Deck Footing Requirements Explained: Size, Depth and Spacing (IRC)
In short: deck footings must be concrete (or another approved system), sized for the load they carry and your soil, with the bottom at least 12 inches below undisturbed ground and below your local frost line. How far apart they go depends on the beam size. Posts must be held at the bottom so they cannot slide off the footing.
1. How big should deck footings be?
A footing must spread the post load over enough soil that the soil is not overloaded. The IRC gives a minimum footing size table for decks (Table R507.3.1) based on how much deck area each post carries and the soil type. The math behind it is simple:
Footing area = load on the post ÷ soil bearing capacity
| Soil type (IRC Table R401.4.1) | Allowed bearing pressure |
|---|---|
| Clay, silt, sandy clay | 1,500 psf |
| Sand, silty sand, clayey sand, silty or clayey gravel | 2,000 psf |
| Sandy gravel, gravel | 3,000 psf |
If you do not know your soil, use 1,500 psf. Run your numbers in the deck footing size calculator.
2. How deep should deck footings be?
- At least 12 inches below undisturbed ground, everywhere.
- Below the frost line in cold areas, so freezing soil cannot lift the footing. Frost depth ranges from a few inches in the South to 4 ft or more in the northern states. Your building department publishes the local number.
- On undisturbed soil, not loose fill. Near the house, the soil was dug out and backfilled when the foundation was built, so footings there often need to go deeper.
Freestanding deck exceptions
Recent editions of the IRC let some freestanding decks (not attached to the house) skip frost depth, for example when joists bear directly on precast pier blocks at grade, the deck is no more than 200 sq ft, and the walking surface is no more than 20 inches above the ground. Check the exact wording your area has adopted before relying on it.
3. How far apart should deck footings be?
Footing spacing is the beam span: the distance between posts along the beam. Bigger beams can span further, so they need fewer posts and footings. Typical residential decks have posts about 6 to 8 ft apart. Fewer posts means each one carries more load, so the footings must get bigger.
| Post spacing (12 ft joists, 1,500 psf soil) | Load per post | Round footing needed |
|---|---|---|
| 6 ft | 1,800 lb | 16" |
| 8 ft | 2,400 lb | 18" |
| 10 ft | 3,000 lb | 20" |
4. How must posts attach to footings?
The post must be held so it cannot slide or lift off the footing. The usual way is a manufactured galvanized post base anchored into the concrete, which also keeps the wood off the wet concrete. Burying wood posts in concrete is allowed in some places but makes them rot faster.
Common questions
Can I use deck blocks instead of footings?
Only for small, low, freestanding decks that meet the exceptions above. Attached decks need footings below the frost line.
Do I need a footing under every post?
Yes. Every post needs its own footing sized for the load it carries.
How much concrete do footings need?
Use the concrete for post holes calculator for the pier and the footing calculator for the pad.
Written and checked by: Engr. Abdul Latif, civil and structural engineer. Based on IRC Sections R403, R401.4 and R507.3 and the AWC DCA 6 deck guide. For guidance only; your local code and building official have the final say.