Joist spacing chart
Joist spacing is usually chosen by habit — 16 inches because that is what framing is done at. It is actually a three-way trade between decking stiffness, joist span and cost.
Span by size and spacing
Maximum clear spans for No. 2 southern pine, 40 psf live / 10 psf dead, L/360 deflection. Other species differ — check the span table for the species group you are buying.
| Joist | 12″ o.c. | 16″ o.c. | 24″ o.c. |
|---|---|---|---|
| 2×6 | 9′ 11″ | 9′ 0″ | 7′ 8″ |
| 2×8 | 13′ 1″ | 11′ 10″ | 10′ 1″ |
| 2×10 | 16′ 6″ | 14′ 11″ | 12′ 9″ |
| 2×12 | 19′ 0″ | 17′ 2″ | 14′ 8″ |
Read the table as: a 2×8 at 16″ centres spans 11′ 10″. Going to 12″ centres buys another 15 inches of span; going to 24″ loses 21 inches. That is the trade in one line.
What spacing does to the decking
- 5/4 deck board: 16″ maximum, and 12″ feels noticeably stiffer.
- 2×6 deck board: 24″ is acceptable, and it is the reason people use 2×6 decking — it lets the whole frame spread out.
- 1″ composite: 12–16″ depending on the board; most require 12″ for diagonal.
- PVC: 12″ in nearly every system.
A 1″ board at 24″ centres will sag visibly between joists on a warm day, and stay sagged. The decking is the spring, the joists are the supports, and the span between supports is what decides whether the spring is soft.
The cost of each direction
On a 12 × 16 ft deck running joists the 12 ft direction:
- 16″ centres: 13 joists, 156 linear feet.
- 12″ centres: 17 joists, 204 linear feet — 31% more joist material, plus 4 more hangers per side.
- 24″ centres: 9 joists, 108 linear feet — 31% less, but you may need 2×6 decking at a higher cost per square foot.
The frame is usually the smaller line item, so going to 12″ centres to allow a thinner deck board rarely pays. Going to 24″ to save joists usually costs more in decking than it saves in framing.
What 16 inches is actually for
16″ is not a structural number. It is 48 divided by 3, which makes it fit sheathing and insulation modules and divides evenly into an 8 ft sheet. On a deck, where there is no sheathing, the module has no meaning — which is why 12″ and 24″ are both legitimate on a deck and only 16″ on a floor.
What this chart does not do
- It does not cover other species. Douglas fir-larch, SPF and hem-fir all have different design values.
- It does not account for cantilevers, which reduce the allowable back span; see cantilever rules.
- It does not cover engineered joists, which have their own span tables from the manufacturer.
- It does not check the beam or footing, which are sized separately from the joist.
Questions people actually ask
What is the standard joist spacing for a deck?
16 inches on centre is the most common, but it is a habit rather than a rule. 12 inches is used for thinner decking or heavier loads, and 24 inches is used with 2×6 decking.
How far can a 2x8 joist span?
About 11 feet 10 inches at 16 inches on centre for No. 2 southern pine, 13 feet 1 inch at 12 inches, and 10 feet 1 inch at 24 inches.
Can I use 24 inch joist spacing with composite decking?
Only with a board rated for it. Most 1-inch composite boards require 12 or 16 inches; thicker premium boards allow 16. PVC requires 12 inches in nearly every system.
Why is 16 inches the standard spacing?
Because it divides evenly into 48-inch sheathing and insulation modules and into an 8-foot sheet. On a deck there is no sheathing, so 12 and 24 inches are equally valid.