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,



 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" 


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.

Laying out the first joist position on an angle.
Laying out the first joist position on an angle.

Using a scrap piece of wood as a guide to locate the first joist in the situation of an angled corner.

Use a sharpie or construction pencil to mark your joists.
Use a sharpie or construction pencil to mark your joists.

Marking joist locations at 16" on center spacing on the header with a marker.

Toe nailing a joist to the rim joist.
Toe nailing a joist to the rim joist.

Installing a joist over a beam. Make sure the top of the joist is flush with the header. Toe nailing will fail in 75+ mile per hour winds. Use hurricane clips in high wind areas.

Use a chisel to notch beams to create a level frame.
Use a chisel to notch beams to create a level frame.

Notching a joist over a beam. This joist was 1/4" higher than all the others. You can notch out a section of the joist or add shims to a narrow joist to even out the top of the deck frame.

Always fill all the nail holes for deck harware.
Always fill all the nail holes for deck harware.

Using a palm-nailer or hammer to install a joist hanger with 1-1/2" teco nails to the ledger board. Reminder: Ensure ledger board flashing is projecting the area it is mounted on to avoid moisture seeping into your foundation.

Inspect the board and trim off the rougher edge.
Inspect the board and trim off the rougher edge.

Measuring the 2x10 pressure treated joist length to prepare for cutting. Use a speed square to square off your joists.

Lift with your knees!
Lift with your knees!

Installing 2x10 pressure treated joists at 16" on center. Be sure to install the joists crown side up.

You can use a pneumatic palm nailer.
You can use a pneumatic palm nailer.

Installing a Simpson Strong Tie H2.5Z Hurricane Tie for every joist to beam connection.