Deck Joist Spacing

A lot of questions revolve around joists when it comes to building a deck. What is the proper placement? How far apart are floor joists placed? How do I keep them even?

In general terms, joists spaced 16 inches on center can span 1.5 times in feet their depth in inches. A 2×8 up to 12 feet; 2×10 to 15 feet and 2×12 to 18 feet. The larger the deck, the larger the joists.

In typical deck construction, with a ledger on one side of the joist and beam on the other, the size of the joists is driven by the size of the deck and based on the general maximum spans mentioned above. For best results, refer to our wood and composite deck joist span table.

Joist Spacing (o.c.)12″16″24″ 
SpeciesSizeAllowable Span
Southern Pine2×69′-11″9′-0″ 7′-7″ 
2×813′-1″11′-10″9′-8″
2×1016′-2″14′-0″11′-5″ 
2×1218′-0″ 16′-6″13′-6″ 
Douglas Fir-Larch,Hem-Fir,Spruce-Pine-Fir2×69′-8″ 8′-4″6′-10″ 
2×8 12′-8″ 11′-1″9′-1″ 
2×10 15′-8″ 13′-7″11′-1″ 
2×12 18′-0″ 15′-9″12′-10″ 
Redwood,Western Cedars,Poderosa Pine, Red Pine2×68′-10″ 8′-0″6′-10″ 
2×8 11′-8″ 10′-7″8′-8″ 
2×1014′-11″ 13′-0″ 10′-7″ 
2×1217′-5″15′-1″12′-4″ 

For conventional guardrail post installation bolted to the side of the framing, larger framing will provide more strength in the rails. For upper-level decks, 2×10 is recommended as the minimum size to use for strong guard post connections. 2×6 joists should only be used on ground-level decks that do not require, and will not provide for, any guards. Similar to guards, if stairs are to be hung from the side of an upper-level deck, 2×10 as a minimum is recommended, as it will exceed the depth of the first step.

Joist hangers are used to attach the ends of joists to the face of a beam or a ledger board. If you identify a crown in the board, you should always install it upwards. The crown will eventually settle after completing construction and should stiffen in the proper position after drying.

The layout term for joists is “On Center,” which is the center-to-center measurement from one joist to the next. Most decks use 16″ on center spacing for joists. Most decking is not strong enough to support longer spans than 16″. Some builders reduce joist spacing to 12″ on center to strengthen the deck frame or to increase maximum allowable joist spans.