Deck joists on a beam, before the boards go down

Reference

Deck joist and beam span tables

How far a deck joist can reach, how far the beam under it can run between posts, how far a joist can hang past that beam, and how closely the ledger has to be bolted to the house. Every figure is from the 2021 International Residential Code, Tables R507.5(1), R507.6, R507.7 and R507.9.1.3(1), 40 psf live load.

Joist span table

The span is the clear distance a joist runs between its supports — ledger to beam, or beam to beam. Closer spacing buys span, because each joist carries a narrower strip of deck. Read the species first: the same size in redwood reaches less than it does in southern pine.

Species	Joist	12 in on center	16 in on center	24 in on center
Southern pine	2 × 6	9′-11″	9′-0″	7′-7″
	2 × 8	13′-1″	11′-10″	9′-8″
	2 × 10	16′-2″	14′-0″	11′-5″
	2 × 12	18′-0″	16′-6″	13′-6″
Douglas fir-larch, hem-fir, spruce-pine-fir	2 × 6	9′-6″	8′-4″	6′-10″
	2 × 8	12′-6″	11′-1″	9′-1″
	2 × 10	15′-8″	13′-7″	11′-1″
	2 × 12	18′-0″	15′-9″	12′-10″
Redwood, western cedars, ponderosa pine, red pine	2 × 6	8′-10″	8′-0″	6′-10″
	2 × 8	11′-8″	10′-7″	8′-8″
	2 × 10	14′-11″	13′-0″	10′-7″
	2 × 12	17′-5″	15′-1″	12′-4″

Sixteen inches on center is the column most decks are built from, and the one the deck calculator opens with. A 12 ft deck ledgered to a house in southern pine needs a 2 × 10 there: the 2 × 8 stops at 11′-10″.

Deck beam span chart

A beam carries half of every joist that lands on it, so the longer the joists, the heavier the beam's load and the closer its posts have to be. Find the joist span along the top, then the beam down the side: the figure is the most the beam may run from one post to the next. A joist span between two columns is read in the next column up.

Southern pine

Beam	6 ft joists	8 ft joists	10 ft joists	12 ft joists	14 ft joists	16 ft joists	18 ft joists
Two 2 × 6	6′-11″	5′-11″	5′-4″	4′-10″	4′-6″	4′-3″	4′-0″
Two 2 × 8	8′-9″	7′-7″	6′-9″	6′-2″	5′-9″	5′-4″	5′-0″
Two 2 × 10	10′-4″	9′-0″	8′-0″	7′-4″	6′-9″	6′-4″	6′-0″
Two 2 × 12	12′-2″	10′-7″	9′-5″	8′-7″	8′-0″	7′-5″	7′-0″
Three 2 × 6	8′-6″	7′-5″	6′-8″	6′-1″	5′-8″	5′-3″	4′-11″
Three 2 × 8	10′-11″	9′-6″	8′-6″	7′-9″	7′-2″	6′-8″	6′-4″
Three 2 × 10	13′-0″	11′-2″	10′-0″	9′-2″	8′-6″	7′-11″	7′-6″
Three 2 × 12	15′-3″	13′-3″	11′-10″	10′-9″	10′-0″	9′-4″	8′-10″

Douglas fir-larch, hem-fir, spruce-pine-fir

Beam	6 ft joists	8 ft joists	10 ft joists	12 ft joists	14 ft joists	16 ft joists	18 ft joists
Two 2 × 6	6′-1″	5′-3″	4′-9″	4′-4″	3′-11″	3′-7″	3′-3″
Two 2 × 8	8′-2″	7′-1″	6′-4″	5′-9″	5′-2″	4′-8″	4′-4″
Two 2 × 10	10′-0″	8′-7″	7′-9″	7′-0″	6′-6″	6′-0″	5′-6″
Two 2 × 12	11′-7″	10′-0″	8′-11″	8′-2″	7′-7″	7′-1″	6′-8″
Three 2 × 6	7′-8″	6′-8″	6′-0″	5′-6″	5′-1″	4′-9″	4′-6″
Three 2 × 8	10′-3″	8′-10″	7′-11″	7′-3″	6′-8″	6′-3″	5′-11″
Three 2 × 10	12′-6″	10′-10″	9′-8″	8′-10″	8′-2″	7′-8″	7′-2″
Three 2 × 12	14′-6″	12′-7″	11′-3″	10′-3″	9′-6″	8′-11″	8′-5″

