Determining Footing Depth
idahobuild
| Posted in Building Code Questions on
Hey all,
General interpretation of
5-1-6: FROST DEPTH:
The frost depth level for the city is hereby declared to be two feet (2′) below the natural surface of the ground. All foundation footings shall be at least two feet (2′) below finish grade or the natural ground surface. (Ord. 786, 1975)
Of course, we will consider the Geotechnical Report when we get it to inform foundation decisions.
1. Does the attached foundation detail look to comply with the intent?
Thanks.
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Replies
The easiest way to think about this is the plane of the bottom of your footings relative to the top of grade. So, if the bottom of your footings (the bottom of your excavation) is 2' or more below the finished ground you will have achieved frost protection.
"The natural surface of the ground" is not the same as "existing grade." They just don't want things like a raised planter to skew depths.
Many people seem to think that the top of the footing needs to be below the frost line; plumbbob's right, it's to the bottom of the footing.
In Los Angeles the standard is 2 foot embedment into undisturbed soil which that diagram would not comply with. Unless the raised grade was compacted fill certified by a soils engineer. (No frost lines here, high of 78 today...)
The "undisturbed soil" portion is what I think I was remembering. I've changed the elevation detail a bit (see atch'd). I'll still be above the Existing Road elevation, though only by about 3". The road is about 70 feet away and the land is quite flat. I'll be sure to pay special attention to the finish grading. We'll see what the building department thinks in a few weeks.
That looks better. As others have said, the bottom of the footing needs to be AT LEAST at frost depth, which is below the existing grade before you change things around for flower beds or drainage. The ground under the footing is supposed to be "undisturbed" too, so that it's reasonably compacted and unlikely to settle after your build. The general idea is to get down deep enough to "old and compacted" earth where you won't have severe issues with frost heave (the "below frost depth" part), or settling (the "undisturbed soil" part).
Bill