Deck joist spacing is usually 12, 16, or 24 inches on center, and the maximum span for each size comes from 2021 IRC Table R507.6. At 40 psf live load and 16" o.c., a No. 2 2x8 spans 11-1 in Douglas fir-larch, hem-fir, or spruce-pine-fir and 11-10 in Southern pine. The decking you choose then caps the spacing through Table R507.7 or the composite manufacturer's label.
Spacing and span work together. Closer spacing lets the same joist span further, and the decking limits how far apart the joists can be. If you are laying out the whole structure, start with the deck framing plan guide, which uses the same drawing conventions as a floor framing plan, and for the general idea of on-center measurement see floor joist spacing explained.
Deck joist span chart (40 psf live load)
2021 IRC Table R507.6, maximum deck joist spans (feet-inches), 40 psf live load, 10 psf dead load, snow load not concurrent with live load, No. 2 grade, wet service factor included, L/Δ = 360 at main span.
| Species group | Joist size | 12" o.c. | 16" o.c. | 24" o.c. |
|---|---|---|---|---|
| Southern pine | 2x6 | 9-11 | 9-0 | 7-7 |
| Southern pine | 2x8 | 13-1 | 11-10 | 9-8 |
| Southern pine | 2x10 | 16-2 | 14-0 | 11-5 |
| Southern pine | 2x12 | 18-0 | 16-6 | 13-6 |
| Douglas fir-larch, hem-fir, spruce-pine-fir | 2x6 | 9-6 | 8-4 | 6-10 |
| Douglas fir-larch, hem-fir, spruce-pine-fir | 2x8 | 12-6 | 11-1 | 9-1 |
| Douglas fir-larch, hem-fir, spruce-pine-fir | 2x10 | 15-8 | 13-7 | 11-1 |
| Douglas fir-larch, hem-fir, spruce-pine-fir | 2x12 | 18-0 | 15-9 | 12-10 |
| Redwood, western cedars, ponderosa pine, red pine | 2x6 | 8-10 | 8-0 | 6-10 |
| Redwood, western cedars, ponderosa pine, red pine | 2x8 | 11-8 | 10-7 | 8-8 |
| Redwood, western cedars, ponderosa pine, red pine | 2x10 | 14-11 | 13-0 | 10-7 |
| Redwood, western cedars, ponderosa pine, red pine | 2x12 | 17-5 | 15-1 | 12-4 |
The Douglas fir-larch, hem-fir, and spruce-pine-fir values include the incising factor used for treated lumber. The redwood, cedar, and pine values do not. Interpolation between columns is permitted. Extrapolation is not.
Snow load rows
Table R507.6 also lists rows for 50, 60, and 70 psf ground snow load, and the deck must be designed for the live load or the ground snow load, whichever is greater (R507.1). The spans drop as the load rises. A Douglas fir-larch, hem-fir, or spruce-pine-fir 2x8 at 16" o.c. goes from 11-1 at 40 psf live load to 10-7 at 50 psf, 9-11 at 60 psf, and 9-5 at 70 psf ground snow load. Use the row that matches your design load.
Some states amend these tables. The Washington edition used as the source for this page deletes the 40 psf rows and requires a 60 psf live load, so check your adopted code before reading a span.
2x6 deck joist span
A 2x6 is the shortest joist in the table. At 40 psf and 16" o.c. it reaches 9-0 in Southern pine, 8-4 in the Douglas fir-larch group, and 8-0 in the redwood and cedar group. At 24" o.c. those fall to 7-7, 6-10, and 6-10. It is a common choice for low, small decks and for freestanding platforms, but Table R507.7 does not allow 24" spacing under 1-1/4-inch wood decking (see the decking table below).
2x8 deck joist span
At 40 psf a 2x8 spans 11-10 (Southern pine), 11-1 (Douglas fir-larch group), or 10-7 (redwood and cedar group) at 16" o.c. Dropping to 12" o.c. adds more than a foot: 13-1, 12-6, and 11-8. That makes 2x8 at 12" o.c. a reasonable alternative to 2x10 at 16" o.c. when headroom under the deck is tight.
