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How far a joist can span before you need another beam.
With these inputs:
| Result | Value |
|---|---|
| Maximum span for 2x8 at 16" on centre | 11.9 ft |
| Span needed | 12.0 ft |
| Verdict | Over the limit |
| Smallest size that works | 2x10 |
| 2x6 at 16" oc | 9.0 ft |
| 2x8 at 16" oc | 11.9 ft |
| 2x10 at 16" oc | 14.0 ft — works |
| 2x12 at 16" oc | 16.6 ft — works |
| Maximum cantilever allowed | 3.0 ft (a quarter of the backspan) |
| Your cantilever | 0.0 ft — OK |
| Total joist length to buy | 12 ft |
2x8 is not enough for 12.0 ft at 16" on centre. Go up to 2x10, which reaches 14.0 ft.
Looks up the maximum allowable span from the AWC DCA 6 prescriptive tables for the species, size and spacing you are using, accounts for cantilever, and tells you the smallest joist that will do the job.
In southern yellow pine, about 13 ft 1 at 12 inch spacing, 11 ft 10 at 16 inch, and 9 ft 11 at 24 inch. Douglas fir is slightly less and western red cedar less again. These are the DCA 6 prescriptive figures for a 40 psf live load, which is what residential deck codes assume.
Roughly 16 ft 2 at 12 inch spacing in southern yellow pine, 14 ft at 16 inch, and 11 ft 5 at 24 inch. The jump from 2x8 to 2x10 buys about two extra feet at typical spacing, which is often cheaper than adding a beam and the footings under it.
The cantilever is the part of the joist that runs past the beam with nothing under it. DCA 6 allows up to one quarter of the backspan — so joists spanning 12 feet to the beam can cantilever 3 feet beyond it. Going further makes the deck bounce and can lift the inboard end.
Yes, by two feet or more at the extremes. Southern yellow pine is the stiffest common treated species and western red cedar the least. Most pressure-treated stock in the eastern United States is southern yellow pine and most in the west is hem-fir or Douglas fir — check the grade stamp rather than guessing.
Only with an engineered design — steel, LVL, or a stamped calculation from a structural engineer. The prescriptive tables exist so that ordinary decks can be permitted without engineering. If your layout needs more span than they allow, that is the code telling you to add a beam or get a design.
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