foundation question

[archimedes]

I talked to a contractor about building my foundation for me.   He suggested a slab on grade and said the it only needed to go below the ground by about a foot.  The frost depth where I'm building is 48" so this surprised me a great deal.  Since I'm from the south and haven't built anything up north before I didn't challenge him on this.    He says he does it all the time.

Is it possible to build a slab on grade this way?   If so why can it be built so shallow.

[rocking23nf]

They do garages like that, I mean if you had 4 feet of concrete through the whole slab, that would be a lot of concrete.

[FrankInWI]

What kind of soil are you building on?  Many houses near me are on a slab, because many are on sandy soil where there is less tendacy for frost heave (like there is on clay).  I wouldn't build a home like that in SE Wisconsin were my home is, but mid state over sand it's the norm.  (I can't put down more anyway, we have high ground water being near a river).

[archimedes]

Thanks for the replies.

The soil is extremey well drained sand.  

So maybe he was right.  I just thought the edge of the foundation still needed to go down 48".  I guess i was wrong.

[FrankInWI]

Let's hear from some of the more experienced guys too....  there's some great experience out there, I'm just learnin as I go.   I think this topic was covered before though and you are ok on sand with that.

[firefox]

One thing you might think of doing is putting a layer of insulation down first. Just make sure it has borate or some kind of insect protection stuff in it to keep termites at bay. There is an article somewhere on this type of construction, but I don't remember where.
Maybe some kind of search for insulated foundation.
Bruce

[John Raabe]

Well drained sand and gravel soils will generally not expand when they freeze. That is the main reasons footings need to be down below frost depth.

Here is an article on the CountryPlans site about shallow insulated foundations. These can be built even in expansive soils: http://www.countryplans.com/Downloads/shallowfound.pdf.

[archimedes]

I had considered a FPSF.  That's why I was surprised when I was tolded that even without insulation that (at least with this contractor) that it was customary to only go down about a foot below grade.

[rocking23nf]

have you considered a full basement?

[firefox]

Thanks John for finding the reference to FPSF.
Bruce

[archimedes]

I wanted to do a basement but unfortunately the property is in a flood zone.  So if I do a basement I wouldn't be allowed to use the space as living area or even use it for mechanical equipment.  So basically it would be a very expensive storage room.

Interestingly, the old cottage that is on the property (at least 60 years old) only has an 8 inch deep perimeter foundation.  And not one bit of settlement or movement.  I attribute this the the extremely porous soil that it sits on.  Tells you how much overkill building codes can be sometimes under the right circumstances

[JRR]

A slab foundation beneath an insulated building would benefit from the same physics that makes the shallow frost proof insulated footings work.  It could be enhanced by additional perimeter insulation, but obviously not needed in your climate.

[archimedes]

OP note: the property in question is a summer residence in Massachusetts.

[chris1199]

Typically here in the northeast, a building would have a perimeter frost wall of usually 8-12" thickness, down to frost depth. In your case, obviously 48". This perimeter wall can be insulated, but generally is enough of a thermal barrier itself to keep frost out of the interior of the building. The wall itself would sit on a footing base, usually 10-12" high by 20-24" wide running the perimeter. The footings bottom is at 48" down.
If a habitable space I would put 2-4" of styrofoam insulation under it. If planning radiant heat in slab go with 4" insulation, then 12" sand or fine gravel for a heat sink, then the slab.
It( the wall) also serves to transfer the buildings structural loads down below frost to minimize movement with seasonal changes. I have been doing building for residential structures for over 25 years and would not reccommend a slab on grade with no frost walls for anything but a storage shed.
Extra cement is extra cost, no doubt about it. But a home is a significant investment as well. No sense putting something nice on top of something that "may" hold it up. Put a good base in and you can put anything on top. A cabin or a mansion.
Good luck with your project.

[archimedes]

Thanks Chris.  That's what I suspected.  That's the reason that I was so surprised when this contractor told me otherwise.  Maybe I shouldn't have told him I was from FL.   

Based on what you told me I might be better off (if I'm gonna do it myself) with an FPSF.    Less digging, less concrete.

[Don_P]

There's an awful lot of stuff you can get away with most of the time, and then I look at the cost difference and ask myself "but why?". I'd agree on checking out the FPSF for that area. For one here it doesn't take much.

Chris