Three different numbers, and only one of them is frost
Most bad advice about footing depth comes from mashing three separate code numbers into one. The Virginia Residential Code sets a footing size at not less than 12 inches in width and 6 inches in depth, and the word depth there means the thickness of the pad, not how far down the trench goes. It sets a minimum burial separately: exterior footings go not less than 12 inches below the undisturbed ground surface. Note the datum, because it is undisturbed ground and not finished grade, and on a site about to be cut or filled those are different elevations. Then, on top of that, comes the frost line depth, which is whatever your locality has entered in its own copy of Table R301.2.
So the floor is 12 inches and it applies whether or not frost is in play. It is a bearing and stability minimum. Where the locality has published a frost figure deeper than that, the deeper figure is the one you dig to and the 12 inch floor stops mattering. Where a locality has published 24 inches, the answer is 24. Where it has published nothing, the number comes out of the building official's mouth and nowhere else. Digging below the frost line is the ordinary way to meet the frost rule, not the only one, and the code excuses some small free standing accessory structures from frost protection altogether, both covered further down.
This is why a bare figure with no jurisdiction attached to it is worthless. Somebody tells you 18 inches. Eighteen inches of what, set by whom, measured from where. Eighteen turns up four separate ways on this page alone: as a published frost line depth, as a published footing depth that never uses the word frost, as a typical footing a county says it determines case by case, and as a maximum height above grade in a shed exception that has nothing to do with digging.
Virginia hands the frost line to your locality, and says so twice
The administrative half of the Virginia Uniform Statewide Building Code lists the provisions of the referenced codes that count as valid parts of the state code, and one entire item reads: "Section R301.2 of the IRC authorizing localities to determine climatic and geographic design criteria." Virginia did not merely leave the locality provision alone. It reached in and named it as good law.
The technical half says the same thing from the other direction. R301.2 says additional criteria shall be established by the local jurisdiction and set forth in Table R301.2, and Virginia publishes that table unfilled. The frost column carries its own footnote b, which reads "The jurisdiction shall fill in the frost line depth column with the minimum depth of footing below finish grade." Read that last clause again. What a Virginia locality is asked to publish is expressed as a minimum depth of footing, which is exactly why every county sheet in the region prints a footing depth rather than a soil temperature.
The edition in force is the 2021 code, effective January 18, 2024, per the Virginia Register historical note on the code chapter and the cover of the Department of Housing and Community Development's own published edition. Nothing in that code sets a statewide frost number. Elevation is written into the Virginia amendments for wind: for localities in special wind regions, near mountainous terrain and near gorges, areas at 4,000 feet and higher use an ultimate design wind speed of 140 mph, and areas lower than 4,000 feet use 110 mph. Higher ground really does behave differently, and that is a fair reason officials land on different numbers, but we found no Virginia provision that makes elevation the input to frost depth.
The county and city documents that carry a figure, and their dates
Every figure below was read off the jurisdiction's own document, not off a chart of frost depths by state. The document and its date travel with the number, because design criteria sheets go stale and several of these already have.
City of Galax publishes a frost depth of 18 inches for residential and commercial work, on an Information for Designers sheet dated September 2025 that says on its face it is written to the 2021 code editions. That is the cleanest document in the set. Floyd County publishes a frost line depth of 18 on its own Building Inspections page, and the page prints the 18 with no unit mark on it, which is worth knowing if you are quoting the county to somebody. The county's other paperwork is gathered in the Floyd County permits guide. One local climatic criteria sheet covers four Roanoke Valley jurisdictions at once and gives two different answers: 18 inches for the City of Roanoke, and 24 inches for Roanoke County, the Town of Vinton and the City of Salem. Its footer reads updated February 2024, but its own list of adopted editions still names the 2018 code, so confirm it before you lean on it.
Montgomery County publishes a frost depth of 24 inches minimum on a short Structural Design Criteria sheet, a file created in 2021 and last modified in January 2023, still linked from the county's own guidelines page today. Pulaski County publishes 24 inches for residential and commercial on an Information for Designers sheet dated February 2025, and the same 24 appears on the county's older design sheet and on its deck details, so the figure has held there while the text around it has not: that older sheet names a different building official and different wind and snow numbers.
Giles County is a different case. The only depth Giles publishes anywhere is a footing depth of 18 inch minimum on a plan requirements sheet for one and two family dwellings, and that sheet is not a climatic criteria table, does not use the word frost, and was created in March 2009, long before the 2021 code now in force took effect. It is a real published figure with a real staleness problem. Confirm it with the county building department at 540-921-2527 before anybody digs to it.
