DCA6-2015 Table 2, p.4
Deck joist span calculator
Your joist
Face of the ledger to the centre of the beam it lands on — not the overall deck depth.
DCA6-2015 prints DF-L, Hem-Fir and SPF as one column.
Leave at zero if the joists stop at the beam.
Table 2 fn.6 puts a 2x8 nominal floor on those members, whatever the span says.
Within the prescriptive tables
Use 2x10 at 16" on centre
Governing cell — DCA6-2015 Table 2, p.4 · 2x10 · Southern Pine · 16" o.c.
Table span
14'-0"
2x10 @ 16" o.c.
Max overhang
3'-0"
lesser of table LO and L/4
| Size | Max span | Max overhang | Your span |
|---|---|---|---|
| 2x6 | 9'-0" | 1'-1" | over span |
| 2x8 | 11'-10" | 2'-0" | over span |
| 2x10smallest that passes | 14'-0" | 3'-0" | passes |
| 2x12 | 16'-6" | 3'-0" | passes |
Spans are floored to the whole inch — the table cells are exact feet-inches, and rounding a maximum up is the one direction that can hurt someone. Load basis: 40 psf live + 10 psf dead, No. 2 grade, wet service, L/360.
The full joist span table
Both species columns, all four sizes, all three spacings — the same data the calculator reads. LJ is the maximum clear span between supports. LO is the maximum overhang the same cell allows before the L/4 rule is applied to it.
Southern Pine
| Size | Max clear span (LJ) | Max overhang (LO) | ||||
|---|---|---|---|---|---|---|
| 12" o.c. | 16" o.c. | 24" o.c. | 12" o.c. | 16" o.c. | 24" o.c. | |
| 2x6 | 9'-11" | 9'-0" | 7'-7" | 1'-0" | 1'-1" | 1'-3" |
| 2x8 | 13'-1" | 11'-10" | 9'-8" | 1'-10" | 2'-0" | 2'-4" |
| 2x10 | 16'-2" | 14'-0" | 11'-5" | 3'-1" | 3'-5" | 2'-10" |
| 2x12 | 18'-0" | 16'-6" | 13'-6" | 4'-6" | 4'-2" | 3'-4" |
Douglas Fir-Larch / Hem-Fir / SPF
| Size | Max clear span (LJ) | Max overhang (LO) | ||||
|---|---|---|---|---|---|---|
| 12" o.c. | 16" o.c. | 24" o.c. | 12" o.c. | 16" o.c. | 24" o.c. | |
| 2x6 | 9'-6" | 8'-4" | 6'-10" | 0'-11" | 1'-0" | 1'-2" |
| 2x8 | 12'-6" | 11'-1" | 9'-1" | 1'-8" | 1'-10" | 2'-2" |
| 2x10 | 15'-8" | 13'-7" | 11'-1" | 2'-10" | 3'-2" | 2'-9" |
| 2x12 | 18'-0" | 15'-9" | 12'-10" | 4'-4" | 3'-11" | 3'-3" |
Load basis: 40 psf live load + 10 psf dead load, No. 2 grade lumber, wet service conditions, L/360 deflection. Values are floored to the whole inch for display; the underlying cells are exact feet-inches.
Three footnotes that fail more decks than the span number
The span number is not the whole answer. The three rules below live in the footnotes of the same table, and each of them can invalidate a span that looks perfectly fine on the chart above.
The overhang is capped twice, not once
DCA6-2015 Table 2, fn.3The allowable overhang is the lesser of the LO value in the table and one quarter of the joist span behind it. A 2x12 tabulates a generous overhang, but on a short back span the L/4 rule governs long before the table does. Both limits are applied here, and the calculator tells you which one bit.
2x8 nominal minimum at the ledger and at guard posts
DCA6-2015 Table 2, fn.6The ledger must be at least 2x8. So must any joist or rim joist that a guard post bolts to — because that post is a lever, and the fastener group needs the depth. A 2x6 can be a perfectly legal field joist on a short span and still be prohibited at those two locations.
18'-0" is the end of the prescriptive road
DCA6-2015 Table 2, fn.7Joist length is prescriptively limited to 18'-0". Past that line there is no table cell to read, so this calculator stops answering and says so. That is not a limitation of the tool: it is the correct output. The fix is an interior beam that shortens the span, or a design professional.
What the table assumes
A span table is only true inside its assumptions, and this one carries four that matter on site. The load basis is 40 psf live plus 10 psf dead — a residential deck, not a hot tub, a planter wall, or a snow-loaded roof structure. The lumber is No. 2 grade. The service condition is wet, which is how outdoor framing actually lives. Deflection is limited to L/360, which is the difference between a deck that feels solid and one that bounces when somebody walks past your drink.
