What does the building code say about deck joist spans, posts and footings?

What R507 and DCA 6 cover

IRC Section R507 is a prescriptive recipe for wood-framed decks: build to its tables instead of designing the deck. They assume a 40 psf live load, or the ground snow load where greater (R507.1, Table R301.5), No. 2 lumber or better, treated or naturally durable, and galvanized or stainless fasteners (R507.2). The joist, post, footing and ledger figures here read the same in the 2021 and 2024 editions.

The American Wood Council's free DCA 6, Prescriptive Residential Wood Deck Construction Guide, is based on the 2015 IRC (2018 printing). It covers single-level decks attached to the house, snow loads up to 40 psf and no hot tubs, and in places asks for more than the 2021 and 2024 IRC: 6x6 posts at least, and shorter beam spans. It says the IRC governs where the two differ.

Deck joist spans and spacing

Joist span is measured between the faces of the supports, ledger to beam, not counting any overhang. Table R507.6, at 40 psf with No. 2 lumber, in feet and inches:

The decking caps the spacing too (Table R507.7): 5/4 wood boards laid square to the joists need 16 in centres where each board crosses three or more joists (12 in for diagonal boards), and 2 in boards allow 24 in. Composite boards follow the span on their label (R507.2.2).

How far apart can deck posts be?

The IRC gives no single post spacing: posts sit one beam span apart, and the beam span comes from Table R507.5(1) for the beam, the species and the joists it carries. For 12 ft joists with no overhang at 40 psf:

The 2021 IRC reads the beam table from an effective joist span (0.66 times the span with no overhang), the 2024 IRC from columns of actual span and overhang; here they agree. DCA 6's spans are shorter because its tables come from the 2015 IRC. A beam may overhang its end post by up to a quarter of its span (R507.5; R507.5.1 in 2024).

Posts and footings

• Depth: at least 12 in (305 mm) below undisturbed ground (R507.3.2), and for a deck attached to a house with frost-protected footings, below the frost line your jurisdiction sets in Table R301.2 (R507.3.3). DCA 6 adds that a footing within 5 ft of the house foundation bears as deep as the house footing.
• Size: Table R507.3.1, by the deck area each post carries and the soil. On 1,500 psf soil, the value the IRC presumes for clay and silt, at 40 psf: 40 sq ft needs a 16 in round or 14 in square footing, 60 sq ft 19 in round or 17 in square, 6 in thick in plain concrete.
• Posts: Table R507.4 limits height, underside of beam to top of footing. At 40 psf a southern pine 4x4 may be 13' 8" tall carrying 40 sq ft and 11' 0" at 60 sq ft; a 6x6 may be 14 ft up to 160 sq ft. The post needs an approved connector or 12 in of embedment to hold it sideways (R507.4.1).

Ledger attachment and lateral load

The ledger is at least a 2x8, treated or naturally durable, fixed to a house band joist of 2 in nominal lumber or 1 in engineered rim board, never to brick or stone veneer, and it may not carry a beam (R507.9.1.1, R507.9.1.2). Table R507.9.1.3(1) spaces 1/2 in lag screws or through-bolts, staggered, by joist span:

The 2024 IRC adds ledger flashing at least 2 in up the wall (R507.9.1.5). For lateral load, the IRC details use hold-downs of at least 1,500 lb within 24 in of each end of the deck, or four of 750 lb (R507.9.2).

Worked example: a 12 x 16 ft deck on a ledger

16 ft (192 in, 4877 mm) along the house on a ledger, joists running 12 ft (3658 mm) out to a beam on three posts, joists at 16 in (406 mm) centres, southern pine, 40 psf, no overhangs, 1,500 psf soil.

