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Guides / Decks and patios · Updated

Deck joist span chart

From IRC 2021 Table R507.6, the same 40 psf spans as the 2018 IRC. Your town can amend it, so the building department has the final say.

Short answer

At the standard 16 in spacing, southern pine joists span 9 ft as a 2x6, 11 ft 10 in as a 2x8, 14 ft as a 2x10 and 16 ft 6 in as a 2x12. Go to 12 in spacing for a little more. Past 18 ft, no joist in the table carries it.

Max joist span, joists stop at the beam

Span is the clear distance from the ledger or rim to the face of the beam. It's not how far the deck sticks out from the house.

Southern pine
Joist12 in o.c.16 in o.c.24 in o.c.
2x69 ft 11 in9 ft7 ft 7 in
2x813 ft 1 in11 ft 10 in9 ft 8 in
2x1016 ft 2 in14 ft11 ft 5 in
2x1218 ft16 ft 6 in13 ft 6 in
Douglas fir-larch, hem-fir, SPF
Joist12 in o.c.16 in o.c.24 in o.c.
2x69 ft 6 in8 ft 8 in7 ft 2 in
2x812 ft 6 in11 ft 1 in9 ft 1 in
2x1015 ft 8 in13 ft 7 in11 ft 1 in
2x1218 ft15 ft 9 in12 ft 10 in
Redwood or cedar
Joist12 in o.c.16 in o.c.24 in o.c.
2x68 ft 10 in8 ft7 ft
2x811 ft 8 in10 ft 7 in8 ft 8 in
2x1014 ft 11 in13 ft10 ft 7 in
2x1217 ft 5 in15 ft 1 in12 ft 4 in

No. 2 grade, wet service. 40 psf live load plus 10 psf dead, deflection limit L/360. Check the stamp on the board for the species.

Joist spacing

16 in on center is the standard. 12 in gets you a longer span, and it's what you use under diagonal boards or composite decking. Composite makers print their own spacing on the install sheet, so go by that.

24 in saves joists but costs span. A southern pine 2x10 drops from 14 ft at 16 in to 11 ft 5 in at 24 in.

When the joists overhang the beam

Joists can run past the beam, but only 1/4 of the back span. A 12 ft back span gets 3 ft of overhang at most. An overhang no longer than the joist is deep doesn't count.

Past that, the back span gets shorter. These are the max back spans with an overhang.

Southern pine
Joist12 in o.c.16 in o.c.24 in o.c.
2x66 ft 8 in6 ft 8 in6 ft 8 in
2x810 ft 1 in10 ft 1 in9 ft 8 in
2x1014 ft 6 in14 ft11 ft 5 in
2x1218 ft16 ft 6 in13 ft 6 in
Douglas fir-larch, hem-fir, SPF
Joist12 in o.c.16 in o.c.24 in o.c.
2x66 ft 3 in6 ft 3 in6 ft 3 in
2x89 ft 5 in9 ft 5 in9 ft 1 in
2x1013 ft 7 in13 ft 7 in11 ft 1 in
2x1218 ft15 ft 9 in12 ft 10 in
Redwood or cedar
Joist12 in o.c.16 in o.c.24 in o.c.
2x65 ft 7 in5 ft 7 in5 ft 7 in
2x88 ft 6 in8 ft 6 in8 ft 6 in
2x1012 ft 3 in12 ft 3 in10 ft 7 in
2x1216 ft 5 in15 ft 1 in12 ft 4 in

Back span is ledger to beam. The overhang is beam to the end of the joist, and it can be up to 1/4 of the back span.

Post heights

Top of footing to bottom of beam, by the deck area the post holds up (IRC Table R507.4). NP means not permitted. Beam spans between posts are in the deck beam span chart.

Max post height by deck area the post holds up
Post20 sq ft40 sq ft60 sq ft80 sq ft100 sq ft120 sq ft140 sq ft160 sq ft
4x4 Southern pine14 ft13 ft 8 in11 ft9 ft 5 in8 ft 4 in7 ft 5 in6 ft 9 in6 ft 2 in
4x6 Southern pine14 ft14 ft13 ft 11 in12 ft10 ft 8 in9 ft 8 in8 ft 10 in8 ft 2 in
6x6 Southern pine14 ft14 ft14 ft14 ft14 ft14 ft14 ft14 ft
4x4 Douglas fir-larch14 ft13 ft 6 in10 ft 10 in9 ft 3 in8 ft7 ft6 ft 2 in5 ft 3 in
4x6 Douglas fir-larch14 ft14 ft13 ft 10 in11 ft 10 in10 ft 6 in9 ft 5 in8 ft 7 in7 ft 10 in
6x6 Douglas fir-larch14 ft14 ft14 ft14 ft14 ft14 ft14 ft14 ft
4x4 Redwood14 ft13 ft 2 in10 ft 3 in8 ft 1 in5 ft 8 inNPNPNP
4x6 Redwood14 ft14 ft13 ft 6 in11 ft 4 in9 ft 9 in8 ft 4 in6 ft 9 in4 ft 7 in
6x6 Redwood14 ft14 ft14 ft14 ft14 ft14 ft13 ft 7 in9 ft 7 in

Deck board joist spacing

Max joist spacing for wood deck boards
BoardsStraight, 3+ joistsStraight, 2 joists45 degrees, 3+ joists45 degrees, 2 joists
1 1/4 in wood (5/4)16 in12 in12 in8 in
2 in wood (2x)24 in24 in24 in18 in
Trex composite, 1 in16 inNot allowed12 inNot allowed

Other composite brands set their own spacing. Go by the installation guide for your boards.

What the chart doesn't cover

These spans are for 40 psf. If your ground snow load is over 40 psf, your town will want the snow columns of the table. Those spans are shorter.

A hot tub is a whole different load. A full one runs 100 psf or more, and no span in this chart is built for it. That deck needs an engineer.

Before you buy

If no size reaches your span at 16 in, try 12 in. Still short? Add a beam in the middle and size the joists for each half.

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