Grayson County is the document most likely to be misread. Grayson prints an 18 inch footing depth in two separate county documents, and they word it two ways. Its Overview of Minimum Requirements calls it typical: 18 inches deep, a minimum of 8 inches of concrete, wide enough for a projection on all sides of the wall or post, and in most cases two runs of number 4 rebar suspended in the lower third, with the requirement determined case by case once the drawings come in. Its plans sheet gives the same 18 inches as a minimum footing depth, with no typically attached. Neither document calls it a frost line, and we found no frost line depth anywhere Grayson publishes. A published footing depth sitting beside a published nothing on frost is the finding, and it is the reason you should not treat the two as the same number.
The spread is the whole argument. Galax publishes 18 and Pulaski publishes 24. Floyd publishes 18 and Montgomery, which shares a line with it, publishes 24. The City of Roanoke publishes 18 and Roanoke County publishes 24 on the same page of the same document. Any single figure for the region would be wrong on one side of a county line.
No frost line figure on paper, and our home county is among them
Carroll, Grayson, Wythe, Patrick and Smyth publish no frost line depth, and each of those was checked on its own site, page by page, and for Carroll and Grayson down into the posted documents. Giles, as above, publishes a footing minimum and no frost line either, which makes six of the eleven with no frost line figure. That absence is a finding rather than a research failure, and saying it plainly is more use to you than borrowing a neighbor's figure and calling it local.
Carroll is the sharpest case, because this crew's shop is in Woodlawn, in unincorporated Carroll County. The only depth figure Carroll County publishes anywhere in its building documents sits on the Manufactured Home Inspection List, under pier holes, and it is about manufactured home piers. It does not generalize to a house footing, a garage footing or a shop, and no page on this site will stretch it that way. For a Carroll County address outside the Town of Hillsville, the number comes from the county's Office of the Building Official, 276-730-3016; inside the town limits the Town of Hillsville issues its own building permits.
The other four desks: Grayson County Building Department, 276-773-2322. Wythe County Building Inspection, 276-223-4505, for addresses outside its towns, and the Town of Wytheville's Building Official, 276-223-3339, for addresses inside that town. Patrick County Building Inspections, 276-694-4143. Smyth County Building and Zoning, 276-706-8317. Ask what depth the office applies at your address and write down the answer, including who gave it to you. Whatever the job, get the depth that applies to your address in writing before anything gets dug, and never take a figure borrowed from the next county over.
Why the depth is not a preference
Water in the soil expands when it freezes, and it lifts whatever bears on it, unevenly, one corner at a time. That is frost heave, and getting the bearing surface below where the freezing reaches is the ordinary way to stay out of it. The service side of that argument already lives on our footings and foundations page and is not worth repeating here. What this page adds is the legal shape of it.
The Virginia frost provision does not say dig. It names four ways to satisfy it: extend below the frost line specified in Table R301.2, build to the insulated shallow foundation section, build to ASCE 32, or erect on solid rock. Digging is the ordinary answer and not the only lawful one, which is why an engineered frost protected shallow foundation is a real path in Virginia rather than a workaround. The insulated section is written for heated buildings, and ASCE 32 is the separately named route, so a detail drawn for a warm house is not a detail for a cold garage.
The same provision carries a winter sentence with teeth, and it carries a qualifier that usually gets dropped: footings shall not bear on frozen soil unless the frozen condition is permanent. And the code's general footing rule requires footings to be supported on undisturbed natural soils or engineered fill, which is the other half of depth and the half that gets skipped. Correct depth into soft fill is not a footing.
The exemptions that get quoted, and what they actually excuse
One that circulates is that a monolithic or thickened edge slab escapes the frost rule because it is a slab and not a footing. Nothing in the Virginia code says that. The frost provision reaches foundation walls, piers and other permanent supports of buildings and structures, and a thickened slab edge carrying a building is a permanent support. Montgomery County writes the same conclusion in plain English in its own accessory structure guidance, a sheet created in February 2024 and last modified that July: over 600 square feet, the structure must sit on continuous foundation walls with a permanent footing or a turndown slab at least 24 inches below finished grade. The turndown is named and it gets the same depth as a footing.
The second one is the accessory structure exception, usually quoted as a shed under 600 square feet needing no footing. Two thresholds are being welded together there. The 600 square foot figure, for light frame construction, and the 400 square foot figure, for other than light frame, both with an eave height of 10 feet or less, excuse frost protection only, and only for free standing structures. The footing itself and the 12 inch minimum below undisturbed ground still apply. A structure attached to the house is not in the exception at all.
