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Stephen Cutler
Stephen Cutler, General Contractor
Category: Home Improvement
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Experience:  35 years of troubleshooting construction, remodel, component & material failures. What to do next
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how deep does a concrete footing need to be for a residential

Customer Question

how deep does a concrete footing need to be for a residential building in the Greensboro NC area for a 30' x 50' A frame wood with aluminium siding and does it need to go below the frost line: 1) to pass inspection and 2) to survive an earthquake similar to the one we had recently i.e. around September 2011
Submitted: 5 years ago.
Category: Home Improvement
Expert:  Stephen Cutler replied 5 years ago.
Hi and thanks for choosing
Footings are placed 12" below frost line. This has nothing to do with seismic events, it has to do with heaving of the supporting soil. If your contractor is experienced, licensed, and knows his codebook, you are in good hands. If he learned to pass his license exam at an adult evening community college class, he could easily have no clue. Go 12" below the worst case senario frost line. This is different at every lattitude & elevation. While the sub is excavating, you can triple check with YOUR local building department. He can alway dig deeper BEFORE forms go up.
Customer: replied 5 years ago.



You did not answer how deep the contractor needs to go to reach the frost line given in the area noted in my question you only stated that he need to go 12 inches below the frost line. How deep does he need to go to reach the fronst line in the Greensboro NC area say 25 mile radius

Expert:  Stephen Cutler replied 5 years ago.

From the N.C. Code:

R403.1.4 Minimum depth.

All exterior footings and foundation systems shall extend below the frost line specified in Table R301.2(1). All exterior footings shall be placed at least 12 inches (305 mm) below the undisturbed ground. In no case shall the bottom of the exterior footings be less than 12 inches below grade.


Frost-protected footings constructed in accordance with Section R403.3 and footings and foundations erected on solid rock shall not be required to extend below the frost line.

In Seismic Design Categories D1 and D2, interior footings supporting bearing or bracing walls and cast monolithically with a slab on grade shall extend to a depth of not less than 18 inches (457 mm) below the top of slab.

Table 301.2(1) is not included in the online version of the code. You can find it in your subcontractors copy of the Code, or the building departments copy. I presume the frost lines are divided into zones.

Expert:  Stephen Cutler replied 5 years ago.
If your sub does not own a copy of the Code, BEWARE, it is a license requirement in most jurisdictions. You do NOT want an unlicensed Contractor messing with your Foundation. If he Oops that, everything on up will be impacted, and you will loose alot of money.
Customer: replied 5 years ago.
you still have not answered my question, how deep does he need to go to reach the frost line? which code are you referring to and how can I get a copy? is it available free on line?
Expert:  Stephen Cutler replied 5 years ago.
The North Carolina Residential Building Code 2009
I have answered as much of your question as can by quoting what the code has to say. I do not have any way to get the Table R301.2(1). If you give me the phone number for your building department I can call them. "25 mi radius may not be enough info - zip code might be more accurate.
Expert:  Stephen Cutler replied 5 years ago.
Free online Codes:

scroll down to free codes > click learn more > click browse catalogs > click North Carolina

See Foundations > footings

Some reference tables are edited out of the free versions. This is to make you purchase the whole thing. Even your State sells them to their contractors. It's a money thing.

I am waiting for your building department phone number
Customer: replied 5 years ago.
The number I believe is XXX-XXX-XXXX in Asheboro NC Zip code is either 27298, 27420 or 27203 area. I believe to reach frost line should be the same for either of these zip codes
Expert:  Stephen Cutler replied 5 years ago.
You need to mask the number so the JA system won't delete it to Xs

like this:

8*1*7-6*4*9*- 1*2*3*4*
Customer: replied 5 years ago.
Expert:  Stephen Cutler replied 5 years ago.
Got it!
Expert:  Stephen Cutler replied 5 years ago.
I asked for the frost line depth for the Greensboro area. The Inspector said 12".

Regardless of the type of account you have with JA, you must successfully click on "ACCEPT" to reward me for my efforts made on your behalf. By clicking ACCEPT you are not giving up your ability to ask more questions pertaining to this subject, and I will be happy to respond to these as well.