Deck footing size and depth: how the IRC table works
A deck footing has two jobs: spread the post’s load over enough soil, and sit deep enough that frost and soft topsoil can’t move it. Here is how to work out both.
1. How much deck does each post carry?
A post carries the tributary area around it: halfway to the neighboring post on each side along the beam, and halfway back toward the house (or the other beam), plus any cantilever past the beam.
tributary area = post spacing × (joist span ÷ 2 + cantilever)
Example: posts 7 ft apart under a beam that carries 12 ft joists from a ledger: 7 × 6 = 42 sq ft.
2. Convert area to load
IRC deck tables use a 40 psf live load (people and furniture) and a 10 psf dead load (the deck itself): 50 psf in total. Our example carries 42 × 50 = 2,100 lb. Hot tubs, roofs, heavy snow and planters are extra loads beyond these tables, and need a design.
3. Divide by the soil’s bearing capacity
Without a soil test, the IRC presumes these allowable bearing values (Table R401.4.1):
| Soil | Presumed bearing |
|---|---|
| Clay, sandy clay, silty clay, clayey silt, silt | 1,500 psf |
| Sand, silty sand, clayey sand, silty or clayey gravel | 2,000 psf |
| Sandy gravel or gravel | 3,000 psf |
| Sedimentary and foliated rock | 4,000 psf |
2,100 lb ÷ 1,500 psf = 1.4 sq ft of bearing. That is a 15″ square or 17″ round footing. These are exactly the steps behind IRC Table R507.3.1. The table also sets minimums of 12″ square or 14″ round.
4. Thickness
An unreinforced footing must be at least as thick as it projects beyond the post, and never less than 6″. A 17″ round footing under a 5½″ post projects (16 − 5½) ÷ 2 = 5¼″, so 6″ thick works.
5. Depth: below frost and on undisturbed soil
- The bottom of the footing must be at least 12″ below undisturbed ground. Fill and loose topsoil don’t count.
- Where the ground freezes, it must be below the local frost depth, which your building department sets. Frost depths run from zero in the far South to 4 ft or more in the northern states.
- A footing that sits above the frost line can heave in winter. A deck attached to the house then racks against the ledger, which is the worst place for movement.
Concrete and forms
A typical approach pours a wide pad at the bottom, then a cardboard tube (8–12″) up to a couple of inches above grade, with a post base set in the top. One-piece bell-shaped plastic forms are faster. Bag yields are printed on the bag. Common figures are 0.60 ft³ for an 80 lb bag, 0.45 ft³ for 60 lb and 0.30 ft³ for 40 lb. The footing calculator does the volume and bag count.