The Standard Building Code average annual frost penetration list NC as ~5 inches (interpolating). Which is less than the min. 12 inch foundation depth.
The info may be online. I looked up something similar for Cedar Rapids IA over the 'net last week. They had it in their local amendments to the 2000 IBC.
Be careful of using "average annual" frost penetration numbers. To me that means 1/2 the time you will exceed it!
North Carolina is under both the SBCCI and the 2000 IBC. As of 1 Jan 03, the 2000 IBC will prevail and the SBCCI will be out of date. According to Section 1805.2.1 of 2000 IBC, the min depth is 12" to avoid frost heave. In Raleigh, I have seen frozen soil to a depth of 9" on January mornings. In the past, I have discussed this with some of my counterpart inspectors in the mountains and they recommend 18" to 24", but enforce 12" as the code indicates. No source is referenced in the code for an exact frost depth (table, chart, or otherwise). I hope this helps, but like the others said, a simple call to the local Building Inspections Department will answer your question and give you a specific requirement for your project's location. As a former inspector, I can say that we don't mind being asked questions as it is just part of the job.
Thanks all for the help. I know the bldg dept is the best source, but no one was available at the time and I wanted to be certain 24" was safe. thanks a 10^6!
Frost penetration and jacking/heaving is also a function of soil type, so (assuming that you have a geotechnical engineer on the project) he may be able to give you some site specific guidance.
