I am building a detached workshop/Garage, I am familiar with SFPF (slab and/or Grade beams). I have built 3 homes using Shallow Frost Protected Foudations w/radiant In-floor heat, but stick built.  I want to build my garage w/12' walls with 6" ICF (Fox Block) I would typically go 8" deep, 12" wide then 45 degree my way up to my 4" slab thickness.  I have been told that  I should go 12" deep to handle the additional weight of the ICF walls.  I am building in sandy soil, putting in Radiant In-Floor heat, and the building site is located here in Western Wisconsin.  Have any of you built a similar project, or does anyone have any suggestions?  FYI-I contacted my local building inspector and he told me that there is not a code that dictates how deep I need my SFPF to be (Detached garage) as long as the concrete under the ICF can handle the load.  I will also be using 2" high density insulation.  Thank you, WI Hank

I've always seen a minimum of 12" depth in references. But are you asking about footing thickness/strength?  As I recall, that is well specified by code.

I am no expert, but as I read it, that is more than enough footer thickness for the footing width and wall thickness you have selected.

Thanks jonr, my building permit should be anyday.  My plan is to from the perimeter of my SFPF - slabe on grade with a 2 x 12 w/2" foam on the sides and bottom of the footing, that will give me a net depth of a 9 1/4 " footing. 

Have you considering using Fast Foot instead of that 2x12 for your footing forms?

Fast Foot link

Arkie6, thanks for the link.  I checked it out, i'll have to do some more thinking. I can see how it would eliminate being so fussy with the bottom footing.

You might try the first row of forms wet set with your rebar wet setin the slab. Just think how the floor and wall can be poured as a beam.

Hank, have you run this by a P Eng ?

The selection and placement of steel re-inforcing plays as big a role as footing depth in your case.