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Re: FNDN: Residential footings considering frost depth

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Another comment on footing depth in cold climates -  Bill Allen asked:

 >I am designing a residential type structure in a climate where there exists
> >a 30" frost depth. I intend to design the foundation such that the footings
> >go at least 18" below the frost depth for proper bearing. My question is: do
> >I only have to do this for the exterior footings or do I also have to do
> >this at the interior footings?
> >
> >Thanks,
> >Bill Allen

In Canada we usually take the footing base only down to the anticipated maximum frost depth, at exterior footings.

Interior footings only have to go down to a solid base, which may be just below the floor slab if you're lucky. However, if the building may be unheated for part of the winter, or for an extended time during construction, there is a risk of frost heaving. Frost heaving requires that there be moisture in the soil, however, so a layer of drain gravel under the footings and good drainage reduces the risk.

It's true that there is usually some heat loss from the house that keeps the frozen zone a little higher at the footing, and is's also true that insulation makes a big difference.

If excavation is difficult, footings can be protected by placing a layer of Styrofoam (usually 2" or 4" thick) in a strip around the building, a foot or so below the surface.

There is a science to frost protection for footings, in areas like our north or Alaska, or even your midwest and mountain areas, where the frost can go down 6 or 8 feet or more. Engineers use degree-days below freezing temperature in their calculations. Dow publishes design information for Styrofoam protection design.

Our Chamber of Commerce will assure you that it never gets that cold in Vancouver. But we do design for the poor people that live in the rest of Canada. We can give you lots of references if you're really interested.

Jim Warne
Vancouver, BC, Canada