Deck Joist Span Calculator: Maximum Joist Span Chart

How to use the deck joist span calculator

1. Pick the species group shown on the grade stamp of your lumber. Douglas fir-larch, hem-fir and spruce-pine-fir share one set of values. Redwood, western cedars, ponderosa pine and red pine share another, lower set.
2. Pick the joist size and spacing. Closer spacing allows a longer span, and a deeper joist allows a longer span still.
3. Enter your planned span. Measure from the face of the ledger (or first beam) to the face of the beam, not counting the part that hangs past the beam.
4. Enter any overhang past the beam. The calculator checks it against the allowed cantilever.
5. Enter the deck width along the ledger to count the joists.

The result shows the table limit for your choice and whether your span and overhang pass. It also shows the smallest joist size that works at that spacing. The size list shows each size’s maximum span, so you can compare them at a glance.

Deck joist span chart (DCA 6 Table 2)

Maximum joist span, face of support to face of support, for No. 2 lumber, from AWC DCA 6-15 Table 2:

The table assumes a 40 psf live load and a 10 psf dead load, wet service conditions, and a deflection limit of L/360. Incising (the small slits in some treated lumber) is assumed for Douglas fir-larch, hem-fir and spruce-pine-fir. Redwood and cedar values are for northern species with no incising. Joist length is capped at 18 ft because of how the guide sizes footings, which is why the longest spans stop at 18′-0″.

Size or spacing? Both buy span. For Southern pine 2×8, moving from 24 in to 16 in spacing adds 2′-2″ of span (9′-8″ to 11′-10″), and moving to 12 in adds another 1′-3″. Going up one size at the same 16 in spacing, to a 2×10, adds 2′-2″ and uses fewer joists than tighter spacing. Price out both before you decide.

How far can a deck joist overhang the beam?

DCA 6 lets joists cantilever past the beam. The overhang is limited to the lesser of the tabulated value and one quarter of the actual joist span (L/4). The table limits come from a cantilever deflection of L/180 with a 220 lb point load at the end. At 16 in on center, the tabulated overhangs are:

The L/4 rule often controls. A 2×10 spanning 8 ft can only overhang 2 ft, even though the table allows more.

The 2021 IRC, unchanged in the 2024 edition, prints the cantilever differently: Table R507.6 keys it to the joist’s back span (4 to 18 ft) instead of the spacing, allows interpolation between columns and marks some long back spans “NP” (not permitted). For a few combinations it is stricter than DCA 6. A Southern pine 2×12 at 12 in spanning 18 ft gets 4′-6″ from DCA 6 but only 4′-1″ from the IRC, and a Southern pine or Douglas fir 2×8 spanning more than 12 ft (redwood or cedar: more than 10 ft) gets no cantilever at all under the IRC. The calculator uses the smallest of the three limits (DCA 6 value, span ÷ 4, IRC back-span value) and rounds down to the inch.

Worked examples

A deck that projects 12 ft from the house, Southern pine at 16 in. A 2×8 reaches only 11′-10″, so it fails. A 2×10 is good to 14′-0″, so it works. With a 12 ft span, the overhang is limited to 3′-0″ (12 ÷ 4), which is less than the table’s 3′-5″.

The same deck in hem-fir with a 2 ft overhang. A 2×10 at 16 in allows 13′-7″, so the 12 ft span passes. The overhang limit is again 3′-0″, so 2 ft is fine.

A 16 ft wide deck at 16 in on center. 192 in ÷ 16 = 12 spaces, so 13 joists, plus the rim joists. Use the deck calculator for the boards, joists and screws on the whole deck.

Other DCA 6 rules that affect joists

• 2×8 minimum in places. The ledger must be at least a 2×8, and joists and rims that carry guard posts must be at least 2×8. A 2×6 may pass on span, but it cannot carry the railing posts.
• Joist hangers. Hangers must be at least 60% as deep as the ledger or beam, and they must be corrosion-resistant.
• Durable lumber. Use preservative-treated wood or a naturally durable species. For redwood or western cedar, that means 90% or more heartwood on each side. Wood in contact with the ground must be rated for ground contact, and cut ends must be field-treated.
• Decking. Each piece of decking must bear on at least 3 joists, and boards may run anywhere from perpendicular to the joists to 45° across them.

