What are minimum footing sizes for foundations?

Old 07-22-02, 10:25 PM
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Question What are minimum footing sizes for foundations?

I had posted the question below concerning pouring a concrete slab over an existing one. I was hoping to add to that question. Question being:

Is there a minimum width and depth for footings for a foundation? I realize that local codes and weather can play a role, but is there some sort of basic formula to use depending on foundation size?

I have seen large shops (say, 20' by 20' built on a simple 4" slab), and they seem to have no problems. Local code in Oregon where I am (we have freezing winters) seems to say that anything over 100 sq. ft. needs footings (if I remember right), but I have seen larger shops around here that have not followed these guidelines. I do have a permit, and my original foundation was approved, but I want to increse it's size about a foot in all dirctions (to about 18' by 16' give or take), and I don't really want to have to repour 18" deep footings if I'm only adding about 1 foot of width and length to the existing foundation (this is a shop only. No cars will be driving on it). Is it necessary to re-dig the the footings for the extra 1 foot all around again to the original depth of 18"? Or, will the existing foundation (which is very solid and reinforced) be enough to support the extra added foot, as long as the sub-base is compact and reinforced in this extra foot? (say I only dig down 8-10" for the new footings).

Needless to say, the sill plates and walls will rest on the new concrete.

Hope I can find an answer (or at least some guidelines before I go to the planning dep't and see what they say). I need to get this done before the snow flies again!

Old 07-27-02, 09:34 AM
Join Date: May 2002
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Change footing and plane require re-apply permit. Otherwise, you may have problems when you call for inspection.

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