A deck footing spreads the load from a post into the ground. Its size depends on how much load that post carries and how strong the soil is — bigger loads or weaker soil need bigger footings.
What drives the size
- Tributary area — the portion of the deck a post supports. A post carrying a large area carries more load and needs a larger footing.
- Total load — the deck’s weight (dead load) plus people and furnishings (live load), often summed to a design load per square foot.
- Soil bearing capacity — how much weight the soil supports per square foot; sandy or weak soils need wider footings than firm soils.
- Frost depth — footings must extend below the local frost line so freeze-thaw doesn’t heave the deck.
Conceptually, multiply the tributary area by the design load to get the load on the post, then size the footing’s bearing area so the soil isn’t overloaded. Codes and prescriptive deck guides (like the IRC deck provisions) provide footing-size tables for common cases — use them, and confirm with your building department.
Frequently asked questions
What determines deck footing size? The load each post carries and the soil’s bearing capacity.
How deep do footings go? Below the local frost line, per code.
Can I size footings from an article? Use code tables and confirm with your inspector — it’s structural.