Footing depth?
user-1137156
| Posted in Building Code Questions on
The location I’ll be building has a ” frost depth” requirement of 42″. I’m planning on “insulated” footings set on a drainage layer of coarse crushed rock. My question is exactly what needs to be 42″ below finished grade? Is that the bottom of the drainage layer, ie undisturbed soil? Is that the bottom of the insulation, ie top of the drainage rock? Or worst possibility the bottom of the poured concrete?
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Replies
Jerry,
I'm not sure what you mean by "insulated footings." Is this a frost-protected shallow foundation? Is it an insulated raft foundation? Something else?
In any case, the only opinion that matters is the opinion of your local building department. If your details differ from ordinary local practice, your plans may need to be stamped by an engineer.
Martin,
As I understand the insulated raft, it uses a slab of uniform thickness with insulation below it and rock below that and little or no perimeter insulation. This would be closet to what I plan & I could do that but it requires what I see as wasted concrete to make the slab extra thick where there is no need. So I would prefer to make the slab have a thickened edge which is actually poured as two parts, a footing part and a slab. As I understand the frost protected shallow foundation, it depends on insulation extending horizontally outside the footings, this is definitely not what I'm thinking of.
Edit: I think I found an answer in the SPFA publication on 'Permanent Wood Foundations' "As with all foundations footings need to be placed on undisturbed soil and the footing excavation should extend below the frost line." Clearly indicating that the drain rock fill is considered part of the footing.