Deck Joist Span Calculator
Max deck joist span by species, size, and spacing per DCA 6.
What it does
Deck joists bridge house to beam, and span tables draw the line where bounce turns to sag. The American Wood Council publishes one table per species group at fixed loads, and inspectors enforce it cell by cell. Guessing a foot past costs a rebuild.
The tool above reads DCA 6 Table 2 without cantilevers. Pick species, size, and spacing. It returns the max span in feet-inches plus metric. Defaults of Southern Pine 2x10 at 16 in open solved at 14 ft 0 in.
Species buys real feet. The same 2x10 at 16 in reaches 14 ft in Southern Pine, 13 ft 7 in fir, and 13 ft even in redwood. Spacing buys nearly as much: pine 2x8 runs 13 ft 1 in at 12 in centers but 9 ft 8 in at 24.
The conditions ride along invisibly: 40 psf live plus 10 dead, L/360 deflection, No. 2 grade, wet service lumber. Dry interior floors, heavier hot tubs, and cantilevers each need their own tables, which is why the page refuses to interpolate.
Use the table for the popular cells, then frame it. Bear fully on beams, hang correctly, flash the ledger, and keep joists at or under the printed span.
How to use
- Pick the species group.
- Pick joist size.
- Pick on center spacing.
- Read max span first, then metric.
- Check overhang separately.
- Size beams for the chosen span.
Formula
- Species
- SP, DF group, or Redwood group.
- Size
- Nominal 2x6 through 2x12.
- Spacing
- 12, 16, or 24 in on center.
- Span
- Table cell in feet-inches.
Maximum joist spans without cantilevers at 40 psf live plus 10 psf dead, L/360 deflection, No. 2 grade, wet service. Southern Pine 2x10 at 12, 16, 24 in: 16-2, 14-0, 11-5. Read the cell exactly; spans do not interpolate between spacings.
Worked example
Example: SP 2x10 at 16 in
Southern Pine 2x10 joists at 16 in on center. Find the maximum span.
- Pick Southern Pine, 2x10, 16 in on center.
- Table 2 row reads 14 ft 0 in.
- Metric: 14 x 0.3048 = 4.27 m.
- At 24 in the same joist reaches only 11 ft 5 in.
- Size up or tighten spacing past the limit.
- Conditions: 40 plus 10 psf, L/360, No. 2, wet service.
Reference table
| Joist at 16 in | Southern Pine | Fir group | Redwood |
|---|---|---|---|
| 2x8 | 11 ft 10 in | 11 ft 1 in | 10 ft 7 in |
| 2x10 (worked) | 14 ft 0 in | 13 ft 7 in | 13 ft 0 in |
| 2x12 | 16 ft 6 in | 15 ft 9 in | 15 ft 1 in |
Limits
- No cantilevers in these cells.
- Fixed loads 40 plus 10 psf.
- No. 2 wet service lumber assumed.
- Read cells exactly; no interpolation.
Common mistakes
- Interpolating between spacings.
- Using dry tables outdoors.
- Exceeding span with heavy tubs.
- Skipping beam sizing after span.
- Wrong species group from the rack.
Try it yourself
Quick checks. Answers sit tucked under each question.
Southern Pine 2x8 at 16 in?
Cell reads 11 ft 10 in, metric 11.83 x 0.3048 = 3.61 m.
Douglas Fir 2x10 at 16 in?
Cell reads 13 ft 7 in, metric 13.58 x 0.3048 = 4.14 m.
Redwood 2x10 at 16 in?
Cell reads 13 ft 0 in, metric 13 x 0.3048 = 3.96 m.
Questions
Which species groups?
Southern Pine, Douglas Fir-Larch with Hem-Fir and SPF, and Redwood with cedar and pines. Pine spans longest per size.
Why does spacing matter so much?
Tighter spacing shares load across more joists: SP 2x8 runs 13 ft 1 in at 12 in but 9 ft 8 in at 24 in.
How far can joists overhang?
About a quarter of the span for cantilevers, with table overhang caps. Keep both within DCA 6 limits.
Can I use this indoors?
No. DCA 6 covers No. 2 grade wet service at 40 plus 10 psf. Drier interior floors use AWC span tables instead.
Is there a length cap?
18 ft prescriptive cap on joist length for footing design. Longer needs engineering.
What about hangers and ledgers?
Boards must bear on beams with proper hangers and flashing. Span tables never replace connection details.
Sources
- AWC DCA 6: prescriptive deck guide, Table 2 joist spans
- AWC: span tables for joists and rafters 2012