How deep must a monolithic slab go?

[yellow_corn]

I am pouring a monolithic slab for a new 16'x24' unheated garage and I am confused about the depth because our state has serious frost codes, usually requiring footings to go down 42 to 48 inches. I didn't think a monolithic slab had to go that deep. One of our subcontractors is suggesting a turned down footing that is only 16 inches wide and 24 inches deep, reinforced with #4 rebar. Since the MI Residential Code typically requires foundations to extend at least 42 inches below grade to get below the frost line, I wonder if the sub's 24 inch deep footing is actually enough, even for an unheated garage. Is a monolithic or turned down slab automatically exempt from the full frost depth requirements or is the sub confusing the proper use of a Frost Protected Shallow Foundation system, which uses insulation to achieve a shallower depth? Thanks!

 

[speed fighter]

What your subcontractor is suggesting at 24 inches wouldn’t meet Michigan’s usual 42–48 inch frost depth requirement unless it’s part of an engineered FPSF plan. For an unheated garage the code treats it like any other building, so unless you insulate and get approval for the shallow foundation method, you’ll have to go with the full frost depth.