How far can a deck joist span?

A deck joist is not an indoor floor joist

The deck tables on this site are transcribed from the 2012 edition of DCA6, the AWC's prescriptive residential wood deck guide, and they carry their own fixed set of assumptions: 40 psf live load, 10 psf dead load, deflection limited to L/360, and No. 2 grade lumber throughout. There is no grade column to shop in - DCA6 publishes No. 2 and nothing else - and the species are grouped differently from the indoor tables, with Southern Pine in one row and Douglas Fir-Larch, Hem-Fir and Spruce-Pine-Fir sharing a second. For that combined group the table also assumes the lumber is incised, the rows of small slits pressed into refractory species so preservative treatment can penetrate.

The load numbers look identical to the indoor 40 psf living-area floor table, and that resemblance is exactly the trap. What changes outdoors is the service condition. Wet-service design values are lower than dry-service ones, so the same size and species group is published at a shorter span on a deck than it is inside a house.

The published deck joist spans

Here is DCA6's joist table as reproduced on this site: both species groups, all four sizes, at the three spacings the guide publishes. Note that DCA6 uses 12, 16 and 24 in on center - there is no 19.2 in column here.

Read it the same way as any span table: find your species group, your size and your spacing, and the cell is the maximum span from the face of the ledger or beam to the face of the next support. If the cell falls short of the deck you have drawn, the moves are the same as indoors - go deeper, go closer together, or add a beam and split the span.

Set it against the indoor table

The comparison that matters is the one people make by accident. Indoors, a Douglas Fir-Larch No. 2 2x8 floor joist at 16 in on center is published at 12 ft 9 in in the IRC 2018 living-area table. On a deck, the DCA6 row that covers Douglas Fir-Larch publishes the same size and spacing at 11 ft 1 in - a foot and eight inches shorter, at the same nominal live load and the same deflection limit. At 2x10 the gap is two feet: 15 ft 7 in indoors against 13 ft 7 in on the deck.

The two tables are not comparable row for row, because DCA6 groups Douglas Fir-Larch, Hem-Fir and Spruce-Pine-Fir into one conservative line while the indoor table gives each species its own row and four grades besides. But the direction of the correction is the point: if you sized a deck from the indoor floor table, the deck table would have given you a shorter answer. Outdoors, the DCA6 figure is the one that applies.

The overhang column is published too

DCA6 does something the indoor tables never need to: alongside every joist span it publishes an allowable overhang, the distance the joists may cantilever past the face of the beam. On this site the overhang values sit next to the spans on the deck joist table page, and the deck calculator reports both together.

The overhang is not "however far looks right". For the Douglas Fir-Larch, Hem-Fir and Spruce-Pine-Fir group at 16 in on center, a 2x8 may overhang the beam by 1 ft 10 in and a 2x10 by 3 ft 2 in; Southern Pine at the same spacing is published at 2 ft 0 in and 3 ft 5 in. Those few feet are a genuinely useful design lever - pulling the beam in from the rim shortens the joist span it has to satisfy and tucks the posts back from the edge - but the limit is a published number, and a deck drawn with a longer cantilever than its row allows is outside the prescriptive guide entirely.

The joist span drives the beam

Deck design in DCA6 is sequential: the joist span you just chose becomes the input to the beam table. Table 3A publishes the maximum beam span for each built-up beam size according to the joist span the beam carries - not tributary-area arithmetic, just the joist span, read against the beam row.

A worked example straight from the reproduced table: a double 2x10 Southern Pine beam carrying joists that span up to 10 feet is published at 7 ft 9 in between posts. The same joist load on a double 2x10 in the Douglas Fir-Larch, Hem-Fir and Spruce-Pine-Fir group is published at 6 ft 3 in. Longer joist spans push the same beam shorter, which is why the joist layout and the beam layout have to be settled together. The deck beam table page carries the full grid, and the deck calculator walks the sequence in order - joists, overhang, then beam.

What DCA6 does not cover here

The joist and beam tables are two pieces of a larger prescriptive system, and the rest of that system is not reproduced on this site. Before a deck is safe, several other things have to be right, and none of them can be read off this page:

• Ledger attachment to the house - fastener type, size, spacing and flashing.
• Footings and posts - size, depth, frost protection and post-to-beam connections.
• Guards and stairs - height, infill, loading and stringer design.
• Lateral load connections tying the deck back to the structure.
• Anything beyond single-span, uniformly loaded joists - hot tubs and other concentrated loads are outside the table.
• These are covered by DCA6 itself and by your locally adopted code - take the whole design, not just the spans, to your building department before you build.

Frequently asked questions