Deck Joist Sizing and Spacing

Having a nice summer day on your deck with your favorite people is a great way to relax and unwind. And even though it's important to have a deck that looks good to you, you should first make sure it's built well. This means you need to know the difference between a joist and a beam and what each one does for your deck's safety and functionality.

What Is a Joist?

Joists are the structural pieces that are used over and over again to build the frame of a deck. The minimum size of the joists used to build a deck depends on how far apart the bearing points are and what kind and grade of treated wood was used to make the joists. There are very strict minimum requirements in the building code for how much weight a deck floor must be able to hold. A span table can be used to figure out the size of the joists.

What Is a Beam?

Your deck's main load-bearing element is a beam. It is not only responsible for holding up the weight of the joists, but also other parts of the building.

The joists sit on top of a beam or are connected to a beam with joist hangers. Vertical posts or columns usually hold up beams that go away from the house. Beam size and construction are very important and should be decided by an architect, a residential structural engineer, or the building department of your local government.

Read more: Reinforcing Deck Rim or Band Joist

How Far Apart Should Deck Joists Be Spaced?

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?

Deck joists are usually spaced 16 inches on center. Before you build a deck, you should check with your local building department and read the decking material's installation instructions. Some decking materials need the joists to be 12 inches apart on center when they are put in at a 45-degree angle to the run of the joists.

How far apart your joists are will depend on how big they are (i.e. 2x8 vs 2x10 vs 2x12). See the span chart below as well as the rules and laws that apply in your area. The joists are bigger the longer the span.

With a ledger on one end of a joist and a beam on the other, the size of the joists is usually determined by the size of the deck and the general maximum spans listed above. For the best results, look at our table of joist spans for composite and wooden decks.

Joist Spacing (o.c.)






Allowable Span

Southern Pine