Deck Joist Sizing and Spacing

Enjoying your deck on a beautiful summer day is the perfect way to entertain guests outside or just unwind and relax. And while it’s important to have a deck that’s aesthetically pleasing to you, the most vital part about your deck is making sure it’s structurally sound. That includes knowing the difference between a joist and a beam and what purpose they serve to your deck and its functionality and safety.

What Is a Joist?

Joists are the repeated structural members that are used to build a deck frame. The minimum size joist to be used in deck construction depends on the number of footings and beams that will be installed.

What Is a Beam?

A beam is the main load-bearing element for the structural soundness of your deck’s roof. It’s not only responsible for supporting the weight of the joists, but other building elements, as well.

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 2x8 up to 12 feet; 2x10 to 15 feet and 2x12 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"
Species	Size	Allowable Span
Southern Pine	2x6	9'-11"	9'-0"	7'-7"
	2x8	13'-1"	11'-10"	9'-8"
	2x10	16'-2"	14'-0"	11'-5"
	2x12	18'-0"	16'-6"	13'-6"
Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir	2x6	9'-8"	8'-4"	6'-10"
	2x8	12'-8"	11'-1"	9'-1"
	2x10	15'-8"	13'-7"	11'-1"
	2x12	18'-0"	15'-9"	12'-10"
Redwood, Western Cedars, Poderosa Pine, Red Pine	2x6	8'-10"	8'-0"	6'-10"
	2x8	11'-8"	10'-7"	8'-8"
	2x10	14'-11"	13'-0"	10'-7"
	2x12	17'-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, 2x10 is recommended as the minimum size to use for strong guard post connections. 2x6 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, 2x10 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 deck 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. When you eventually lay the floor, remember to leave proper spacing between deck boards as they will also expand and contract as the weather changes.

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.

Joist Spacing for Composite Decking

Before building a composite deck, always read the installation instructions from the manufacturer, paying special attention to the required joist spacing for composite decking. Most composite decking materials, like Trex composite decking, require minimum joist spacing for composite decking at 16" on center spacing for straight decking and 12" on center joist spacing for 45-degree angle diagonal decking.

Also, many composite decking materials require 12" or even 10" on center stair stringers spacing to support composite stair treads. If you are installing composite decking over an existing frame, you may need to install new intermediate joists or stair stringers to meet the installation requirements.

Since most composite products aren’t as inflexible as their wood counterparts, they don’t hide imperfections in the framing quite as well. This can sometimes cause an uneven surface. That’s why stretching a string across the joist spacing for composite deck boards can help detect spots that may be higher than others. These spots can be addressed with a power hand planer to give you a nicer, smoother surface for the deck of your dreams.