A typical 10x12 deck attached to the house needs 3 posts on 3 footings: one beam along the outer edge, with a post at each end and one in the middle. Build the same deck freestanding and you need 6 posts, because a second beam replaces the ledger. With a heavier beam you can sometimes get down to 2 posts (or 4 freestanding). Smaller beams with tighter post spacing can push the count up.
Below are the counts for each layout, the code tables behind them, and what to check before you dig.
The short answer by layout
All counts below come from our deck framing calculator, with joists at 16” on center, a 12” joist overhang past the beam, two-ply beams and posts at both ends of every beam.
| Layout | Max post spacing | Joists | Beams | Posts per beam | Posts and footings |
|---|---|---|---|---|---|
| 12 ft along house, attached | 8 ft | 10 × 10 ft | 1 | 3 | 3 |
| 12 ft along house, attached | 6 ft | 10 × 10 ft | 1 | 3 | 3 |
| 12 ft along house, attached | 5 ft | 10 × 10 ft | 1 | 4 | 4 |
| 12 ft along house, freestanding | 8 ft or 6 ft | 10 × 10 ft | 2 | 3 | 6 |
| 10 ft along house, attached | 8 ft or 6 ft | 9 × 12 ft | 1 | 3 | 3 |
| 10 ft along house, attached | 10 ft | 9 × 12 ft | 1 | 2 | 2 |
| 10 ft along house, freestanding | 8 ft or 6 ft | 9 × 12 ft | 2 | 3 | 6 |
| 10 ft along house, freestanding | 10 ft | 9 × 12 ft | 2 | 2 | 4 |
Note that 8 ft and 6 ft limits give the same count on a 12 ft beam: either way you need a middle post, so the posts land 6 ft apart. And the spacing column only works if your beam can actually span that far, which is the next section.
The first row is the layout on our 10x12 deck page: 3 posts, 1 beam and 10 joists.
How many posts do I need for a 10x12 deck? It depends on the beam
Posts hold up the beam, and the beam can only span so far between them. The 2021 IRC sets that distance in Table R507.5, based on the beam size and the length of the joists it carries. Here are the southern pine values for a 40 psf live load, as reproduced in Anne Arundel County’s 2021 IRC deck guide:
| Beam (southern pine) | Joist span up to 10 ft | Joist span up to 12 ft |
|---|---|---|
| 2-ply 2×8 | 6’-9” | 6’-2” |
| 2-ply 2×10 | 8’-0” | 7’-4” |
| 2-ply 2×12 | 9’-5” | 8’-7” |
| 3-ply 2×10 | 10’-0” | 9’-2” |
| 3-ply 2×12 | 11’-10” | 10’-9” |
Other species span less. Washington State’s edition of the table, for example, lists a Douglas fir or hem-fir 2-ply 2×8 at 5’-3” for a 10 ft joist span (Island County, WA 2021 deck handout). AWC’s DCA 6 deck guide (Table 3A) gives its own, slightly different spans. Use the table your building department enforces.
Reading the table for a 10x12 deck:
- 3 posts, 6 ft apart, on the 12 ft side. The joists span about 9 ft from ledger to beam, so a 2-ply 2×8 (6’-9”) is enough.
- 3 posts, 5 ft apart, on the 10 ft side. The joists span about 11 ft, which falls in the 12 ft column. A 2-ply 2×8 (6’-2”) still covers 5 ft.
- 2 posts, 10 ft apart, on the 10 ft side. The beam has to span 10 ft with 11 ft joists. Only the 3-ply 2×12 (10’-9”) makes it.
- 2 posts, 12 ft apart, on the 12 ft side. Nothing in the table spans 12 ft, even with the shorter joists, so that layout needs an engineered beam.
The code also lets a beam overhang its end posts by up to a quarter of the span (Anne Arundel County guide). That lets you set the posts in from the corners. Our calculator puts a post at each end of the beam, so check overhang layouts against the table yourself.
Which side runs along the house changes things
“10x12” doesn’t say which way the deck faces. The orientation changes three things:
12 ft along the house, 10 ft out. The beam is 12 ft long and the joists are short. The calculator counts 10 joists cut to about 9’-10-1/2” from 10-ft stock. Short joists make the beam’s job easier, but a 12 ft beam almost always needs a middle post.
10 ft along the house, 12 ft out. The beam is only 10 ft long, but the joists are longer. The calculator counts 9 joists cut to about 11’-10-1/2” from 12-ft stock. With the beam 1 ft in from the edge they span about 11 ft. That fits southern pine 2×8s at 16” on center, which the IRC allows up to 11’-10” (Anne Arundel County guide). Move the beam out to the edge and the 12 ft span needs 2×10s. In return, the short beam is the one layout where 2 posts can work, with a big 3-ply beam.
If you’re torn, the 12-ft-along-the-house layout is the easier build: every part stays small.
Attached (ledger) vs freestanding
On an attached deck, a ledger bolted to the house carries the inner ends of the joists. You need only the outer beam and its posts.
