Deck joist span calculator
A southern pine 2x8 joist 16 inches apart spans up to 11 ft 10 in and can overhang the beam up to 2 ft. Choose your wood, joist size and spacing, or type your span to see which joists reach it.
Longest joist span
11 ft 10 in
The longest span for 2x8 southern pine joists 16 in apart, from the face of one support to the face of the other.
- Overhang past the beam: up to 2 ft, and never more than a quarter of the span.
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 joist span chart
| Joist | Span, joists this far apart | Overhang | ||||
|---|---|---|---|---|---|---|
| 12 in | 16 in | 24 in | 12 in | 16 in | 24 in | |
| 2x6 | 9 ft 11 in | 9 ft | 7 ft 7 in | 1 ft | 1 ft 1 in | 1 ft 3 in |
| 2x8 | 13 ft 1 in | 11 ft 10 in | 9 ft 8 in | 1 ft 10 in | 2 ft | 2 ft 4 in |
| 2x10 | 16 ft 2 in | 14 ft | 11 ft 5 in | 3 ft 1 in | 3 ft 5 in | 2 ft 10 in |
| 2x12 | 18 ft | 16 ft 6 in | 13 ft 6 in | 4 ft 6 in | 4 ft 2 in | 3 ft 4 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. Spans run from the face of one support to the face of the other and leave out the overhang.
How to measure the span
Measure from the face of the ledger on the house to the face of the beam, not to the outside edge of the deck. Joists that sit on top of the beam can run past it as an overhang.
The overhang is the shorter of the table’s overhang and a quarter of your span: a joist spanning 10 ft can overhang no more than 2 ft 6 in, even where the table allows more.
Closer joists, longer spans
Moving joists from 24 to 16 inches apart adds about a fifth to the span. Closer joists also make the deck boards feel stiffer, and many composite boards need 16 inches or less: check the board maker’s spacing before you choose 24.
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.