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.
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. Ground snow, wind speed, seismic category, weathering, frost line depth: the jurisdiction fills them in, per the table's 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, and nothing in it ties frost depth to elevation either. 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 no Virginia provision 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. 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 publishes what it calls a typical footing: 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. It says that in two separate county plan documents. It also hedges it in its own words, as typical and determined case by case once the drawings come in. And Grayson publishes no frost line depth at all. 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. There is no regional number here, let alone a state one.
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 Hillsville is where this crew is based. 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. 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. On a job of ours, the depth that applies to your address goes on your free written estimate before anything gets dug, and it is never 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 people quote at us, and what they actually excuse
The one we hear most 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.
The only Virginia provision that excuses a footing altogether stops at 256 square feet of building area for one story detached sheds, playhouses and similar uses, and it carries five conditions that have to all be true together: eave height of 10 feet or less, a maximum height from finished floor level to grade not exceeding 18 inches, bearing elements in ground contact set level on firm soil and pressure treated for ground contact if they are wood, anchorage against wind loads, and light frame construction. That last one is one more sense of 18 inches, and it is a height above grade rather than a depth below it.
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, and in no case shall the bottom of the exterior footings be less than 12 inches below the finished grade, with one exception for footings erected on solid rock. 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 there is no prescriptive shallow path in the North Carolina residential code at all. A shallow insulated design there is an engineered alternate that the local code enforcement official has to pass on case by case, and nobody can tell you in advance which way that goes. 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 we do about it is unglamorous. Before a footing job we confirm what your jurisdiction publishes and what document the figure came off, and where it publishes nothing we say so and point you at the desk above rather than hand you a number borrowed from next door. The depth we are digging to goes on your free written estimate before anybody breaks ground, so the number is agreed in writing 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.