Deck Footings and How Deep and Wide to Dig Them

A deck that looks perfect in July can have a 2 cm dip at one corner by March, and the cause is almost always underground. Deck footings carry every pound of lumber, furniture and people down into the soil, and if they are too shallow, too small or sitting on loose fill, no amount of careful framing above will save the structure.

Footings are also where most DIY deck projects cut corners, because nobody sees them once the deck is built. This guide covers how deep to dig, how wide to make each footing, which types suit which projects, and the mistakes inspectors flag most often. The numbers below are typical for North American residential decks. Your local building department has the final word, and it is worth a phone call before you buy a single bag of concrete.

How deep deck footings need to go

The short answer is: below your frost line, and never less than about 12 inches (30 cm) into undisturbed soil. The frost line is the depth to which the ground freezes in a normal winter. When wet soil freezes it expands, and any footing that sits within that frozen layer can be lifted, a process called frost heave. Over a few winters a heaving footing will rack a deck out of level and loosen its connections.

Frost depths vary widely by region. Parts of the southern United States require 12 inches or less. Much of the Midwest and Northeast sits in the 36 to 48 inch range. Northern Minnesota and parts of Canada can exceed 60 inches. Your building department publishes the required depth for your area, often on a single-page deck guide you can download.

Why undisturbed soil matters as much as depth

The bottom of each hole must rest on soil that has never been dug up. Fill dirt, old garden beds and backfill around a house foundation compress over time. If you hit loose or dark, organic soil, keep digging until you reach firm native ground, even if that takes you past the frost line. Near the house, remember the foundation excavation may extend 3 to 5 feet out from the wall, so footings placed close to the house often need to go deeper than the rest.

How wide to make each footing

Depth stops heaving. Width stops sinking. A footing spreads the load from a post over enough soil that the ground can carry it without settling.

The size depends on two things: how much weight each post carries, and how much your soil can bear. Building codes assume a typical residential deck carries 40 pounds per square foot of live load plus 10 pounds of dead load. Many codes also assume a presumptive soil bearing capacity of 1,500 pounds per square foot unless a test shows better.

Here is a rough example. A post that supports a tributary area of 8 by 8 feet carries 64 square feet times 50 pounds, which is 3,200 pounds. Divide by 1,500 and you need just over 2 square feet of bearing area, which works out to a round footing about 20 inches across, or a square pad about 17 by 17 inches. That is far wider than the 8 inch cardboard tube many people pour and call done.

Values assume soil bearing of 1,500 psf. Treat them as a planning guide only and confirm sizes with your local code tables.

Types of deck footings and when to use each

• Poured concrete with a flared base. A cardboard form tube sits on top of a wider bell or square pad at the bottom of the hole. This gives you bearing area at the base and a neat round pier above grade. It is the most common choice for attached decks.
• Full-width poured pier. The whole hole is filled with concrete at the bearing width. Simple, but it uses more concrete and is heavier work.
• Precast deck blocks. Concrete blocks set on the surface or a shallow gravel bed. Some codes allow them only for low, freestanding decks that are not attached to the house and can tolerate a little movement.
• Helical piles. Steel shafts screwed into the ground by a contractor with a machine. They cost more, often $250 to $500 per pile installed, but go in quickly, work in tight spots and avoid hauling soil.

Why a bigger concrete pier is not always the safer choice

A common instinct is to pour a big, chunky pier all the way up, thinking more concrete means more strength. In frost-prone ground that can backfire. A wide, rough-sided pier gives freezing soil more surface to grip, and frost can lift the whole pier by its sides even when the base sits below the frost line. This is called adfreeze.

A smoother, narrower column above a wide base resists this better. Some builders wrap the upper part of the form with a plastic sleeve so frozen soil slides past it. The lesson is that footing design is about shape, not just volume. Bearing area belongs at the bottom, where it does its job.

Steps to dig and pour footings that pass inspection

1. Call your utility locating service a few days before digging. In the US the number is 811.
2. Lay out post locations with batter boards and string, and check your corners for square using the 3, 4, 5 method.
3. Dig with a post hole digger or a rented auger. A one-person auger rents for about $80 to $120 a day.
4. Flatten and compact the bottom of each hole. Remove loose crumbs by hand.
5. Book the footing inspection before you pour. Most jurisdictions want to see open, empty holes.
6. Pour, set a galvanized post base or anchor bolt while the concrete is wet, and let it cure at least several days before loading.

If you are hiring help instead, ask exactly how deep and wide the footings will be. A good builder will answer with numbers, and that question belongs on any list of things to know before hiring your contractor. Water management matters here too. Footings sitting in soil that stays saturated behave much like a wall with poor retaining wall drainage, so grade the area to shed water away from each post.

For more practical building guides, browse our construction articles.

Frequently asked questions

Do deck footings have to go below the frost line?

For decks attached to a house, almost always yes. Some codes allow freestanding, low decks on shallow footings or deck blocks because a little movement will not damage the house. Check with your local building department before choosing shallow footings.

How far apart should deck footings be?

It depends on the beam size and joist span, but 6 to 8 feet between posts along a beam is common for residential decks. Longer spans need larger beams and larger footings. Code span tables list the allowed distances for each lumber size.

Can I put deck footings on top of the ground?

Only for small freestanding decks where your code permits surface blocks. Footings on top of the ground can shift with frost and settle into soft soil. For any deck attached to a house or more than a couple of feet high, dig proper footings.