Which code applies?

DCA 6 is a prescriptive guide based on the International Residential Code. The edition used here, DCA 6-15, is based on the 2015 IRC and is still the current edition on AWC’s site as of October 2026. We compared it value by value with the 2021 IRC Table R507.6 at 40 psf live load, as printed in the Town of Prospect, Connecticut deck guide (2022 Connecticut code, based on the 2021 IRC) and the Fairfax County, Virginia 2021 deck details. All 36 allowable spans are identical. Only the cantilever limits differ, as explained above. The 2024 IRC made no changes to Table R507.6, so its 40 psf spans and back-span cantilever limits are the same as the 2021 values used here; we checked all 36 spans and 96 cantilever cells against a 2024 reprint of the table. The 2024 edition did revise the deck beam table (R507.5), which this calculator does not use. The 2021 table also has 50, 60 and 70 psf snow-load rows with shorter spans; this calculator covers the 40 psf row only, so if your area’s ground snow load is above 40 psf, use your local table. Heavier loads such as a hot tub are outside both tables. Many local guides measure span to the beam centerline rather than its face; measuring to the centerline is the safer choice. Your building department has the final say.

If the deck has steps, the stair stringer calculator lays out the rise, run and stringer length. For post footings, use the concrete bag calculator.

Sources & assumptions

How this calculator gets its numbers. Full method: our methodology.

• Maximum spans and overhangs are AWC DCA 6-15 Table 2 values: 40 psf live load, 10 psf dead load, No. 2 grade lumber, wet service, L/360 deflection.
• Span is measured from the face of the ledger or beam to the face of the next support, not counting the overhang. The allowed overhang is the lesser of the tabulated value and the actual span ÷ 4.
• Incising is assumed for Douglas fir-larch, hem-fir and spruce-pine-fir; northern species are not incised. Joist length is capped at 18 ft in the table.
• Joists = deck width ÷ spacing, rounded up, + 1. Rim joists, blocking and doubled joists are extra.
• Checked value by value on 2026-10-27 against the 2021 IRC Table R507.6 (40 psf live load): all 36 spans match DCA 6-15. The 2021 IRC keys the cantilever to the back span; the calculator applies the smallest of the DCA 6 overhang, span ÷ 4 and the interpolated IRC value, rounded down to the inch.
• Code editions (checked 2026-10-28): DCA 6-15 is still AWC’s current DCA 6. The 2024 IRC did not change Table R507.6; its 40 psf spans and back-span cantilevers equal the 2021 values used here (all 36 spans and 96 cantilever cells compared). The 2024 change to deck beam spans (R507.5) does not affect this calculator.
• Covers the 40 psf live-load row only. The 2021 IRC has shorter spans for 50, 60 and 70 psf ground snow load; use your local table there.

Sources

1. American Wood Council, DCA 6 – Prescriptive Residential Wood Deck Construction Guide (2015 IRC). Table 2 maximum joist spans and overhangs, with its footnotes; Figure 5 span and overhang measurement.
2. Town of Prospect, Connecticut, Building Department, Residential Wood Deck Construction Guide (2022 Connecticut State Building Code, based on the 2021 IRC). Reprints 2021 IRC Table R507.6: 40 psf spans for all three species groups and maximum cantilever by back span (4–18 ft, NP = not permitted, interpolation allowed).
3. Fairfax County, Virginia, Typical Deck Details (version 2021.0). Table 2 (Southern pine): same spans and back-span overhangs as the 2021 IRC; overhang never more than a quarter of the joist span.
4. Fine Homebuilding, How Far Can You Cantilever Deck Joists? (Fine Homebuilding, October 2024). Glenn Mathewson: after the 2021 revision of the cantilever method, no changes were proposed or made to deck joist Table R507.6 for the 2024 IRC.
5. Deck Estimate Tools, Deck Joist Span Table (IRC 2024, Table R507.6). Reprint of 2024 IRC Table R507.6 at 40 psf (checked against ICC Digital Codes, Sep 24, 2026): spans and back-span cantilevers identical to the 2021 values used here.

Last reviewed October 29, 2026. Product figures vary by brand; the label on what you buy wins.