Deck Footing Depth and the Frost Line in Northern Virginia

The short answer: 24 inches

Fairfax County's Typical Deck Details say footings shall bear on solid ground 24 inches below grade, and deeper if solid ground isn't found at that depth. Loudoun County's Typical Residential Deck Detail also requires footings at least 24 inches below grade and to solid ground. If you're in either county, 24 inches is the minimum to plan around.

Where the frost line comes in

When soil freezes, the water in it expands and can lift anything sitting in it. A footing that's above the frost line can heave in winter and settle in spring, which over the years tilts posts, opens up connections and makes decks sag or bounce. Footings are set below the frost depth so they sit in soil that doesn't freeze.

The Virginia Residential Code doesn't print one frost depth for the whole state. Each locality sets its own climate design values, including frost line depth, in the code's Table R301.2. For a homeowner in Fairfax or Loudoun, the practical answer to "what's the frost line in Virginia?" is the one the county puts in its deck handout: 24 inches.

Solid ground matters as much as depth

Depth alone isn't enough. The footing has to bear on solid, undisturbed soil. Loudoun County lists inadequate soil bearing at footings among the most common reasons deck inspections fail, and Fairfax's handout says inspectors verify bearing before concrete. Around many houses, the soil near the foundation is backfill that was pushed in after the basement walls were built, and it can be softer than the native soil below.

Footings near the house

Fairfax's details add a rule that surprises people: a footing within 5 feet of the house wall must bear at the same elevation as the house's own footings. On a home with a basement, that can be far deeper than 24 inches. The handout doesn't explain the rule, but the usual concern is that soil next to a basement wall is backfill, and a shallow footing there would bear on it and add load to the wall. It's one reason deck layouts near the house get planned carefully.

Concrete and inspection

Fairfax's details call for concrete of at least 3,000 PSI. In both counties, footings are the first inspection: Fairfax inspects with the footing holes dug and the ledger attached; Loudoun inspects footings before concrete is placed. Once concrete is poured, nobody can verify depth or bearing, which is why this inspection comes first.

Before you dig

Call Miss Utility at least 48 hours before digging, as Loudoun County's deck page reminds owners. Footing holes go deep enough to reach buried lines.

Signs of footing trouble on an existing deck

• Posts that lean, or a deck surface that slopes in a way it didn't used to.
• Gaps opening between the deck and the house, or between the ledger and its joists.
• Concrete piers that are cracked, tilted, or sitting above ground on loose soil.
• Posts resting on flat stones, blocks or pavers rather than a footing.
• Wood posts buried directly in soil, a common source of rot at the base.

What we look at on an existing deck

Footing depth usually can't be confirmed without digging, so on an existing deck we look for evidence instead: whether posts are plumb, whether the deck is level against the house, how the posts sit on their piers, and whether any permit records exist for the deck. A deck that was permitted and inspected had its footing holes checked before concrete. One that wasn't may have been built on whatever the original builder chose.

Can footings be fixed without replacing the deck?

Sometimes one or two footings can be replaced while the deck is supported temporarily. When most footings are shallow, or the posts are rotted at the base too, fixing the foundation means rebuilding most of what sits on it. That's when a deck replacement, built to the county's current details, is usually the better use of the money.