building?? If so, the building technique my builder used was a "post" building where he used pressure treated posts in the ground placed on top of concrete footers under the posts (a bag of Quickcrete); left the holes open, then they poured the reinforced slab (which was on top of a leveled gravel base, which was in turn, on top of local "bank-run" which is a sandy-gravely dirt mixture dug out of river banks.) The concrete filled in the rest of the post holes as the posts were inside the completed building and therefore, part of the completed concrete floor. After completion, we then insulated the metal building in the walls and ceiling. It never gets below 40 degrees F inside and the floor never cracked (unbelievably!) after the pour. I guess because it was in shade and we kept wetting it down during the cure.

[Mingy]

Re: Why dont I need to dig footer below frost line?

Nate
I believe they use pea gravel specifically because it doesn't compact which allows the slab to drain. So, my basement floor its (top to bottom) concrete, 6 mil plastic, 2" styrofoam, pea gravel, rock hard clay. I believe for your application you'd want compacted fill. I put a layer of geotextile between the ground and rock.

I replied to your PM about the plastic. In this area, its considered 'best practices' to put plastic under concrete. I did find one article which suggested that, in very dry climates, it negatively impacts concrete curing, presumably because the bottom of the concrete can't get moisture from the ground. Here, you usually 'mist' curing concrete if drying is an issue (only after it is hard).

[Laku]

Re: Why dont I need to dig footer below frost line?

For keeping the slab dry, not filling out the gaps and cracks is better. If the gaps and cracks get filled then water can climb to your slab by capillary action.

[Pompeii Nate]

Re: Why dont I need to dig footer below frost line?

So I have more gravel questions now. I followed the FB plans and have been putting down pea gravel (approximately 8" so far) but a friend told me that it isn't the best stuff to use. He mentioned that road base is better because it fills in all the cracks and gaps. My concern with the road base is how much dirt is in it and if the gravel is for drainage why do I want all that dirt in there? Does that make sense? I really don't want to take out all that I have compacted down so far. Help!?

Also, I heard that using 6mL of plastic causes the concrete to "curl" on the edges so I was told to go with the black gardening fabric as a barrier and told that will do better. Thoughts?

Nate

[Mingy]

Re: Why dont I need to dig footer below frost line?

Cool! Thanks for the update!

[Pompeii Nate]

Re: Why dont I need to dig footer below frost line?

So I was able to track down a structural engineer about a slab (luckily I have a friend who works with one and it didn't cost me anything). I sent him my google Sketchup drawings and he said a 6" floating slab with rebar would be plenty. So now my plan is to lay down 12" of gravel (or maybe road base) and then build a floating slab on top of that. This should cut way down on my costs. Thanks again Mingy for the suggestion. I owe you one.

Now to just line up the equipment to get my build started...

[Laku]

Re: Why dont I need to dig footer below frost line?

Frost heave is caused by frost, freezing water in wet soil. No water, no frost heave. Hence the layer of gravel under the slab, to drain away moisture. If your oven is at the lowest, wettest place in your property, then this won't work, but it does work most of the time.

Figured as much. What I'm wondering is that if you don't dig up below frost line or insulate beneath and around, the water will still get below the oven and freeze and give some frost heave. Which in turn might lead to the slab being slanted to some direction and cause lateral strain on the oven stand.


Insulation below and around heated(and cold) buildings in here is added to prevent frost forming below the foundation and to act as first capillary break. The idea as I understand it is that you insulate around the house (as well as beneath) as far away from the walls as the frost line is deep (or more don't remember exactly). That will prevent the frost ever reaching below the house, thus preventing any damage by frost heave.

[Mingy]

Re: Why dont I need to dig footer below frost line?

Nate
It depends on the structure. For my garage, its about 12" thick around the edges (where the structure weight is) and about 6" in the middle. I made my oven slab about 6" thick around the edges and about 5" in the middle. I put 6" mesh and 1/2" rebar as well. I think I put rebar 12" on center in a grid with 6" around the edges. Rebar is cheap.

So, the process I used is: strip off the topsoil. I live on a farm, so thats about 12" to 18". If you live in a suburb, they usually only leave a few inches of topsoil. So, I had to fill in with gravel, which I compacted. Good thing I have a tractor with a loader and a backhoe!

I don't believe you have to insulate the slab for an oven. If you were building a house, or building a heated garage, you'd want to so you don't spend money heating the ground in the winter.

To find an engineer, you might look for a civil or structural engineer. First I'd call or visit my town's planning department and speak with a plan inspector or building inspector and see what they have to say. They also are likely to know an engineer in your area.

The folks in my town are pretty helpful so I ask them stuff all the time. You may be hesitant to discuss your oven, but here a structure less than 100 square feet doesn't need a permit. So if I had any questions about building a storage shed, for example, I'd call up and say I want to build a slab on grade (nobody would expect a foundation for a shed), do you have any suggestions, example plans, etc?

When I build my house, every now and then they would pull out a 'how to' sheet of paper (like how to tie wood to concrete, how to install a vapor barrier properly, and so on).