Redwood, western cedars, ponderosa pine, red pine

Beam	6 ft joists	8 ft joists	10 ft joists	12 ft joists	14 ft joists	16 ft joists	18 ft joists
Two 2 × 6	6′-2″	5′-4″	4′-10″	4′-5″	4′-0″	3′-8″	3′-4″
Two 2 × 8	7′-10″	6′-10″	6′-1″	5′-7″	5′-2″	4′-10″	4′-5″
Two 2 × 10	9′-7″	8′-4″	7′-5″	6′-9″	6′-3″	5′-10″	5′-6″
Two 2 × 12	11′-1″	9′-8″	8′-7″	7′-10″	7′-3″	6′-10″	6′-5″
Three 2 × 6	7′-8″	6′-9″	6′-0″	5′-6″	5′-1″	4′-9″	4′-6″
Three 2 × 8	9′-10″	8′-6″	7′-7″	6′-11″	6′-5″	6′-0″	5′-8″
Three 2 × 10	12′-0″	10′-5″	9′-4″	8′-6″	7′-10″	7′-4″	6′-11″
Three 2 × 12	13′-11″	12′-1″	10′-9″	9′-10″	9′-1″	8′-6″	8′-1″

That is why the deck calculator's post count moves when the deck gets wider without getting any longer: the joists grow, the beam's allowed span shrinks, and another post goes in. A beam may also run past its end post by a quarter of the span next to it.

How far a joist can cantilever

A joist can run past its beam, and the overhang is measured against the back span — the part of the joist between its supports. The rule of thumb is a quarter of the back span: a joist with a 12 ft back span may hang 3 ft past the beam. The 2021 IRC prints the limit for each size, and for shallow joists over long back spans it is less than the quarter, or none at all (NP).

Species	Joist	4 ft	6 ft	8 ft	10 ft	12 ft	14 ft	16 ft	18 ft
Southern pine	2 × 6	1′-0″	1′-6″	1′-5″	NP	NP	NP	NP	NP
	2 × 8	1′-0″	1′-6″	2′-0″	2′-6″	2′-3″	NP	NP	NP
	2 × 10	1′-0″	1′-6″	2′-0″	2′-6″	3′-0″	3′-4″	3′-4″	NP
	2 × 12	1′-0″	1′-6″	2′-0″	2′-6″	3′-0″	3′-6″	4′-0″	4′-1″
Douglas fir-larch, hem-fir, spruce-pine-fir	2 × 6	1′-0″	1′-6″	1′-4″	NP	NP	NP	NP	NP
	2 × 8	1′-0″	1′-6″	2′-0″	2′-3″	2′-0″	NP	NP	NP
	2 × 10	1′-0″	1′-6″	2′-0″	2′-6″	3′-0″	3′-3″	NP	NP
	2 × 12	1′-0″	1′-6″	2′-0″	2′-6″	3′-0″	3′-6″	3′-11″	3′-11″
Redwood, western cedars, ponderosa pine, red pine	2 × 6	1′-0″	1′-4″	1′-1″	NP	NP	NP	NP	NP
	2 × 8	1′-0″	1′-6″	2′-0″	1′-11″	NP	NP	NP	NP
	2 × 10	1′-0″	1′-6″	2′-0″	2′-6″	3′-0″	2′-9″	NP	NP
	2 × 12	1′-0″	1′-6″	2′-0″	2′-6″	3′-0″	3′-6″	3′-8″	NP

Joist spacing for deck boards

The boards set a limit of their own. A diagonal board crosses the joists at a slant and spans further between them, so the joists have to come closer.

Wood decking	Square to the joists	45° diagonal
1¼ in thick (5/4)	16 in	12 in
2 in thick (2 ×)	24 in	24 in

For boards continuous over three or more joists. Composite and PVC are not in the code table — their makers set the spacing, commonly 16 in square to the joists and 12 in on a diagonal. The installation guide for the board you buy decides.

Ledger fastener spacing

The ledger is what holds a deck to the house, and it is where decks fail. The longer the joists it carries, the closer the ½ in lag screws or bolts have to be.

Joist span	½ in lag screw	½ in bolt	½ in bolt, 1 in sheathing
6 ft	30 in	36 in	36 in
8 ft	23 in	36 in	36 in
10 ft	18 in	34 in	29 in
12 ft	15 in	29 in	24 in
14 ft	13 in	24 in	21 in
16 ft	11 in	21 in	18 in
18 ft	10 in	19 in	16 in

On-center spacing along the ledger. The ledger also has to be flashed so water never reaches the house's band joist.

What these figures assume

40 psf live load and 10 psf dead load; snow load not assumed to act with live load. No. 2 grade lumber, wet service factor included. Joist deflection limited to L/360 on the main span and L/180 at a cantilever with a 220-pound point load at the end. Douglas fir-larch, hem-fir and spruce-pine-fir values include the incising factor; redwood, western cedar, ponderosa pine and red pine values do not. Decking spacing is for boards continuous over three or more joists. Interpolation is permitted; extrapolation is not.

These are the prescriptive figures from the model code at a 40 psf live load. Where the ground snow load is higher, the code's snow tables give shorter spans. Your city or county may amend any of it, and the building department has the final word on your deck.

Check your joists in the deck calculator →

The posts stand on footings: deck footing sizes and spacing covers how big and how deep, and the footing calculator works out the concrete.

The rest of the deck

These tables set how far the frame can reach. Every deck tool on the site works inside them.