Deck post spacing by beam size
Posts can be as far apart as the beam can span: about 6 to 7 ft for a 2-ply 2x10 under 12 ft joists and 8 to 10 ft for a 3-ply 2x12. Longer joists put more deck on the beam, so the posts move closer. These spans come from this calculator, for No. 2 wet-service lumber and a joist overhang of a quarter of the joist span:
| Beam | 8 ft joists | 10 ft | 12 ft | 14 ft |
|---|---|---|---|---|
| 2-ply 2x8 | 7-2 | 6-5 | 5-11 | 5-5 |
| 2-ply 2x10 | 8-7 | 7-8 | 7-0 | 6-6 |
| 2-ply 2x12 | 10-1 | 9-0 | 8-3 | 7-7 |
| 3-ply 2x8 | 8-10 | 7-11 | 7-2 | 6-8 |
| 3-ply 2x10 | 10-6 | 9-4 | 8-7 | 7-11 |
| 3-ply 2x12 | 12-4 | 11-0 | 10-1 | 9-4 |
| Beam | 8 ft joists | 10 ft | 12 ft | 14 ft |
|---|---|---|---|---|
| 2-ply 2x8 | 6-9 | 6-1 | 5-6 | 5-1 |
| 2-ply 2x10 | 8-3 | 7-5 | 6-9 | 6-3 |
| 2-ply 2x12 | 9-7 | 8-7 | 7-10 | 7-3 |
| 3-ply 2x8 | 8-3 | 7-5 | 6-9 | 6-3 |
| 3-ply 2x10 | 10-1 | 9-1 | 8-3 | 7-8 |
| 3-ply 2x12 | 11-9 | 10-6 | 9-7 | 8-10 |
Set the joist span first with the deck joist span calculator; the full frame is on the deck framing calculator.
Deck beam span: how it is worked out
A deck beam span is the distance between post centres that the beam can carry, and it depends on how much deck rests on it. The beam carries the joist reaction: with joist span L and overhang a, each foot of beam holds 50 psf × (L + a)² ÷ 2L, which is 0.78 L when the overhang is L/4. The beam is then checked like a joist[5]:
- Bending with the No. 2 design value, wet-service factors and incising for DF-L, hem-fir and SPF; built-up beams get no repetitive-member bonus.
- Deflection limited to span ÷ 360 under the 40 psf live load.
- Shear, which rarely governs.
- DCA 6 caps beam spans at 18 ft and lets the beam run past its end posts by up to a quarter of the beam span[1].
| Beam | Joist span | DCA 6 | Calculator |
|---|---|---|---|
| 2-ply 2x8 | 10 ft | 6-6 | 6-5 |
| 3-ply 2x10 | 12 ft | 8-9 | 8-7 |
The calculator lands an inch or two short of DCA 6, on the safe side, because it applies the full joist reaction. Use DCA 6 Table 3A, or the beam table in your adopted code, as the final word.
How many posts does a deck need?
A deck needs one post per beam span plus one: posts = (beam length − 2 × end overhang) ÷ allowed spacing, rounded up, + 1. A 20 ft beam with 1 ft overhangs and a 7 ft 0 in allowed span needs (240 − 24) ÷ 84 = 2.57, rounded up to 4 spaces, so 4 posts at 6 ft. A ledger-attached deck has posts only along the beam; the house carries the other end of the joists.
Deck footing calculator: footing size for each post
A deck footing must spread the post load over enough soil: footing area = post load ÷ (soil bearing capacity − the footing's own weight). DCA 6 assumes 1,500 psf soil, the presumptive value for clay and silt, 150 pcf concrete and 2,500 psi concrete strength[1][4]. In DCA 6's example of an 8 ft beam span with joists up to 14 ft, the footing is 24 in round or 21 in square and 10 in thick. The calculator gives 25 in round or 22 in square for the same deck, again slightly larger because of the full joist reaction.
- Inner posts carry a full beam span; end posts carry half a span plus the beam overhang, so their footings are smaller.
- Better soil (sand or gravel at 2,000 to 3,000 psf) allows smaller footings. Choose it in the tool.
- Corner posts may use footings 0.9 times the tabulated size under DCA 6.
Post size, height and footing depth
Deck posts must be at least 6x6, and footings must bear on undisturbed soil at least 12 in below grade and below the frost line, whichever is deeper[1][2].
- Post height: DCA 6 Table 4 allows 6x6 posts up to 14 ft, less for weaker species: with an 8 ft beam span and joists up to 14 ft, 14 ft in southern pine but 11 ft in ponderosa pine, red pine or SPF.
- Near the house: footings closer than 5 ft to the house foundation must bear at the same depth as the house footing.
- Connections: notch the beam into the post or use a post cap; 3-ply beams need a post cap. Corner posts over 2 ft tall get diagonal bracing.
- Stairs and guards: stringers and guard posts have their own rules; see the deck stair calculator and deck railing height.
Planning numbersBeams, posts and footings are where decks fail. Confirm sizes with DCA 6, your building department's deck guide or an engineer before you dig.
Frequently asked questions
How far apart should deck posts be?
As far as the beam can span, usually 6 to 8 ft. A 2-ply southern pine 2x10 under 12 ft joists allows 7 ft 0 in; a 3-ply 2x12 allows 10 ft 1 in.
How many posts for a 12 × 20 ft deck?
With 12 ft joists and a 2-ply southern pine 2x10 beam along the 20 ft side, 4 posts at 6 ft centres, plus the ledger on the house.
Can deck posts be 10 feet apart?
Yes, with a heavy beam and short joists: a 3-ply southern pine 2x12 under 10 ft joists spans 11 ft 0 in.
How big should deck footings be?
Post load ÷ soil bearing. On 1,500 psf soil, the 4-post 12 × 20 ft deck needs about 18 in square (20 in round) under each inner post.
How deep should deck footings be?
At least 12 in below undisturbed grade and below the local frost line, whichever is deeper; at the house footing depth within 5 ft of the house.
Sources
- DCA 6: Prescriptive Residential Wood Deck Construction Guide (2015 IRC basis)
- 2021 International Residential Code, Chapter 5: Floors (R502 joists, R507 decks)
- 2024 International Residential Code, Chapter 5: Floors (R502.1.2, R502.3, R507.6)
- 2021 International Residential Code, Chapter 4: Foundations (R401.4 soil bearing, R403.1 footings)
- 2018 National Design Specification for Wood Construction, Chapter 4: Sawn Lumber (adjustment factors)