How Deep Should Deck Footings Be? A Frost Line Guide
Footings are the most important part of a deck you’ll never see. Get them right and the deck stands solid for decades; get them too shallow and winter will heave them out of the ground. Here’s how deep deck footings need to be and why.
The frost line is the rule
When wet soil freezes, it expands and pushes upward — a force called frost heave. If your footing sits above the depth where the ground freezes (the frost line), it can lift, crack, and throw your whole deck out of level.
The rule everywhere cold: footings must extend below the local frost line. That depth is set by your local building department, so always confirm before you dig.
Typical footing depths by region
These are general ranges — your jurisdiction has the final say:
- Warm / southern climates: 0–12 inches (little or no frost)
- Mild climates: 12–24 inches
- Cold / northern climates: 36–48 inches
- Very cold regions: 48 inches or more
Many northern building departments require 42 inches as a common standard.
How many footings do you need?
Footings sit under the posts that carry your beams. As a rule of thumb:
- Space posts no more than about 8 feet apart on center along each beam.
- Add a footing at each end of every beam.
- Bigger beams and lighter loads can allow wider spacing — your local span tables and code decide.
The Deck Footing & Concrete Calculator works out the count for you from your deck length and post spacing.
How much concrete per footing?
A poured footing is a cylinder, so its volume is:
volume = π × radius² × depth
For example, a 12-inch-diameter footing poured 42 inches deep holds about 2.7 cubic feet of concrete — roughly six 60-lb bags. A standard 60-lb bag yields about 0.45 cubic feet, and there are 27 cubic feet in a cubic yard (about 60 bags). The calculator converts your dimensions into footings, cubic feet, cubic yards, and bags automatically.
Footing tips
- Dig below the frost line, then add a few inches of compacted gravel at the bottom for drainage.
- Use a footing form tube for a clean, consistent cylinder.
- Set anchors, not posts, in concrete — galvanized post bases keep wood off the concrete so it doesn’t wick moisture and rot.
- Buy one or two extra bags for spillage and any over-dig.
When in doubt, ask
Footing requirements are the part of a deck most tied to local code and soil conditions. A quick call to your building department — or pulling the permit — will tell you the exact depth, diameter, and inspection steps for your area before you pour.