Deck joist spacing and span tables (AU and US)
Most decks have joists at 450 mm centres in Australia and 16 in. on centre in the US. The decking sets the spacing: 19 mm hardwood can go to 500 mm, 22 mm treated pine to 450 mm, and many composites need 400 to 450 mm. How far each joist can then span depends on its size. A 140 × 45 MGP10 pine joist at 450 mm centres spans about 2.6 m between two supports, and a 2×10 Southern pine joist at 16 in. spans 14 ft 0 in.
The figures below are typical values from published tables. Real spans depend on timber grade, load, wind and how the deck is built, so treat them as a starting point and have your builder, certifier or building department check the final design.
What joist spacing does decking need?
Joist spacing is about the boards, not the joists. A thin board between joists that are too far apart feels springy and can crack or sag, so every board type has a maximum distance between joist centres.
Australia
- 19 mm hardwood decking: up to 500 mm (WoodSolutions Technical Design Guide 21).
- 22 mm H3 treated pine decking: up to 450 mm.
- Composite and PVC: whatever the maker says. Ekodeck Essentials and Trex Select need 400 mm, as does Eva-Last Infinity under its Australian load table; most others in Deckulator's catalogue allow 450 mm, and Ekodeck Summit allows 500 mm.
- Boards laid at 45° need closer joists: TDG 21 works 19 mm hardwood at 45° down to 350 mm.
United States
- 5/4 wood decking laid square to the joists: 16 in. on centre, or 12 in. where a board only spans two joists (IRC Table R507.7).
- 2 in. thick wood decking: 24 in.
- 5/4 decking laid diagonally: 12 in.
- Composite and PVC: the span on the board's ASTM D7032 label (IRC R507.2.2). The Trex, TimberTech and Fiberon lines in Deckulator are all set to 16 in.
How far can deck joists span?
A single span joist sits on two supports, such as the ledger on the house and one bearer. A continuous joist runs unbroken over three or more supports and can span a little further between each.
Australia: MGP10 treated pine at 450 mm centres
| Joist | Single span | Continuous span |
|---|---|---|
| 90 × 45 | 1.5 m | 1.8 m |
| 120 × 45 | 2.1 m | 2.5 m |
| 140 × 45 | 2.6 m | 3.0 m |
| 190 × 45 | 3.6 m | 3.6 m |
| 240 × 45 | 4.8 m | 4.8 m |
These are supplier figures (Hyne T3 Green MGP10, and TABMA Technical Note 1 for 120 × 45) built on AS 1684 for a 2.0 kPa deck load. AS 1684 itself isn't freely published. Where sources disagree, Deckulator uses the lower figure. Hardwood and higher stress grades span differently, so use the table for the timber you are actually buying.
United States: No. 2 Southern pine, 40 psf live load
| Joist | 12 in. o.c. | 16 in. o.c. | 24 in. o.c. |
|---|---|---|---|
| 2×6 | 9 ft 11 in. | 9 ft 0 in. | 7 ft 7 in. |
| 2×8 | 13 ft 1 in. | 11 ft 10 in. | 9 ft 8 in. |
| 2×10 | 16 ft 2 in. | 14 ft 0 in. | 11 ft 5 in. |
| 2×12 | 18 ft 0 in. | 16 ft 6 in. | 13 ft 6 in. |
These come from IRC Table R507.6 (2021, unchanged in 2024) and match the American Wood Council's DCA 6 guide. Douglas fir, hem-fir and SPF span a little less: a 2×10 at 16 in. reaches 13 ft 7 in. Where the ground snow load is above 40 psf, the code's heavier tables apply and spans get shorter.
How far can a joist overhang the bearer?
Letting joists run past the outer bearer (a cantilever) hides the posts and gives a lighter look. It has limits:
- Australia: Deckulator allows an overhang of up to a quarter of the joist's single-span figure, with at least twice the overhang on the inside of the bearer. For a 140 × 45 joist that is up to 650 mm. It prefers 300 mm.
- US: Table R507.6 sets the overhang by the length of joist behind the beam. A 2×10 can overhang 2 ft 0 in. with an 8 ft back span, or 3 ft 0 in. with 12 ft. DCA 6 also caps it at a quarter of the actual joist span.
Worked example: one bearer or two?
Australia. A deck 4.8 m wide and 3.6 m out from the house, on a ledger, with joists at 450 mm. With one bearer set 300 mm in from the front edge, the joists span about 3.3 m. That is too far for 140 × 45 (2.6 m single span), but within 190 × 45 (3.6 m). The other option is a second row of posts: two bearers cut each span to about 1.65 m, which even 90 × 45 joists can cover as continuous joists (1.8 m). Deckulator found the 90 × 45 frame used the least timber. It needs six posts and footings instead of three, so price both.
United States. A deck 16 ft wide and 14 ft out, on a ledger, with joists at 16 in. With the beam about 1 ft in from the edge, the joists span about 13 ft. That rules out 2×8 (11 ft 10 in.) unless you add a second beam, but a 2×10 (14 ft 0 in.) works with one beam and three posts.
What else changes the span?
- Timber grade and species: the tables above are for MGP10 pine and No. 2 Southern pine only.
- Heavy loads: hot tubs, large planters and snow need an engineered design. DCA 6 doesn't cover hot tubs.
- Bounce: a joist at its full table span meets the code but can still feel lively. Going one size up, or spacing joists closer, firms it up.
- Deep joists need blocking: Timber Queensland calls for solid blocking at supports once a joist is more than four times as deep as it is wide.
- Joist ends need support: in the US, joist hangers or blocking at the ledger and beam (IRC R507.6.2). See the footings and bearer guide for what carries the joists.
Common questions
What is the standard joist spacing for a deck?
In Australia it is usually 450 mm centres, the limit for 22 mm treated pine decking; 19 mm hardwood allows 500 mm. In the US it is 16 in. on centre for 5/4 wood and most composite decking.
Can deck joists be 24 inches on center?
Only with 2 in. thick wood decking (IRC Table R507.7) or a composite whose label allows 24 in. Standard 5/4 wood decking and most composites need 16 in.
How far can a 2×8 deck joist span?
In No. 2 Southern pine at a 40 psf live load, IRC Table R507.6 allows 13 ft 1 in. at 12 in. on centre, 11 ft 10 in. at 16 in. and 9 ft 8 in. at 24 in.
How far can 140 × 45 deck joists span?
In MGP10 treated pine at 450 mm centres, supplier tables built on AS 1684 give about 2.6 m between two supports, or 3.0 m per span when the joist runs continuously over three or more supports.
Does composite decking need closer joists?
Often, yes. Some Australian composites, such as Ekodeck Essentials and Trex Select, need joists at 400 mm, and most US composites are rated for 16 in. Always use the spacing in the maker’s installation guide.
This guide explains common rules in plain words. It isn’t engineering advice: span tables, footings and barriers depend on your timber grade, soil, wind and local rules. Have a builder, certifier or building department check your design before you build.