Virginia's exception that lets a shed skip the footing altogether stops at 256 square feet of building area for one story detached tool and storage sheds, playhouses and similar uses, and it applies only where "all of the following conditions are met": an eave height of 10 feet or less, a maximum height from finished floor level to grade not exceeding 18 inches, supporting elements in direct contact with the ground placed level on firm soil and, where they are wood, approved pressure preservative treated for ground contact, anchorage to withstand wind loads as the code requires, and light frame construction with walls and roof of light weight material, not slate, tile, brick or masonry. The floor height condition is one more sense of 18 inches, and it is a height above grade rather than a depth below it. The same Virginia amendment carries another exception: "Footings are not required for ramps serving dwelling units in Groups R-3 and R-5 occupancies where the height of the entrance is no more than 30 inches (762 mm) above grade." And the deck section, R507.3, excuses footings for some free standing decks under conditions of its own.
Montgomery County's own accessory structure sheet shows where this lands in practice, and it is worth reading for what it actually is. Its tiers run no building permit up to 256 square feet, then from 256 to 600 square feet a permit and a structure sitting on a continuous foundation wall or grouted masonry piers anchored to permanent footings at least 12 inches below finished grade, then 24 inches over 600. The 256 on that sheet is a building permit trigger, not the state's footing exception, so the two share a number without being the same rule. Read the county sheet for what the county requires and the state amendment for the footing question.
North Carolina fills the cell in once, for the whole state
Across the line the mechanism inverts, and this is the part nobody carries correctly. North Carolina fills its Table R301.2(1) with a single statewide row and the frost line depth cell reads 12 inches. Its residential code says all exterior footings and foundation systems extend below that frost line, except footings and foundations erected on solid rock, and that in no case shall the bottom of the exterior footings be less than 12 inches below the finished grade. Note the datum has moved: North Carolina measures from finished grade where Virginia measures from the undisturbed ground surface.
North Carolina also deleted two sections Virginia keeps. Its frost protection subsection reads Deleted, and its frost protected shallow foundation section reads Deleted, so we found no prescriptive shallow path in the North Carolina residential code. A shallow insulated design there is an engineered alternate that the local code enforcement official has to pass on case by case, so ask that official before anyone draws one. That is the opposite of the Virginia answer, and a homeowner in Surry or Alleghany gets a genuinely different answer from a homeowner in Carroll.
It is not that North Carolina refuses to go county by county. It does, for wind and for seismic. Alleghany County is listed as a special mountain region on the wind speed table and runs through an elevation ladder from 115 mph up to 150 mph, while Surry, Stokes and Wilkes are not listed and take the default. None of the four appears on the list of counties in Seismic Design Category C. The state built county tables and then deliberately did not build one for frost.
One edition caveat, because it will matter soon. The code in force is the 2018 edition, effective January 1, 2019, and the Office of the State Fire Marshal's own current codes page still says so. The 2024 residential code is published, and it changes that cell from 12 inches to minimum 12 inches and adds a new footnote reading "Check with local jurisdiction for frost line depth." No effective date for it is published. That is North Carolina moving part of the way toward Virginia's posture, and it is a reason to ask rather than assume. More on how the two states differ on paperwork is on the North Carolina permits page.
A published number still has to survive the trench
Even where a locality has published a figure, that figure is a table and a table cannot see your lot. The code requires the footing to bear on undisturbed natural soil or engineered fill, and no design criteria sheet knows about the soft low corner, the fill pad somebody pushed in last week, or rock coming up shallower than the plan wanted. The footing inspection exists for exactly that. Virginia's minimum inspection list, which the building official conducts when applicable to the construction or permit, opens with inspection of footing excavations and reinforcement material for concrete footings prior to the placement of concrete. What the official standing at the open trench decides is that official's call, and it gets made on your dirt rather than off a table.
What to do about it is unglamorous. Before a footing job, find out what your jurisdiction publishes and which document the figure came off, and where you cannot find a published figure, take the question to the desk above rather than borrowing a number from next door. Get the depth in writing before anybody breaks ground, so the number is agreed on paper rather than discovered on pour day.
The pour itself is the other half. Footings and foundation walls are the work most likely to need the pump on this crew, because a loaded ready mix truck backed to the lip of an open trench is how a trench wall ends up in the bottom of the footing. We run our own trailer line pump on 2.5 and 3 inch hose, so the truck stays out on firm ground and the concrete goes down the trench in the order we want it. When another crew has the excavation and the steel and just needs the concrete placed, we do the pumping for them.