Is it possible to "foot" a floating slab building?

[not2old]

If the garage is separate from the house have you considered raising it, removing the existing floor/slab and constructing a foundation under it as deep into the ground as necessary?

 

[jimgerken]

Not2old: yea, that would be the best way to do it I suppose. That also gives the opportunity to relocate it if desired.

 

[TonyF]

I would try this method before digging a new footer. Maybe you can get the insulation under the existing slab.

http://www.countryplans.com/Downloads/shallowfound.pdf

 

[arsenix2001]

Your basically going to be under pinning, proper way is to dig to frost next to the edge of slab and dig back at least 12 inches, rebar and form the outsides and pump a peagravel mix right to the top, and small joints left can be packed with a portland sand mixture.

 

[inveresk]

There are a number of ways to do this but depends on the wall material and whether the slab edge is carrying a lot of weight. Does the slab edge support framed walls or brick? How much weight is added to the wall or slab edge by the roof? How much reinforcement do you have in the slab? If you excavate along a full side, is there a risk of subsidence? Could the load on the slab from roof and walls risk it cracking if 12" or so is cantilevered over the excavation during the works? If either of the last two, you have to undertake excavation on a 'hit and miss' basis - excavate a length of about 4 feet at a time and leave 4 feet undisturbed. Corners, too, have to be excavated one side at a time, not in a L shape.

Other option is digging down locally and use sona tubes at, say, 4 foot centres to provide concrete piles. This wouldn't give you a continuous foundation but it might be enough to do the job. And it would be a lot less work. Depends, again, on thickness of slab, load on it and how much reinforcement it contains.

 

[pat32rf]

Normally the footings support the structure. In your case the floating slab is designed to do this. All you really need to do is insulate the slab edges, down a foot or two, then a sloping skirt of insulation that extends out from the footing and carrys away any ground (rain)water to a weeping tile. If you keep the soil dry a bit of frost won't heave things. Its the expanding moisture (ice) that causes problems.. Blue styrofoam is wonderful, but seal the seams...

 

[dstig1]

If the garage is separate from the house have you considered raising it, removing the existing floor/slab and constructing a foundation under it as deep into the ground as necessary?

Bingo! Just what I was going to suggest. House lifting/moving isn't too expensive, really.

And then if you want to turn it into a shallow frost-protected footing instead of full 4' depth, our own U of Minn has this to say:
Tomorrow

 

[boxygen]

In this case, I would do nothing at all or I would jack the house up and get rid of the existing slab completely. You will spend much more time and effort retrofitting the slab to be frost free than the original slab would cost to replace. If you want your house on a frost wall or full basement I would design the connector between the two to be a shallow frost protected slab. You would form and pour the basement/frostwall then backfill and prep and form the shallow frost protected slab connector between the house and garage. This is all without seeing your site conditions and what you want for a building to connect to the garage, which can dictate a lot.

 

[not2old]

Not2old: yea, that would be the best way to do it I suppose. That also gives the opportunity to relocate it if desired.

If you want to move the garage, it isn't that big of a deal either. 40 years ago, my uncle raised his garage onto blocks and then lowered it onto a bunch of old telephone poles lined up in the direction he wanted to go. He then towed it into place with The Big D. (late 1960's Datsun pick-up)

 

[CurlyDave]

...The expansion joint idea is kind of the default plan if nothing else seems workable. for the joint, I was thinking building the house with atttached breezeway pretty close to the garage, like a foot away, and around the doorway there would be an extension that could screw to the footed breezeway side, like a large extension jamb, and a layer of foam on the side where the garage door would be.

I would use the breezeway idea, but make it much longer than a foot. At least 5 or 6 feet and maybe as much as 8'.

That way, if the garage rises and settles with the season, the angular adjustment needed in the breezeway is much less than if it is only a foot long. You should be able to enclose, heat and insulate the breezeway, which makes the distance seem much easier to live with.

Secondly, if the breezeway idea doesn't work out and you eventually want to jack up the garage and add a footer, you have room to get a piece of equipment between the house and the garage. Otherwise you are stuck digging a 4' deep trench in an alley only 1' wide between the house and the garage.