Deck joist spacing and how far deck joists can span

Two numbers set out a deck's frame: how far apart the joists are, and how far each joist can reach between its supports. The first is set mostly by the decking that goes on top; the second by the joist's size, species, and spacing. This guide quotes both from the documents that set them: the installation guides from Trex, TimberTech, and Fiberon, and the American Wood Council's DCA 6 Prescriptive Residential Wood Deck Construction Guide, which is based on the 2015 International Residential Code (IRC). Then it counts the joist lumber for the default deck in the deck calculator.

Joist spacing vs. joist span

Joist spacing is measured on center (o.c.): from the center of one joist to the center of the next. DCA 6's span table lists spacings of 12, 16, and 24 in, and decking makers publish a maximum spacing for their boards.

The joist span is how far a joist reaches between supports. DCA 6 measures it “from the face of support at one end of the joist to the face of support at the other end of the joist,” and it “does not include the length of the overhangs.” On a deck attached to a house, the supports are usually the ledger fastened to the house and a beam on posts.

The overhang is the part of the joist that runs past the beam. DCA 6 lets joists “bear on the beam and extend past the beam face up to the lesser of LO or L/4,” where LO is the allowable overhang in its Table 2 and L/4 is a quarter of the actual span. Its Figure 1A measures the overhang from the outer face of the beam to the outside of the rim joist, and it shows blocking between the joists at the beam “at overhanging joists only.”

Joist spacing by decking type

Deck boards have to bridge the gap from one joist to the next, so for composite and PVC boards the joist spacing comes from the board maker. These are the residential figures from each maker's installation guide: Trex, TimberTech composite, TimberTech Advanced PVC, and Fiberon composite.

On center. *TimberTech's diagram caption allows up to 16 in for MAX boards on the diagonal, while its text says 12 in for all angled installations; ask TimberTech which applies. Commercial projects have their own, often closer, spacing in each guide.

Composite and PVC boards

The figures are maximums. TimberTech says joist spacing “should never exceed 16" on center” and that for a more rigid feel 12 in may be preferred. Near glass and other spots with high heat build-up, it recommends tighter joist spacing, 6 to 8 in at most. Trex adds that its decking “must span at least three joists,” and that heavy items such as hot tubs and planters need span recommendations from a local building engineer or inspector. Fiberon reminds builders that the joists themselves “should meet all applicable IRC / IBC, state and local codes.”

Lumber and span-rated decking (DCA 6)

DCA 6 does not give a joist spacing for wood decking. It requires decking of “dimension lumber (2" nominal thickness) or span rated decking” under the American Lumber Standard Committee's policy, and its commentary says those span-rated products “are assigned a recommended span of usually 16" or 24".” Ask your supplier for the rated span of the decking you buy. Wood decking is attached “to each joist with 2-8d threaded nails or 2-#8 screws.” Choosing a drill/driver for the decking screws? See DEWALT vs Milwaukee drill/driver kits compared.

Diagonal decking needs closer joists

Angles here are measured the way DCA 6 measures them: between the boards and the joists, so 90° is square to the joists. DCA 6 allows lumber decking “from an angle perpendicular to the joists to an angle of 45 degrees to the joists,” with each piece bearing on at least 3 joists.

A board laid at an angle crosses each joist on a slant, so it spans farther between joists than the spacing. By geometry, the length of board between two joists is the spacing divided by the sine of the angle. At 45° and 16 in on center, that is 22.6 in of board between supports; with joists at 12 in it drops to 17 in, close to the 16 in a square board spans.

Each maker states its own rule, in its own terms:

• TimberTech (composite and Advanced PVC): all angled installations need joists 12 in on center at most, and “More severe angles may require closer joist spacing.”
• Fiberon: 12 in on center for a “Diagonal (up to 45-degree angle)” layout on a residential deck.
• Trex: at 60°, joist spacing is 2 in less than its chart; at 45°, 4 in less (12 in for its 1 in boards on a residential deck); at 30°, “1/2 of the distance listed in the chart.”

Trex's 30° layout is shallower than DCA 6 allows for lumber decking, which stops at 45°. Decide the board angle before the framing goes in, because the angle sets the joist spacing. Angled boards also mean more angled cuts; see deck board spacing and layout for gaps, joints, and allowance.

How far deck joists can span (DCA 6 Table 2)

Table 2 of DCA 6 (based on the 2015 IRC; American Wood Council, 2018) gives the maximum span, LJ, for No. 2 grade joists by species group, size, and spacing. Your joist's actual span, L, must be less than or equal to LJ. Reproduced as published:

Source: AWC DCA 6, Table 2, Maximum Joist Spans and Overhangs. DF-L, Hem-Fir, SPF = Douglas fir-larch, hem-fir, spruce-pine-fir. Redwood, cedars, PP, RP = redwood, western cedars, ponderosa pine, red pine. The 18'-0" entries are the prescriptive joist length limit.