On a freestanding deck, a second beam on its own posts replaces the ledger, usually set a little way from the wall. That doubles the posts and footings, from 3 to 6 in the common layout. You also give up the joist hangers: the calculator drops from 10 hangers to none, and hurricane ties go from 10 to 20, because every joist now sits on two beams.
Freestanding is the usual answer when the house has brick veneer, an overhang, or a rim joist you can’t verify. The ledger vs freestanding guide explains the rules and the trade-offs.
One catch on freestanding decks: the posts near the house often land in loose backfill. DCA 6 says deck footings closer than 5 ft to the house foundation wall must bear at the same elevation as the house footing (AWC DCA 6). That can mean much deeper holes on the house side.
Footings and frost depth
Every post sits on its own footing, so the footing count matches the post count. The size of each footing depends on how much deck area its post carries (its tributary area).
For the common attached layout (12 ft along the house, beam 1 ft in from the edge, posts 6 ft apart), each beam post carries half the 9 ft joist span plus the 1 ft overhang, or 5-1/2 ft of depth:
- Middle post: 6 ft × 5-1/2 ft = 33 sq ft
- Corner posts: 3 ft × 5-1/2 ft = 16-1/2 sq ft each
The 2021 IRC footing table (R507.3.1), for a 50 psf load on 2,000 psf soil, calls for an 11” round footing at 20 sq ft and a 15” round at 40 sq ft (Anne Arundel County guide). A 15” hole in the middle and 12” holes at the corners covers it. DCA 6 is more conservative: its Table 4 assumes weaker 1,500 psf soil and gives bigger footings (AWC DCA 6). Your building department will tell you which soil value to use.
Depth is the number that varies most. The 2021 IRC requires footings at least 12” below undisturbed ground. Where the deck is attached to a frost-protected house, they also have to go below the local frost line (2021 IRC R507.3 via Island County, WA). The frost depth is set locally. Anne Arundel County, Maryland, for example, requires 30” for attached decks (Anne Arundel County guide).
Concrete from the deck footing calculator, poured as full-depth round columns with 10% extra:
| Footings | 36” deep | 48” deep |
|---|---|---|
| Attached: one 15” + two 12” | 9.2 cu ft, 16 × 80 lb bags (21 × 60 lb) | 12.3 cu ft, 21 × 80 lb (28 × 60 lb) |
| Freestanding: two 15” + four 12” | 18.5 cu ft, 31 × 80 lb bags (42 × 60 lb) | 24.6 cu ft, 42 × 80 lb (55 × 60 lb) |
A pad with a narrower pier above it uses less concrete than a full-depth column. The footings and frost depth guide shows both methods.
4×4 or 6×6 posts?
The 2021 IRC sets post size by height and tributary area (Table R507.4). For southern pine at a 40 psf live load, a 4×4 is allowed up to 14’-0” at 20 sq ft and 13’-8” at 40 sq ft. A 6×6 is allowed up to 14’-0” at every area in the table (Anne Arundel County guide). On paper, then, 4×4s are permitted for a 10x12 deck’s roughly 33 sq ft posts.
AWC’s DCA 6 is stricter. It requires 6×6 posts minimum. Its commentary says a 4×4 “would often be overstressed” in deck applications, and that a 4×4 is too small for the notched beam connection, so it needs a post cap instead (AWC DCA 6). DCA 6 also calls for diagonal bracing at corner posts taller than 2 ft.
In practice, 6×6 is the safe default.
Ask your building department
Before you buy posts or rent an auger, call your local building department or download its deck handout. Ask:
- Do I need a permit? Do you inspect footing holes before the pour?
- What is the frost depth here, and the soil bearing value to use for footing size?
- Which beam and post tables do you enforce: the IRC, DCA 6, or a local version?
- Are 4×4 posts accepted, or do you require 6×6?
- For a ledger deck, what are the house-attachment and lateral connection rules?
Their answers can change the post count, hole size and depth.
Frequently asked questions
How many posts does a 10x12 deck need?
Three posts on three footings for a typical deck attached to the house with one beam on the 12 ft side. A freestanding 10x12 deck needs a second beam, so six posts. With the 10 ft side on the house and a 3-ply 2×12 southern pine beam, two posts 10 ft apart can work under the IRC beam table.
How far apart should deck posts be on a 10x12 deck?
As far as your beam can span under your local beam table, and no farther. In southern pine at 40 psf, a 2-ply 2×8 beam carrying joists up to 10 ft long spans 6’-9”. That covers the 6 ft spacing of a 3-post, 12 ft beam. Longer post spacing needs a bigger beam.
Can I use 4×4 posts for a 10x12 deck?
The 2021 IRC post table allows southern pine 4×4s up to about 13’-8” tall at 40 sq ft of tributary area. AWC’s DCA 6 requires 6×6 minimum, though, and many building departments follow it. Check locally. 6×6 is the common choice.
How deep should footings be for a 10x12 deck?
At least 12” below undisturbed soil under the 2021 IRC. For an attached deck, they also go below your local frost line, which your building department sets: 30” in one Maryland county, deeper in colder areas.
Run your own layout, with your beam, spacing and attachment, in the deck framing calculator.