1. Joists: 192 ÷ 16 = 12 spaces, plus one for the far end: 13 joists, both outside joists included, plus the ledger and a 16 ft rim joist across their ends.
2. Joist size: at 16 in a 2x8 spans 11' 10", short of 12 ft ✗; a 2x10 spans 14' 0" ✓.
3. Ledger: a 2x10, as deep as the joists as DCA 6 asks, with 1/2 in lag screws every 15 in (381 mm) or through-bolts every 29 in (737 mm) for a 12 ft joist span.
4. Beam: posts at 0, 8 and 16 ft make 8 ft beam spans. A southern pine 2-2x10 spans 9' 0" under the 2021 and 2024 IRC ✓, but 7' 1" under DCA 6 ✗, so a DCA 6 deck takes a 2-2x12 (8' 4") or a 3-2x10 (8' 9").
5. Area per post: the beam carries half the joist span, 6 ft. Middle post: 6 × 8 = 48 sq ft (4.5 m²); end posts: 6 × 4 = 24 sq ft.
6. Footings: reading the next row up (interpolation is also allowed), the middle post takes the 60 sq ft row, 19 in round or 17 in square, and the end posts the 40 sq ft row, 16 in round or 14 in square, all 6 in thick and below your frost line. DCA 6 Table 4, which assumes full overhangs, asks for 24 in round footings 10 in thick.
7. Posts: in the 60 sq ft column a southern pine 4x4 may be up to 11' 0" tall and a 6x6 up to 14 ft; DCA 6 requires 6x6.

Guards and stairs on a deck

A deck more than 30 in above grade within 36 in of its edge needs a guard at least 36 in high with no opening a 4 in sphere can pass (see the guards page). Guard loads must reach the joists, and 4x4 guard posts must not be notched at the deck (R507.10). DCA 6 shows 4x4 guard posts on 2x8 or deeper joists with hold-downs of at least 1,800 lb for a 36 in guard.

Deck stairs follow the house rules: 7 3/4 in risers, 10 in treads and a handrail at four risers (see the stairs page). DCA 6 adds 2x12 stringers, at least three where only cut stringers are used, and a landing once a stair rises more than 12 ft. The 2024 IRC lets a deck stair to grade of three risers or fewer end on a bottom landing just 36 in wide, unless it is the egress route (R318.7.6).

Counting and marking out in MeisterCalc

MeisterCalc has no span tables; it does the counting and layout. In feet and inches, Stud Counter with Wall Length 16', On-Centre Spacing 16" and Stud Height 12' gives Number of Studs 13 (read studs as joists) and Total Linear 156' 0". Running Measurements with Start 0", Interval 16" and Count 12 lists the joist centres for the ledger and rim, 1' 4", 2' 8" and on to 16' 0", and Speak reads them out. Both come with Pro (Stud Counter, Running Measurements). Concrete, which is free, works out footing volumes, and the free concrete calculator turns post holes into cubic yards and 80, 60 or 40 lb bags.

Before you build

Local rules differ most for decks: frost depth, snow load, permits, guards, and whether DCA 6 is accepted. These are the model IRC and DCA 6 figures; check the edition and amendments with your local building department and get the permit. DCA 6 has the building official check footing holes before the concrete goes in. Neither this page nor the app makes a deck pass inspection.

Sources

Checked on 2 November 2026. These pages explain the rules; the code itself, your plans and your council, building consent authority, building control body or certifier decide what a job needs.

• 2021 IRC, Chapter 5 Floors (ICC): R507 decks: R507.3 footings and Table R507.3.1, Table R507.4 posts, Table R507.5(1) and R507.5(5) beams, Table R507.6 joists, Table R507.7 decking, R507.9 ledgers and lateral load, R507.10 guards (checked 1 November 2026)
• 2024 IRC, Chapter 5 Floors (ICC): R507: the same joist, post, footing and ledger tables; beam table by joist span and overhang; R507.9.1.5 ledger flashing added (checked 1 November 2026)
• 2024 IRC, Chapter 3 Building Planning (ICC): Table R301.2 frost line depth set by the jurisdiction, Table R301.5 deck live load, R318.7.6 landing exception, R321 guards (checked 1 November 2026)
• 2024 IRC, Chapter 4 Foundations (ICC): Table R401.4.1(1): 1,500 psf presumed for clay and silt soils (checked 2 November 2026)
• American Wood Council, DCA 6 Prescriptive Residential Wood Deck Construction Guide (PDF): based on the 2015 IRC, 2018 printing: Tables 2, 3A, 4 and 5, 6x6 posts, guard posts, stairs, footings near the house (checked 1 November 2026)

Do the maths in MeisterCalc

MeisterCalc does the arithmetic behind these rules on an Android phone, with no signal needed: Stairs, Concrete and Check Square are free, and the other calculators come with Pro. It runs on Android 7.0 or later.

Related

• Guards and baluster spacing under the IRC
• IRC stair rules: risers, treads and headroom
• How many joists do I need for a deck or floor?
• How do you space deck posts evenly?
• Concrete calculator in cubic yards and bags
• How do I square a deck frame?