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Guides / Decks and patios · Updated

How far apart should deck posts be?

From IRC 2021 Tables R507.5(1) and R507.4, 40 psf live load. Your building department has the final say.

Short answer

As far apart as your beam can span, and no farther. With 10 ft joists in southern pine, that's 8 ft for a double 2x10 beam and 9 ft 5 in for a double 2x12. Longer joists put more load on the beam, so the posts move closer.

Post spacing for common beams

Pick the column for how far your joists run to the beam. The number is the max distance between posts, center to center.

Southern pine: max beam span by joist span
Beam6 ft8 ft10 ft12 ft14 ft16 ft18 ft
2-2x88 ft 9 in7 ft 7 in6 ft 9 in6 ft 2 in5 ft 9 in5 ft 4 in5 ft
2-2x1010 ft 4 in9 ft8 ft7 ft 4 in6 ft 9 in6 ft 4 in6 ft
2-2x1212 ft 2 in10 ft 7 in9 ft 5 in8 ft 7 in8 ft7 ft 5 in7 ft
3-2x1013 ft11 ft 2 in10 ft9 ft 2 in8 ft 6 in7 ft 11 in7 ft 6 in
3-2x1215 ft 3 in13 ft 3 in11 ft 10 in10 ft 9 in10 ft9 ft 4 in8 ft 10 in
Douglas fir-larch, hem-fir, SPF: max beam span by joist span
Beam6 ft8 ft10 ft12 ft14 ft16 ft18 ft
2-2x88 ft 2 in7 ft 1 in6 ft 4 in5 ft 9 in5 ft 2 in4 ft 8 in4 ft 4 in
2-2x1010 ft8 ft 7 in7 ft 9 in7 ft6 ft 6 in6 ft5 ft 6 in
2-2x1211 ft 7 in10 ft8 ft 11 in8 ft 2 in7 ft 7 in7 ft 1 in6 ft 8 in
3-2x1012 ft 6 in10 ft 10 in9 ft 8 in8 ft 10 in8 ft 2 in7 ft 8 in7 ft 2 in
3-2x1214 ft 6 in12 ft 7 in11 ft 3 in10 ft 3 in9 ft 6 in8 ft 11 in8 ft 5 in
Redwood, western cedar, ponderosa or red pine: max beam span by joist span
Beam6 ft8 ft10 ft12 ft14 ft16 ft18 ft
2-2x87 ft 10 in6 ft 10 in6 ft 1 in5 ft 7 in5 ft 2 in4 ft 10 in4 ft 5 in
2-2x109 ft 7 in8 ft 4 in7 ft 5 in6 ft 9 in6 ft 3 in5 ft 10 in5 ft 6 in
2-2x1211 ft 1 in9 ft 8 in8 ft 7 in7 ft 10 in7 ft 3 in6 ft 10 in6 ft 5 in
3-2x1012 ft10 ft 5 in9 ft 4 in8 ft 6 in7 ft 10 in7 ft 4 in6 ft 11 in
3-2x1213 ft 11 in12 ft 1 in10 ft 9 in9 ft 10 in9 ft 1 in8 ft 6 in8 ft 1 in

The full table, with single beams and 2x6s, is in the beam span chart.

How tall the posts can be

Height runs from the top of the footing to the bottom of the beam. The more deck a post holds up, the shorter it has to be. A middle post carries one full beam span, half on each side, times half the joist span.

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

NP means not permitted. 6x6 posts in southern pine and the fir group go 14 ft at every area in the table.

Under the post

The footing goes at least 12 in below undisturbed ground. If the deck is attached to the house, it goes below the frost line too. The frost depth by state guide has a starting number for your area.

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