2x10 and 2x12 deck joist spans
A 2x10 at 16" o.c. reaches 14-0 in Southern pine and 13-7 in the Douglas fir-larch group. A 2x12 at 16" o.c. reaches 16-6 and 15-9. At 12" o.c. the table lists 18-0 for a 2x12 in both species groups.
Deck joist cantilever limits
Table R507.6 also limits how far joists may cantilever past the beam, based on the joist back span (the span on the other side of the beam). Cantilever values use L/Δ = 180 with a 220-pound point load at the end.
2021 IRC Table R507.6, maximum cantilever (feet-inches), Southern pine, 40 psf live load. NP = not permitted.
| Joist | 4 ft back span | 6 ft | 8 ft | 10 ft | 12 ft | 14 ft | 16 ft | 18 ft |
|---|---|---|---|---|---|---|---|---|
| 2x8 | 1-0 | 1-6 | 2-0 | 2-6 | 2-3 | NP | NP | NP |
| 2x10 | 1-0 | 1-6 | 2-0 | 2-6 | 3-0 | 3-4 | 3-4 | NP |
| 2x12 | 1-0 | 1-6 | 2-0 | 2-6 | 3-0 | 3-6 | 4-0 | 4-1 |
Other species have their own cantilever columns in the same table. Draw the cantilever and the back span as separate dimensions on the plan so the reviewer can find the right cell.
Deck joist spacing chart for wood decking
Joist spacing is limited by the decking, not just the joist. Table R507.7 sets the maximum spacing for wood deck boards. A board over two joists is single span. A board over three or more joists is multiple span.
2021 IRC Table R507.7, maximum on-center joist spacing for wood decking (inches).
| Decking | Perpendicular, single span | Perpendicular, multiple span | Diagonal, single span | Diagonal, multiple span |
|---|---|---|---|---|
| 1-1/4-inch-thick wood | 12 | 16 | 8 | 12 |
| 2-inch-thick wood | 24 | 24 | 18 | 24 |
Diagonal means up to 45 degrees from perpendicular. Wood decking must be fastened to each joist with at least two 8d threaded nails or two No. 8 wood screws (R507.7). A grading or inspection agency may publish other spans for 1-1/4-inch boards.
Deck joist spacing for composite and Trex decking
Composite and plastic decking is not in Table R507.7. R507.7 sends it to Section R507.2, and R507.2.2.1 requires the boards or their packaging to carry a label showing the allowable load and the maximum allowable span determined under ASTM D7032. The installation must follow the manufacturer's instructions (R507.2.2.5).
In practice that means the joist spacing comes from the product's install guide for the exact board line. Manufacturers commonly call for tighter spacing for diagonal or herringbone layouts and for stair treads than for boards laid perpendicular. Decide the decking before you size the joists, because a tighter spacing changes the joist count and can let you use a smaller joist.
Using the chart on a framing plan
On the plan, label every joist bay with size, species, and spacing, for example 2x10 SPF JOISTS @ 12" O.C., and dimension the span and cantilever. State the design load and decking type in the notes. Rim joists and blocking at joist ends need their own callouts; the rim joist guide covers what R507.6.2 asks for.
Do not borrow interior floor values. Interior joists use Tables R502.3.1(1) and R502.3.1(2), summarized in the floor joist span table, which use different loads and no wet service factor. Inside or out, blocking requirements are in joist blocking and bridging.
Limits of the table
These spans are prescriptive limits for No. 2 sawn lumber under the stated loads only. The adopted code edition and local amendments govern, and engineered lumber, steel joists, and composite decking use manufacturer tables. The American Wood Council's DCA 6 deck guide presents the same family of tables with details.
Framing Plan is a preliminary drawing aid that draws joist layouts, beams, and openings and exports PNG, PDF, and DXF. It does not check spans against Table R507.6, so read the values here and label them on your drawing. Open the framing studio to lay out a deck joist field.