According to the Michigan Building Code, what is the minimum depth for residential footings in areas where the frost line extends to 42 inches below grade?
Correct Answer
C) 48 inches below grade
Michigan Building Code requires footings to extend at least 6 inches below the frost line. With a 42-inch frost line, footings must be at least 48 inches below grade.
Why This Is the Correct Answer
Michigan Building Code requires footings to extend at least 6 inches below the established frost line. With a frost line at 42 inches, the minimum footing depth is 42 + 6 = 48 inches below grade. This extra depth ensures the footing bears on soil that will not heave during freeze-thaw cycles, which could otherwise crack foundations.
Why the Other Options Are Wrong
Option A: 36 inches below grade
36 inches is shallower than the frost line itself (42 inches). Placing a footing above or at the frost line means the soil beneath could freeze, expand, and push the footing upward, causing structural damage. This depth does not meet code.
Option B: 42 inches below grade
42 inches matches the frost line depth exactly but does not include the required 6-inch safety margin below the frost line. Footings placed exactly at frost depth remain vulnerable to frost heave if frost penetrates slightly deeper in an unusually cold winter.
Option D: 54 inches below grade
54 inches exceeds the code minimum of 48 inches. While deeper footings are not prohibited, they are not required. This option is a distractor that exaggerates the calculation.
Memory Technique
Use the phrase 'Six below the freeze.' The frost line is where soil freezes — your footing must go 6 inches BELOW that freeze line. 42 (frost) + 6 (below the freeze) = 48 inches.
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