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mfiore 03-22-2008 12:38 PM

Another Foundation question
After speaking with several people, including local building inspectors (less than helpful), it seems that in this part of Michigan, a foundation needs to extend 42 inches deep to get below the frost line. I've gotten variable responses about the best way to proceed with the foundation. Several have suggested the need to dig below the frost-line for the perimeter of the slab. This seems to be the safest bet. I plan to dig down (that's almost 4 feet!) the entire perimeter of the foundation, then tie the foundation into the concrete slab with rebar. I'd like to do the whole thing with one pour.

Does anyone have a photo of this deep of a foundation? I found one from thebadger (thank you), however his foundation is of block. If I'm pouring the slab, would it be easier to pour the whole thing? How much concrete is this going to require?


thebadger 03-22-2008 02:07 PM

Re: Another Foundation question

I dug down past my frost line (~30 inches in my area) and I poured a 6+" footer with rebar. Once that dried, I dry stacked 4 courses of cinder block (below ground) and 4 courses above ground. I then poured every core (overkill) with concrete and every other one with rebar.

Yes, you can do this using all concrete and just extend your walls up. I opted myroute as I could work at my own pace, didn't have to rent concrete truck and I could do all the work my self. I thought it would be "easier" than trying to build forms...

The advice you're getting sounds right. I have heard some new techniques were you dig down (not as far) and you insulate the area below... Not to familiar with it.

I would definitely get advice from others but I'm glad you're thinking about digging deep as I think "historically" that's the way to go...

I'm hoping to pour my hearth on my base this coming weekend.

Keep us posted.


thebadger 03-22-2008 02:09 PM

Re: Another Foundation question

Not sure where the post is, but one FB member uses the "form" approach and poured his footers (deep like yours)/footers in one swoop.

I would do some searching as there were some good pics as well.


mfiore 03-23-2008 04:17 PM

Re: Another Foundation question
Thanks, Dick.

Several have mentioned putting the foundation deep, below the frost line (42 inches here), however it doesn't look like anyone is actually doing that. All the foundation photos I've seen (with the exception of yours, thanks!), are of a reinforced thick slab.

Ken524 03-23-2008 04:49 PM

Re: Another Foundation question
There is a discussion on here somewhere about a "floating slab" vs. the "deep foundation". Might be in one of Badger's threads??

I have a floating slab... but then I'm in KY. Way milder than Mich.

CanuckJim 03-23-2008 05:22 PM

Re: Another Foundation question
The easiest, sure fire,way of doing this is to find out what code is for a garage floor treatment. A WFO will generate about as much load per sq ft as a garage, then build heavier. Go to that in your area. Depends on your frost line.


dmun 03-23-2008 07:07 PM

Re: Another Foundation question
The reason that structures have footings is the phenominon of frost heave. Certain types of soil contain and transmit water, that freezing, create ice lenses, that lift the structure out of the soil.

There are two ways to combat frost heave. One is brute force. You dig down below the frost line, and you pour footings 12 inches thick, protruding six inches beyond the outside of the structure.

The other is to avoid the problem by creating an artificial soil that is not subject to water wicking. You do this by laying down six inches of well drained crushed rock below your slab. This porous layer doesn't get saturated with water, so even if it freezes, it doesn't form the dreaded ice lenses. In order for this to work, the area for the oven must not be in the lowest part of the garden where puddles form in heavy rains.

Here's a quote from the last link, the Canadian Building Digest:


Where a detached building is located on a concrete slab on grade, protection will be provided by placing the slab on a mat of coarse granular material, which will act as a buffer against any movement of the soil under the mat. A mat 12 to 18 in. in thickness is usually adequate.
Just my opinion, but if it's good enough for Canada, it's good enough for Michigan.

All this is moot, if the oven is part of the house or other structure, or if you plan a tall masonry chimney. Then you must have proper footings, as per masonry fireplace building code.

dbhansen 03-24-2008 02:05 PM

Re: Another Foundation question

Here in Wisconsin, I dug a hole (by hand) about 60 inches deep through solid clay and put in a 12" x 16" concrete footer with rebar, and then built a 44" mortared cinderblock wall up from there. This spring (if it arrives this year) I'm going to proceed with the slab on top of that. I do have a few pictures of the hole, if you're interested.

I only dug the perimeter, and left a column of dirt in the middle, for a 42" oven. The hole measured 88" x 96" (outside dimensions) and it took 1.25 yards of concrete.


mfiore 03-24-2008 04:04 PM

Re: Another Foundation question

Thank you. I would love to see photos. Is the footer 12" wide or deep? Are you pouring the slab on top of the concrete blocks, or between the blocks (using the blocks as forms)? What do you think the benefit was of putting a blocked foundation, as opposed to pouring the whole thing? This also seems to be what theBadger did in his posts.

Again, very helpful information. I really appreciate it. It is clear I have no concrete or masonry experience, but am willing to do whatever is needed for a stable foundation.

james 03-24-2008 04:11 PM

Re: Another foundation question
Hello all,

This is a good thread -- and I have moved it to Getting Started. A good spot for a good discussion on foundations and footings.

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