Deck Post Spacing: How Far Apart Should Posts Be?

"How far apart do deck posts go?" is one of the most searched deck questions, and most of the answers online are a rule of thumb rather than the code. The reason is that the IRC never publishes a post-spacing table. It publishes a beam-span table, and the posts are simply placed under the beam — one at each end and one at every point the beam needs support. So the honest way to answer the question is to follow the load path down: the decking sets the joist spacing, the joist span sets the beam span, and the beam span sets the post spacing.

The short answer — post spacing is your beam span

The distance between deck posts is the maximum span the beam they carry is allowed, straight out of Table R507.5(1) of the IRC for a 40 psf live load. That span depends on three things: the species group, the number of plies, and the size of the beam — and on one more that people miss, the effective joist span, which is the actual joist span scaled by a factor (0.66 for a plain deck with no joist overhang) from Table R507.5(5).

The table below is that lookup for Southern pine at the four joist spans a typical deck actually falls into. The cell is the maximum distance between posts, in feet-inches — 9-0 means 9 ft 0 in.

Read a row and a column and you have the spacing. A doubled 2×10 beam on a deck whose joists enter the table at an 8 ft effective span gives 9-0 — so the posts may be 9 ft on centre. Add a ply (3-ply 2×10) and the same column gives 11-2, or about 11 ft 2 in. Nothing about the posts changed; only the beam did, and the spacing followed it. The full beam chart — all three species groups, 1-, 2- and 3-ply, every column from 6 ft to 18 ft — is on the deck beam span chart.

Where the "8 feet apart" rule of thumb comes from

The number that dominates the internet — never space deck posts more than 8 ft apart — is not in the code. It is a carpenter's convention that happens to be safe for the most common deck: a doubled 2×8 or 2×10 beam on a fairly shallow deck. Look at the table above. A 2-ply 2×8 holds 8-9 at a 6 ft effective joist span, which is a 10 ft joist span with no overhang (10 × 0.66 = 6.6 ft). That is the deck the rule of thumb was built for.

The danger is treating a convention as a limit in both directions. Go the wrong way and 8 ft is too far: on a deck with longer joists, a 2-ply 2×8 beam allows only 7-7 at an 8 ft effective span, so 8 ft spacing would exceed the beam. Go the other way and 8 ft is needlessly tight: a 3-ply 2×10 clears 11-2, so posts 11 ft apart are legal and save concrete and labour. The rule of thumb is a starting point, not the rule.

Post size and maximum height — Table R507.4

Once the spacing is set, the post itself has to be tall enough to hold the beam at deck height and strong enough not to buckle. IRC 2021 Table R507.4 answers both from three inputs: the species group, the post size (4×4, 4×6, 6×6 or 8×8), and the tributary area — the deck area that one post and its footing carry. Height is measured from the underside of the beam down to the top of the footing or pier, with a 10 psf dead load assumed and a No. 2 grade, wet-service basis.

Two things fall out of it. First, a 6×6 post in Southern pine or in Douglas fir-larch / hem-fir / spruce-pine-fir is good to the full 14 ft at every tributary area in the table — which is why 6×6 is the default on almost every deck. Second, the 4×4 is the one that runs out: a Southern pine 4×4 drops to 6-2 at a 160 ft² tributary area, and a Douglas fir-larch / hem-fir / spruce-pine-fir 4×4 to 5-3, while a redwood or cedar 4×4 is not permitted at all past a 100 ft² tributary area. If you are building anything taller than a garden step, 6×6 is the cheap insurance. Note also that a notched post loses capacity at the notch and must be sized to carry the beam it receives (R507.5.2), so set the beam on top of the post or notch it deliberately, not accidentally.

The footing under each post — Table R507.3.1

A post is only as good as the pad it stands on. Table R507.3.1 sizes the footing from the same tributary area that sized the post, against the allowable soil bearing from Table R401.4.1. For a deck with joists running from the ledger to a single beam, the tributary area for an interior post is (half the joist span) × (the post spacing) — because the beam picks up one end of each joist, so it carries half of the joist span, not the whole deck depth.

Footing thickness runs 6 in for the smaller pads and steps up as the area grows — the 100 ft² row at 1,500 psf soil is an 8 in thick pad — so check the thickness column in the code table, not just the footprint. Clay and silty soils sit in the 1,500 psf column, sand and gravelly soils in the 2,000 psf column, and clean gravel or rock at 3,000 psf and up. If you are unsure what you have, 1,500 psf is the conservative assumption. The deck footing calculator runs this lookup for any tributary area, including the layout mode that computes the area for you.

The post-to-footing connection — R507.4.1

A deck fails at its connections more often than at its members, and the base of the post is a favourite spot. IRC R507.4.1 requires that where posts bear on concrete footings, lateral restraint comes from either a manufactured post base connector or a minimum of 12 inches of post embedment in the surrounding soil or concrete pier. The exception matters: where expansive, compressible or otherwise questionable soils are present, you may not rely on surrounding soil for the restraint at all — use a connector. A post sitting loose on a pad, held only by gravity, is the failure the section exists to prevent.

