Joist & Beam Spans

How far a joist or beam actually spans, by species, size and spacing — from IRC Table R507.

Deck joist spans come from IRC Table R507.5, and the table is written around four assumptions that all have to hold: 40 psf live load plus 10 psf dead, No. 2 grade lumber, wet service conditions, and no cantilever.

Break one and the table no longer applies. A hot tub, a heavy planter wall, a roof over the deck or a high-snow jurisdiction all put you outside it, and outside it the answer comes from an engineer rather than a chart.

Species is worth about a foot

A 2×10 at 16″ on center spans 14′ 0″ in Southern Pine and 13′ 0″ in redwood or cedar. Same size, same spacing, same code — a foot of difference that nobody mentions when you buy whatever the yard has in stock.

Douglas Fir-Larch, Hem-Fir and SPF share a column in the table and sit between the two.

Spacing is the expensive way to buy span

Halving the spacing from 24″ to 12″ doubles the number of joists and buys you a couple of feet. Going up one size — a 2×8 to a 2×10 — usually buys more for less money and less labor.

Work out which you need before you order, because changing your mind after the lumber is delivered is expensive in both directions.

Cantilever is a quarter of the actual span

Not a quarter of the maximum. If the table allows 16 ft and your joists run 10, the overhang is 2½ ft, not 4. This is the single most misread line in the deck section of the code.

The ledger is what actually fails

Span tables get the attention; ledger attachment kills people. Most deck collapses are ledger failures — nailed instead of bolted, or fastened through rotten sheathing into nothing. IRC R507.9 specifies the fastener pattern, and flashing above the ledger is what stops the rot that undoes it.

The inspector decides

Local codes amend the IRC and some jurisdictions use higher live loads. Pull the permit before you buy lumber, not after.

6 articles

How far can a Double (2-ply) 2 × 8 beam span carrying 12 ft joists?

A double 2x8 Southern Pine beam carrying 12-ft joists

5′ 11″between posts

AWC DCA 6 Table 3A / IRC R507.5 — 40 psf live, 10 psf dead, No. 2 grade, wet service

Maximum beam span, post to post	5′ 11″
Maximum overhang past the postone quarter of the actual span, not of this maximum	1′ 6″
Posts needed across a 20 ft side	5posts
Posts needed across a 30 ft side	7posts
Species groupthe strongest of the common deck species and the usual choice in the south-east; it gets its own column in DCA 6	Southern Pine
— Same size, other ply counts —
Double 2x8the beam on this page	5′ 11″
Triple 2x8	7′ 6″
— Other sizes, double —
double 2x6	4′ 7″
double 2x8	5′ 11″
double 2x10	7′ 1″
double 2x12	8′ 4″
— Same beam, other joist spans —
Joists spanning 6 ft	8′ 6″
Joists spanning 8 ft	7′ 4″
Joists spanning 10 ft	6′ 6″
Joists spanning 12 ft	5′ 11″
Joists spanning 14 ft	5′ 6″
Joists spanning 16 ft	5′ 1″
Joists spanning 18 ft	4′ 9″

Notes

The column is the joist span, not the load width. This page answers for joists that span 12 feet. A common mistake is to halve the joist span first because the beam only carries half of it — the table has already done that. Read the column that matches the joists you are actually building.
Adding a ply is usually cheaper than going up a size. A triple 2x8 reaches 7′ 6″ against 5′ 11″ for the double — 1′ 7″ more span for one extra board. Going up to a double 2x10 instead reaches 7′ 1″.
Species is worth a foot. The same beam is 5′ 11″ in Southern Pine and 4′ 7″ in the Douglas Fir / cedar group. Buying whatever the yard has in stock quietly changes the post spacing you are allowed.
Joists from both sides double the column. DCA 6 assumes joists bear from one side only. If joists frame in symmetrically from opposite sides of the same beam, look up twice the joist span — 12 ft joists from both sides means reading the beyond the 18 ft column, which is off the table and needs a designer.
Four assumptions, and all four have to hold. 40 psf live plus 10 psf dead, No. 2 grade, wet service, single level. A hot tub, a roof over the deck, a planter wall, or a high-snow jurisdiction all put you outside this table entirely.
The building official has the last word. Local amendments to the IRC are common and are frequently stricter. Pull the permit before buying lumber — a beam sized from the wrong column fails with people standing on it.

Open the full page for this calculation →

Built-up beam span by ply, size and joist span

112 calculations · browse all

Deck joist span chart by size

4 calculations · browse all

Deck joist span

36 calculations · browse all

Deck Beam Span Table: Why the Span Depends on the Joists, Not the Deck
A beam span table has two inputs, and the one people forget is the joist span the beam is carrying. Read it with one input and the answer is wrong by feet.
Deck Post Size by Height: When 4×4 Stops Being Allowed
The 8 ft line is where 4×4 stops. Above it the question is not only size but bracing, and the connection at each end matters more than the post between them.
How to Tell if a Deck Is Unsafe: The Seven Checks, in Order of What Kills People
Deck collapses are almost never a broken board. They are the ledger letting go, all at once, with everyone standing on it.
Deck Joist Cantilever: One Quarter of the Actual Span, Not the Maximum
This is the most misread line in the deck code, and it is misread in the direction that puts the beam too far in.
Ledger Attachment: The Detail That Causes Nearly Every Deck Collapse
Span tables get the attention. Ledger failure is what actually drops decks, and it is almost always one of four specific mistakes.
Deck Joist Span Tables: The Four Assumptions That Have to Hold
The span table is not a rule about lumber. It is a rule about lumber carrying 40 psf, dry, at No. 2 grade, with no cantilever, and any of those can be false on your deck.