Speaking of which I usually put 6 mill plastic down before I pout the concrete because it keeps the water/cream with the concrete.

[Pompeii Nate]

Re: Why dont I need to dig footer below frost line?

Mingy,

you make a very good point. Thanks for the input. I was actually considering a slab like the one in the diagrams you linked to. I will really have to reconsider after your comments. I had never really thought about talking to an engineer so thanks for that idea. Now I just need to track one down. Thanks again. A question for you though. I believe those are monolithic slabs (I was under the impression that was another name for slab-on-grade) in the links you supplied. Whatever their names are, do they need to go down to the frost line or do you just go down a bit and insulate? Thanks again for your help.

Nate

[Mingy]

Re: Why dont I need to dig footer below frost line?

Laku I see what you are saying: they make sure there is a solid foundation for the sidewalk. I thought you meant they did below the frost line (about 1.5M) which they never do here for sidewalks. Obviously, you want a good base for a slab, but, if you think about it, the slab riding up or down a bit will not put any more stress on the slab than the weight bearing down. If your slab is properly made with rebar and mesh, it should be able to support the weight either way. If you think about the suspended slab the oven sits on, it spans about 2M with all the weight of the oven on it, and it should be (and is) strong enough to support that. The bottom slab has the added advantage of being supported across its area, so there are less stresses.

As I said, they build large structures here slab on grade, insulation optional (I believe code calls for it for a heated building for the same reason you have to insulate the walls).

[dmun]

Re: Why dont I need to dig footer below frost line?

Frost heave is caused by frost, freezing water in wet soil. No water, no frost heave. Hence the layer of gravel under the slab, to drain away moisture. If your oven is at the lowest, wettest place in your property, then this won't work, but it does work most of the time.

The exceptions are for extra tall, or non-free-standing structures. If it's part of a building, it needs footings like the building.

[Laku]

Re: Why dont I need to dig footer below frost line?

Laku - I would like of see pictures of that. The strength or durability should not be affected by a foundation unless their is real funky soil. Here they put a 16" slab with 1" epoxy coated rebar in the walkway for the commuter train. You could park a battleship on that slab. I suspect the reason they did so was because it isn't their money.

They don't use concrete for walkways in here but asphalt. What they do is same what they do for roads. Dig the soil away, add drainage and replace soil with different grades of gravel, with the coarsest on the bottom, for about 0,5 meters or more I think. I suppose depending from the soil it could vary.

But I think there is a point in making a proper foundation. The way I figure it is that although you might only get inch of frost heave, it might not be uniform under your slab. which in turn might lead it to being little slanted on one direction, causing a lateral strain on your oven/structure. Which in time could stress it beyond breaking point. I'm not an engineer so take it with a grain of salt.

How ever what I do know is that building code in here require proper foundation to all buildings, with base below frost line and/or insulation layer to prevent frost from forming under the building, for any house/building.

[Mingy]

Re: Why dont I need to dig footer below frost line?

Nate : A footer will add to the load on the soil below because it will add tremendously to the amount of weight which has to be supported. Also, chances are the load per square foot will be even higher because the entire structure will be supported by a smaller area. You should always check with building codes or structural engineers rather than trades. I have seen/heard trades recommend all kinds of things which are contrary to good practice. Here in Canada, where we know a thing or two about cold, many structures are slab on grade. I have a 40x24' garage slab on grade, and in the summer I'll start construction of a 100'x 40' workshop. Both slabs were 'engineered' only because the city needed to be sure the structures could take the snow load).

Mooch - concrete cracks, its a fact of life. The best defense against serious crack which could affect structure is rebar. I put 6" mesh and 1/2" bars around the edges where it will have to support the blocks. Rebar holds things together and makes the concrete much stronger. Proper placement is important: about 1/3 of the way off the bottom.

Laku - I would like of see pictures of that. The strength or durability should not be affected by a foundation unless their is real funky soil. Here they put a 16" slab with 1" epoxy coated rebar in the walkway for the commuter train. You could park a battleship on that slab. I suspect the reason they did so was because it isn't their money.

Here are some relevant links
Slab-On-Grade Construction (CMHC Canada Mortgage and Housing Corporation)
http://publications.gc.ca/collection...7-1998-eng.pdf


There are loads of others. Note that the insulation shown is so a heated dwelling doesn't lose heat to the ground.

[Laku]

Re: Why dont I need to dig footer below frost line?

The example I use is sidewalks: do you ever see the city making a foundation for a sidewalk? Make sure your slab is thick enough and use plenty rebar and make sure you remove all the topsoil and make a packed gravel base. I always put geotextile below the gravel because it stops the gravel from sinking in to the soil.

Actually here in Finland they do make foundations for side walks. As well as for anything else they intend to last a bit longer.

[mooch]

Re: Why dont I need to dig footer below frost line?

I will be making more of an outdoor kitchen area and it will all be tied together. Since the footprint may be a 10' x 15' L shape i am worried about it cracking relative to each other.