What the table assumes

• A 40 psf live load and a 10 psf dead load, No. 2 grade lumber, and wet service conditions, with deflection limited to L/360.
• Incised lumber for Douglas fir-larch, hem-fir, and spruce-pine-fir; northern species design values, without incising, for ponderosa pine and red pine.
• The ledger must be at least a nominal 2×8, and so must joists and rim joists that guard posts are attached to. The 2×6 rows don't apply to those.
• Joist spans are limited to a maximum of 18 ft “to ensure appropriate design of beams and footings.” The commentary adds that “3" was subtracted from each allowable span” so that the table lists clear spans, measured between the faces of the supports.

The commentary also notes that this table format “was adopted in the 2018 IRC,” so spans in DCA 6-15 are slightly different from earlier editions of the IRC. Check which edition your jurisdiction uses.

Overhangs past the beam

The overhang is limited to the lesser of LO and a quarter of the actual span (Table 2's footnote: “Maximum allowable overhang cannot exceed L/4 or ¼ of actual main span”). For a Southern Pine 2×10 at 16 in on center, LO is 3'-5". If its actual span is 12 ft, a quarter of that is 3 ft, which is less than LO, so the overhang may be at most 3 ft.

Source: AWC DCA 6, Table 2. Assumes cantilever length/180 deflection with a 220 lb point load. The overhang may never exceed a quarter of the actual span.

Overhangs don't always grow as joists get closer. A Southern Pine 2×8 may overhang 1'-10" at 12 in on center but 2'-0" at 16 in, so look up the spacing you will actually build. Measure the overhang the way Figure 1A does, from the beam face. DCA 6's commentary words it differently (joists “can cantilever past the joist bearing centerline up to LO or L/4”), so ask your building department which reading it uses. Where joists overhang the beam, DCA 6 requires blocking between them at the beam.

Blocking between joists

Blocking is short pieces of joist lumber fastened between neighboring joists. Where it is needed, and in what form, depends on the document:

• DCA 6: joists may overhang the beam only when “blocking is provided between joists at beam bearing.”
• TimberTech composite: “For best results, install solid wood blocking between each joist, placed every 4' - 6' within the structure,” to reduce movement or twisting of the joists.
• TimberTech Advanced PVC: “You MUST install SOLID WOOD BLOCKING between each joist, placed in rows at maximum every 4’ - 6’ within the structure using exterior grade screws.” If the joists twist, the PVC boards follow them and look wavy.
• Fiberon: “solid blocking (blocking laid flat) will inhibit water flow and result in buildup of organic materials over time,” so “When blocking is required, install ladder style for best results.” It also recommends 2× blocking at butt joints where temperature swings are extreme.

Note that “solid blocking” means different things here: TimberTech means solid wood blocks between the joists, while Fiberon uses the phrase for blocking laid flat. Where board ends meet over a joist, see deck board spacing and layout for Trex's butt joint rules. TimberTech's video shows its spacing and blocking layout:

Worked example: joists for a 16 × 12 ft deck

Take the deck calculator's default deck, 16 × 12 ft, with the boards running the 16 ft length. The joists run the other way, 12 ft out from the house, and repeat along the 16 ft ledger. Assume the plans call for 2×10 joists bought as 12 ft lumber. This example counts lumber; it does not size the joists, which is Table 2's or your designer's job.

The joist count is layout arithmetic: one joist per spacing plus one to close the end. The lumber is measured in board feet with the board foot calculator's own formula. Blocking rows are counted only for decking that requires them, using TimberTech Advanced PVC's 4–6 ft limit, and each block is the spacing minus the 1-1/2 in thickness of a dry 2× joist.

Joists        = 192 in ÷ 16 in = 12 spaces, + 1 = 13 joists
Joist lumber  = 13 × (2 × 10 × 12 ÷ 12) = 13 × 20 = 260 board feet
Blocks        = 13 − 1 = 12 per row, each 16 − 1.5 = 14.5 in long
Per block     = 2 × 10 × 14.5 ÷ 144 = 2.014 board feet
Rows ≤ 6 ft apart = 1 row × 12 = 12 blocks = 24.17 board feet
Rows ≤ 4 ft apart = 2 rows × 12 = 24 blocks = 48.33 board feet

The same deck with the boards at 45°, framed at 12 in on center as TimberTech and Fiberon require:

2×10 joists bought as 12 ft lumber (an assumption). Board feet use nominal sizes, as the board foot calculator does. Ledger, beam, rim joist, posts, and footings are not included.

Framing at 12 in adds 4 joists and 80 board feet of joist lumber. The blocks get shorter but more numerous, so the blocking stays about the same. Measure your joists before cutting blocks, since actual thickness can vary. The decking itself doesn't change for square boards: the deck calculator gives 26 rows and 29 boards of 16 ft with a 10% allowance at any joist spacing. It does not model diagonal boards, so raise the allowance for a 45° layout.

Before you build or order

• Get the installation guide for the exact decking you will buy, and read its joist spacing chart.
• Decide the board angle before framing; angled boards need closer joists.
• Ask your building department which code edition it uses and whether it accepts DCA 6 for your deck.
• Look up your joist size in Table 2, or your code's table, for your species, grade, and spacing. Don't stretch a span the table doesn't list.
• Plan blocking where your decking maker requires it, and at the beam wherever joists overhang it.
• Count boards with the deck calculator and joist lumber with the board foot calculator; check actual lumber dimensions in nominal vs. actual lumber sizes.