Deck Footing Size & Depth: A Complete Guide
A practical, code-aware guide to sizing deck footings, digging below the frost line, and choosing the right footing type for your soil and loads.
Footings are the part of a deck nobody sees and everybody depends on. They carry the entire weight of the structure plus everything on it, then spread that load into the soil so the deck doesn't settle, tip, or heave. Get them wrong and you'll see sloping boards, racking, and gaps at the ledger within a season or two. Get them right and the deck outlives the house's first roof.
There are two questions every footing has to answer: how deep, and how wide. Depth is mostly about frost. Width is mostly about load and the soil under it. This guide walks through both, covers the common footing types, and points you to the code rules that govern the whole thing.
One disclaimer up front: footing requirements are set by your local building department, and they can be stricter than the model code. Frost depth, soil assumptions, and required permits all vary by jurisdiction. Use the numbers here to plan and budget, then confirm everything with your local inspector before you dig.
Why footings matter: load transfer and frost heave
A deck footing does two jobs. First, it transfers vertical load. Posts concentrate the deck's weight onto a few small points; the footing's wide base spreads that load over enough soil that the ground can carry it without compressing. Second, it resists frost heave. When water in the soil freezes, it expands and pushes upward with enormous force. A footing that stops above the frost line will be lifted every winter and dropped every spring, slowly jacking your deck apart.
The combined load matters. The IRC uses 40 psf live load plus 10 psf dead load for residential decks, so 50 psf total. A footing carrying a 10-by-10-foot tributary area is holding roughly 5,000 pounds. That single number drives both how big the footing base needs to be and how deep it has to sit.
Digging below the frost line: depth by region
The rule is simple: the bottom of the footing must sit below the local frost line so the soil under it never freezes. The number is not. Frost depth depends on your climate and is published by your building department.
Typical minimum footing depths run roughly:
- Warm/southern climates (Gulf Coast, Florida, southern California): 12 inches or a no-frost minimum
- Mid-Atlantic and temperate zones: 24 to 36 inches
- Northern states (Upper Midwest, New England, Mountain West): 42 to 48 inches or more
- Far-north/Alaska interior: 60 inches or deeper
Even in frost-free regions, codes usually require a minimum embedment (often 12 inches) for bearing and lateral stability. Always pull the official frost depth for your county rather than guessing, and dig to undisturbed soil, not loose backfill.
Footing diameter vs. load and soil bearing capacity
Width is set by load divided by the soil's bearing capacity. The IRC default presumptive value is 1,500 psf for unknown soil; many soils do better, but you can't claim more without a soils report. Required base area equals the post load divided by that capacity.
Take that 5,000-pound post on 1,500 psf soil: you need about 3.3 square feet of bearing, which is roughly a 25-inch-diameter or 24-by-24-inch footing. Drop the load to 3,000 pounds and a 16- to 18-inch base is plenty. This is why bigger spacing means bigger footings.
Footing thickness matters too. A common minimum is 6 to 8 inches of concrete for a poured pad, thicker as diameter grows, so the pad doesn't crack and punch through under the post.
Common footing types
Several footing systems all satisfy code; the right one depends on soil, frost depth, and budget.
- Poured concrete pier: concrete poured directly into the hole, often belled at the bottom for extra bearing. Cheap and strong, but harder to keep plumb.
- Sonotube (cardboard form): a tube sets a clean round pier above grade. Pair with a flared footing form like a Bigfoot at the base to widen bearing without a huge hole.
- Footing-plus-pier: a wide poured pad at the bottom carries load while a narrower pier rises to the post. Best for high bearing demands.
- Helical/screw piles: steel shafts driven below frost with hydraulic equipment. No concrete, no curing, excellent in poor or wet soil, but installer-dependent and pricier.
For most residential decks, a Sonotube with a flared base over good soil is the practical sweet spot.
Spacing, beam span, and footing count
Footing layout follows the framing. Joists span to beams, beams span between posts, and posts land on footings, so beam size and post spacing set how many footings you need and how much each carries.
Wider post spacing means each footing carries more, demanding a deeper or wider base. The IRC deck provisions (Section R507) include span tables for joists, beams, and post spacing based on lumber species and load. As a rough planning rule, beams of doubled 2x10 or 2x12 commonly span 6 to 9 feet between posts at residential loads.
Don't freelance the spans. Pick your beam and post layout from the code tables or an engineer, then size each footing for its actual tributary load. Closer footings are smaller and cheaper to dig but add labor and concrete count.
How much concrete per footing, and code/permit notes
Concrete volume is straightforward. A cylindrical pier's volume in cubic feet is pi times radius squared times depth, divided by 1,728 for inches. A 12-inch-diameter pier 48 inches deep holds about 3.1 cubic feet, roughly 0.12 cubic yards, or about five 60-pound bags. A 16-inch pier the same depth nearly doubles that.
For 2026, bagged concrete runs about $6 to $8 per 60-pound bag, and ready-mix delivered is roughly $160 to $200 per cubic yard plus short-load fees. Below about one yard, bags usually win; above it, call a truck.
On code: decks generally require a permit and inspection. The IRC (2021/2024, Section R507) governs deck footings, and the inspector typically checks the open holes before you pour. Always confirm frost depth, soil bearing, and footing size with your local building department first.
Frequently Asked Questions
- How deep do deck footings need to be?
- Deep enough that the footing base sits below your local frost line so the soil under it never freezes. That ranges from about 12 inches in frost-free southern regions to 42 to 48 inches or more in northern states. Frost depth is set by your building department, so pull the official number for your county and dig to firm, undisturbed soil.
- What size diameter should a deck footing be?
- Divide the post's load by the soil bearing capacity to get the required base area. Using the IRC default of 1,500 psf, a post carrying 5,000 pounds needs roughly 3.3 square feet, about a 24-inch diameter. Lighter loads from closer post spacing can use 16- to 18-inch footings. Wider spacing always means a bigger base.
- How much concrete is in one deck footing?
- For a round pier, multiply pi by the radius squared by the depth, then divide by 1,728 for inches. A 12-inch-diameter pier 48 inches deep holds about 3.1 cubic feet, roughly five 60-pound bags or 0.12 cubic yards. A 16-inch pier the same depth needs close to double. Add the flared-base volume if you use one.
- Do deck footings require a permit?
- Almost always, yes. Most jurisdictions require a building permit for an attached or freestanding deck, and the inspector typically checks the open footing holes for depth and size before you pour. The IRC Section R507 covers deck footings, but local rules can be stricter. Confirm requirements with your building department before you start digging.