Worked example — a 15 ft beam, 10 ft joists, checked by hand

A Southern pine deck 15 ft long along the house and 10 ft deep, with a ledger on the house and one beam out at the far edge. Joists are 2×10 at 16 in on centre, spanning the 10 ft from the ledger to the beam, with the beam flush at the outer edge — no joist overhang.

• Joist check. SP 2×10 at 16 in on centre spans 14-0 in Table R507.6, so a 10 ft joist is fine.
• Effective joist span. No joist cantilever means C/J = 0 and a Table R507.5(5) factor of 0.66: 10 × 0.66 = 6.6 ft.
• Post spacing from the beam. A doubled 2×10 Southern pine beam reads 10-4 at the 6 ft column and 9-0 at the 8 ft column. Interpolating at 6.6 ft: 10-4 minus 0.6 × (10-4 − 9-0) ÷ 2 = 10.33 − 0.4 = 9.93 ft, about 9 ft 11 in.
• Post count. 15 ft ÷ 9.93 ft = 1.51 → two bays of 7 ft 6 in need three posts, and 7 ft 6 in is inside the 9 ft 11 in limit.
• Post size. The tributary area for an interior post is 5 ft (half the joist span) × 7.5 ft = 37.5 ft², read as the 40 ft² row. A 6×6 Southern pine post is good to 14 ft there, so it carries the deck height easily.
• Footing. The 40 ft² row at 2,000 psf soil is a 12 in square or 14 in round pad, 6 in thick; at 1,500 psf soil it is 14 in square or 16 in round. Bear it at least 12 in below undisturbed grade (R507.3.2), and to the local frost depth because the deck is attached to the house (R507.3.3).

Every step is one table cell or one multiplication, so you can check the whole thing with a pencil — which is the point. If you want a wider spacing, the lever is the beam: a 3-ply 2×10 reaches 11-2 at the 8 ft column and would let two posts cover the 15 ft.

Common mistakes with deck post spacing

• Believing there is a code post-spacing number. The IRC does not publish one. Post spacing is a consequence of the beam span, so it changes the moment the beam does.
• Using the raw joist span in the beam table. The beam tables take the effective joist span (Table R507.5(5); 0.66 with no joist overhang). Reading the raw span sends you to a longer column and a shorter than necessary beam.
• Sizing the footing from the post size. Footing size follows the tributary area and the soil, not the post. A 6×6 post on a big tributary area can still need a wide pad.
• Mixing up railing posts and support posts. Railing (guard) posts are sized by the guard load — 200 lb concentrated per IRC Table R301.5 — and their spacing comes from the guard system and its infill, not from the beam span.
• Setting the posts at the very ends of the beam. The beam may cantilever up to one quarter of its span past the end post (R507.5), so the outer posts can move inboard and the beam can overhang — a common way to reduce the number of holes to dig.
• Notching the post and ignoring it. A notched post must be sized to accommodate the beam it carries (R507.5.2), and R507.10.2 forbids notching a 4×4 post where it carries guard loads.
• Trusting the 8 ft rule on a deep deck. On long joists the effective span rises and a small beam no longer reaches 8 ft. Check the table; do not assume.

Where these numbers came from

Table R507.5(1) and Table R507.5(5) were last re-verified on 2026-10-09 against the up.codes 2021 IRC chapter 5 viewer (as adopted in Connecticut) and a second ICC-embedded reproduction at advantagelumber.com, which agree on all 36 rows. Table R507.4 and its footnotes (height measured from the underside of the beam, 10 psf dead load, No. 2 grade wet-service, tributary area, interpolation permitted and extrapolation not) were fetched and parsed on 2026-10-12 from the same up.codes chapter-5 viewer in two jurisdictions (Connecticut and Virginia, identical) and confirmed against the ICC digital-code snippet for R507.4. Table R507.3.1 was verified cell-by-cell on 2026-09-02 against the same viewers. R507.4.1 and the guard load in Table R301.5 are read verbatim from the chapter 3 and chapter 5 pages fetched on 2026-10-12. The IRC is adopted state by state and amended locally, so check your own edition before you build — the authority having jurisdiction always wins.

This is a working reference, not engineering. A table lookup is the right answer only while your deck sits inside the table — inside the span, the load and the conditions. Anything outside it, a second storey, a roof over the deck, or unusual soils, needs a designer or an engineer.

Deck post spacing FAQ

Related pages

• Deck beam span chart — IRC Table R507.5(1) — the full beam table behind every post-spacing number here
• Deck joist span table — IRC Table R507.6 — sizes the joists that set the beam load, with the maximum-cantilever column
• Deck footing calculator — footing size from tributary area, and the frost-depth rule
• Deck beam & joist span calculator — both span tables applied to one layout, including the R507.5(5) factor
• Deck stairs guide — step count, stringer span and the footing at the bottom of the run
• Deck board spacing guide — the decking and its gaps, which set the maximum joist spacing
• Reference tables — every code table on the site, with its source and verification date