Deck beam span calculator
A beam of two southern pine 2x10s carrying joists that span up to 10 ft can rest on posts up to 7 ft 9 in apart. Choose your wood, beam and joist span, or type your post spacing to see which beams reach it.
Longest distance between posts
7 ft 9 in
The longest distance between posts for a 2-2x10 southern pine beam carrying joists that span up to 10 ft.
- The beam may run past the end post up to 1 ft 11 in.
Your local building code decides. These are the guide's prescriptive sizes for 40 psf live load, 10 psf dead load, No. 2 grade lumber, wet service.
Deck beam span chart
| Beam | Joists spanning up to | ||||||
|---|---|---|---|---|---|---|---|
| 6 ft | 8 ft | 10 ft | 12 ft | 14 ft | 16 ft | 18 ft | |
| 2-2x6 | 6 ft 8 in | 5 ft 8 in | 5 ft 1 in | 4 ft 7 in | 4 ft 3 in | 4 ft | 3 ft 9 in |
| 2-2x8 | 8 ft 6 in | 7 ft 4 in | 6 ft 6 in | 5 ft 11 in | 5 ft 6 in | 5 ft 1 in | 4 ft 9 in |
| 2-2x10 | 10 ft 1 in | 8 ft 9 in | 7 ft 9 in | 7 ft 1 in | 6 ft 6 in | 6 ft 1 in | 5 ft 9 in |
| 2-2x12 | 11 ft 11 in | 10 ft 4 in | 9 ft 2 in | 8 ft 4 in | 7 ft 9 in | 7 ft 3 in | 6 ft 9 in |
| 3-2x6 | 7 ft 11 in | 7 ft 2 in | 6 ft 5 in | 5 ft 10 in | 5 ft 5 in | 5 ft | 4 ft 9 in |
| 3-2x8 | 10 ft 7 in | 9 ft 3 in | 8 ft 3 in | 7 ft 6 in | 6 ft 11 in | 6 ft 5 in | 6 ft 1 in |
| 3-2x10 | 12 ft 9 in | 11 ft | 9 ft 9 in | 8 ft 9 in | 8 ft 3 in | 7 ft 8 in | 7 ft 3 in |
| 3-2x12 | 15 ft | 13 ft | 11 ft 7 in | 10 ft 6 in | 9 ft 9 in | 9 ft 1 in | 8 ft 7 in |
From the American Wood Council, Prescriptive Residential Wood Deck Construction Guide (DCA 6), based on the 2015 IRC, which assumes 40 psf live load, 10 psf dead load, No. 2 grade lumber, wet service. The beam may run past the end posts up to a quarter of its span.
The joist span decides the beam
The longer the joists, the more deck each beam carries, so the posts must stand closer. Use the joist span from the house to the beam; the tool reads the next column up.
Building the beam
Two or three boards nailed together make one beam, as the guide shows. A beam sits in a notch cut into the post or in a post cap, never bolted to the side of a post, and a three-ply beam always sits in a post cap.
If the joists hang from the side of the beam, the beam must be at least as deep as the joists.
What the table assumes
Joists and beams of No. 2 grade or better, treated for wet service, carrying 40 pounds per square foot of people and furniture plus 10 of the deck’s own weight. A hot tub, a roof over the deck or heavy snow needs an engineered design. Douglas fir-larch, hem-fir and SPF figures assume incised lumber, the small slits made for treatment.
Your building department decides what applies at your address, and most decks attached to a house need a permit.