What size joist do I need for my deck span?
Short answer
- For a 12 ft joist span in southern pine at 16 in on center (measured from the middle of one joist to the middle of the next), use 2 × 10 joists. They can span up to 14' 0".
- Find your span in the tables below. Each cell shows the smallest joist that DCA 6 allows, and its maximum span.
- Joist span is the clear distance from the ledger or beam to the next beam, not the overall deck depth if there's an overhang.
The tables list joist spacing as o.c., short for on center. Each cell shows the smallest joist that works and, in parentheses, the longest span DCA 6 allows for it.
Southern pine
| Joist span | 12 in o.c. | 16 in o.c. | 24 in o.c. |
|---|---|---|---|
| 8 ft | 2 × 6 (max 9' 11") | 2 × 6 (max 9' 0") | 2 × 8 (max 9' 8") |
| 10 ft | 2 × 8 (max 13' 1") | 2 × 8 (max 11' 10") | 2 × 10 (max 11' 5") |
| 12 ft | 2 × 8 (max 13' 1") | 2 × 10 (max 14' 0") | 2 × 12 (max 13' 6") |
| 14 ft | 2 × 10 (max 16' 2") | 2 × 10 (max 14' 0") | Over 2 × 12: add a beam |
| 16 ft | 2 × 10 (max 16' 2") | 2 × 12 (max 16' 6") | Over 2 × 12: add a beam |
| 18 ft | 2 × 12 (max 18' 0") | Over 2 × 12: add a beam | Over 2 × 12: add a beam |
Douglas fir-larch, hem-fir, SPF
| Joist span | 12 in o.c. | 16 in o.c. | 24 in o.c. |
|---|---|---|---|
| 8 ft | 2 × 6 (max 9' 6") | 2 × 6 (max 8' 4") | 2 × 8 (max 9' 1") |
| 10 ft | 2 × 8 (max 12' 6") | 2 × 8 (max 11' 1") | 2 × 10 (max 11' 1") |
| 12 ft | 2 × 8 (max 12' 6") | 2 × 10 (max 13' 7") | 2 × 12 (max 12' 10") |
| 14 ft | 2 × 10 (max 15' 8") | 2 × 12 (max 15' 9") | Over 2 × 12: add a beam |
| 16 ft | 2 × 12 (max 18' 0") | Over 2 × 12: add a beam | Over 2 × 12: add a beam |
| 18 ft | 2 × 12 (max 18' 0") | Over 2 × 12: add a beam | Over 2 × 12: add a beam |
Redwood, western cedar, ponderosa or red pine
| Joist span | 12 in o.c. | 16 in o.c. | 24 in o.c. |
|---|---|---|---|
| 8 ft | 2 × 6 (max 8' 10") | 2 × 6 (max 8' 0") | 2 × 8 (max 8' 8") |
| 10 ft | 2 × 8 (max 11' 8") | 2 × 8 (max 10' 7") | 2 × 10 (max 10' 7") |
| 12 ft | 2 × 10 (max 14' 11") | 2 × 10 (max 13' 0") | 2 × 12 (max 12' 4") |
| 14 ft | 2 × 10 (max 14' 11") | 2 × 12 (max 15' 1") | Over 2 × 12: add a beam |
| 16 ft | 2 × 12 (max 17' 5") | Over 2 × 12: add a beam | Over 2 × 12: add a beam |
| 18 ft | Over 2 × 12: add a beam | Over 2 × 12: add a beam | Over 2 × 12: add a beam |
The spans come from Table 2 of the American Wood Council's DCA 6 prescriptive deck guide. They assume No. 2 grade lumber, the common lumber grade, rated for outdoor, wet conditions. They also assume a 40 psf live load plus a 10 psf dead load. Psf means pounds per square foot. Live load is people and furniture; dead load is the deck's own weight. Beams, posts, footings, the ledger connection and guards need their own design. Check your local code and get a permit where one is required. The deck calculator counts boards, joists and screws and checks the span for your exact deck.
Common questions
What size joist do I need for a 12 ft deck span?
Southern pine joists set 16 in on center need to be 2 × 10s, which can span up to 14' 0". Douglas fir-larch, hem-fir or SPF at the same spacing need 2 × 10s, good for up to 13' 7". These are DCA 6 maximum spans for No. 2 grade lumber (the common lumber grade) and a 40 psf (pounds per square foot) live load.
Can 2 × 8 joists span 12 ft?
Not at 16 in on center under DCA 6: southern pine 2 × 8s reach 11' 10". At 12 in on center they reach 13' 1", which covers 12 ft.
What is the span of a 2 × 10 deck joist?
Southern pine 2 × 10s span 14' 0" at 16 in on center and 16' 2" at 12 in. Douglas fir-larch and SPF 2 × 10s span 13' 7" at 16 in.