Addition to a building on floating slab

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BobInWisc - 12 May 2005 17:14 GMT

I'm contemplating having a 3-season cabin shell put up by a (mostly)
garage-building firm that only builds on slab-on-grade (no frost footings).
The site is on gently sloping sandy soil in NW Wiscsonsin. If I add to it
at some time -- a deck, another room, etc -- what problems are likely?
Can I build on posts anchored below frost line? On piers just resting on
the gound? Do I need to extend the slab? Pour an adjacent slab?

TIA

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Bob Morrison - 12 May 2005 20:52 GMT

In a previous post BobInWisc says...

> I'm contemplating having a 3-season cabin shell put up by a (mostly)
> garage-building firm that only builds on slab-on-grade (no frost footings).
> The site is on gently sloping sandy soil in NW Wiscsonsin. If I add to it
> at some time -- a deck, another room, etc -- what problems are likely?
> Can I build on posts anchored below frost line? On piers just resting on
> the gound? Do I need to extend the slab? Pour an adjacent slab?

Bob:

The fact that it is 3-season is immaterial. Over time it is quite
likely that the building will exhibit the problems of frost heave, which
you will ultimately have to correct.

I recommend that you find a different contractor or that you at least
have someone else put in a proper foundation, the the "cabin" guys can
build on top of that.

Signature

Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA

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cs - 28 May 2005 10:42 GMT

> I recommend that you find a different contractor or that you at least
> have someone else put in a proper foundation, the the "cabin" guys can
> build on top of that.

It appears quite normal that "cabin companies" don't do "proper"
foundations so don't let that deter you from the cabin you really
want. What the cabin guys should do is give you the exact dimensions
of the cabin's footprint, you then get a good local contractor to
build you a proper foundation that suits the area you're in if the
default

When I was looking for a garden building myself this was the case
for all the companies I spoke to.

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Homeowner Forum / Construction / May 2005

Addition to a building on floating slab