The span itself is the clear span between supports — from the face of the ledger to the centre of the beam the joist lands on. It is not the overall depth of the deck, and it does not include the overhang. Getting that measurement wrong is the single most common reason a joist that "checks out" on a chart is undersized in the ground.
Anything outside those assumptions — a hot tub, a roof over the deck, an outdoor kitchen, a second storey — puts you outside the prescriptive tables entirely, no matter what size the joist is.
Why there are only two species columns
Because DCA6-2015 publishes 2 of them. The guide prints Douglas Fir-Larch, Hem-Fir and Spruce-Pine-Fir in a single column, at the governing minimum of the three, and gives Southern Pine its own higher column. That is the whole grid: 2 columns, four sizes, three spacings — and a dropdown longer than that has to get the extra rows from somewhere other than this guide.
If your lumber is cedar, redwood or a western species, you are outside this table and the answer is a different document — not a lower number in the same one.
One more thing worth knowing: the 2015 edition matters. It carries the reduced Southern Pine design values published after 2013. A span table sourced from the 2012 edition returns noticeably longer Southern Pine spans, and they are not conservative. This engine is built on the 2015 edition specifically for that reason.
Common questions
How far can a 2x8 joist span on a deck?
A 2x8 No. 2 Southern Pine deck joist spans 11'-10" at 16" on centre, 13'-1" at 12" and 9'-8" at 24". In Douglas Fir-Larch, Hem-Fir or SPF the same joist spans 11'-1" at 16". Those are clear spans between supports under 40 psf live plus 10 psf dead load, from AWC DCA 6-2015 Table 2, page 4. An overhang past the beam is separate and additional.
How far can a 2x10 deck joist span?
14'-0" at 16" on centre in Southern Pine, and 13'-7" in the Douglas Fir-Larch / Hem-Fir / SPF group. At 12" on centre the Southern Pine value rises to 16'-2"; at 24" it drops to 11'-5" (DCA6-2015 Table 2, p.4).
What is the maximum deck joist span?
A 2x12 at 12" on centre reaches 18'-0", which is also the hard ceiling: DCA6-2015 Table 2 footnote 7 prescriptively limits joist length to 18'-0". Past 18 feet there is no prescriptive answer — the framing has to be designed by a licensed professional, or the span has to be broken with an interior beam.
Can I use 2x6 deck joists?
Only in the field, and only for short spans: 9'-0" at 16" on centre in Southern Pine. DCA6-2015 Table 2 footnote 6 puts a 2x8 nominal floor on the ledger and on any joist or rim joist a guard post attaches to, so a 2x6 is not permitted at those locations regardless of the span.
How far can a deck joist overhang past the beam?
The lesser of the overhang value in Table 2 for that size, species and spacing, or one quarter of the joist span behind it (L/4). Both caps apply — DCA6-2015 Table 2 footnote 3 and the guide body text on page 3. A 2x10 Southern Pine at 16" on centre tabulates 3'-5", but on a 10-foot back span the L/4 rule cuts that to 2'-6".
Does 12-inch joist spacing let me span further than 16-inch?
Yes, and the table says by how much. A Southern Pine 2x10 goes from 14'-0" at 16" on centre to 16'-2" at 12" — 2'-2" more — at the cost of roughly a third more joists. The gain is not the same for every size: a 2x8 picks up 1'-3" over the same change and a 2x12 only 1'-6". Tightening spacing is also what composite decking usually needs for a 45-degree board layout.
Next: size the beam those joists land on with the beam and footing calculator, or lay out the steps down to grade with the deck stair calculator.
Keep going
Beam & footing calculator
Beam size, post spacing and footing diameter for one deck — including the post/footing combinations the table marks Not Permitted.
DCA6-2015 Table 3A + Table 4 · IRC R507
Deck stair calculator
Riser height, tread depth, stringer count and cut layout, screened against IRC R311.7 — including the 3/8" riser uniformity rule.
IRC R311.7 · R312.1
Ledger fastener spacing
On-centre spacing for ½" lags and through-bolts in a deck ledger, by the joist span it carries and the band joist behind it.
DCA6-2015 Table 5, p.15
Now do the whole deck
The calculator sizes one member. The free designer runs the same tables across a real footprint — every joist, beam, post and footing sized together, with the full material list underneath. No signup.
Source & limits
AWC DCA 6-2015, Prescriptive Residential Wood Deck Construction Guide (based on the 2015 IRC), © 2018 American Wood Council. The guide is published free by AWC.
Estimate only — not an engineered design. Spans, sizes, and quantities are screened against the AWC DCA 6-2015 / IRC prescriptive baseline at 40 psf live + 10 psf dead load. Local codes and amendments vary; your Authority Having Jurisdiction (AHJ) and a licensed design professional govern. Verify everything with a permit and a licensed pro before ordering or building.