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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.)

12"

14"

16"

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"

15'-5"

2x12

18'-0"

16'-6"

13'-6"

​Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir

2x6

9'-6"

8'-4"

6'-10"

2x8

12'-6"

11'-1"

9'-1"

2x10

15'-8"

13'-7"

11'-1"

2x12

18'-0"

15'-9"

12'-10"

​Redwood, Western Cedars, Ponderosa 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"

Joist Spacing for Composite Decking

Before you build a composite deck, you should always read the instructions from the manufacturer, paying special attention to how far apart the joists need to be. Most composite decking has a maximum joist spacing of 16 inches on center for straight decking and 12 inches on center for diagonal decking at a 45-degree angle.

Also, many types of composite decking need stair stringers that are 12" or even 9" on center to support composite stair treads. If you are installing composite decking over an existing frame, you may need to add new intermediate joists or stair stringers to meet the installation requirements.

Since most composite products are more flexible than their wood counterparts, they don't hide imperfections in the framing as well. This can sometimes make the surface not even. So, putting a string across the joists for composite deck boards can help find spots that are higher than others. You can fix these spots with a power hand planer to make the surface of your dream deck more even.


How to Space and Lay Joists For Decking

As floor joists are an important part of your deck's structure, it's important to take the time to learn how to space them out. Start by making sure that any existing structures are solid.

You'll also need your deck plan, including the dimensions and layout of the railing, especially if the way you plan to install the railing would otherwise get in the way of blocking your frame. With a plan and some hard work, figuring out how to lay joists for decking can be pretty easy.

Materials & Tools Needed

You may need:

  • Mitre Saw

  • Hammer

  • Tape measure

  • Drill

  • Chalk Line

  • Level

  • Pencil/marker

  • Wood screws

  • Fasteners

  • Spare Lumber

  • Safety Glasses

Step 1: Layout the first joist position on an angle.

Start with one end of the rim joist and work your way to the other. The position of your first joist will help you figure out where to put the rest, making it easier to put each board after that. For a corner with an angle, you can use a scrap piece of wood as a guide for where to put the first joist.

Step 2: Use a sharpie or construction pencil to mark your joists

Use a pencil, marker, or chalk line to put marks on the header at 16" on the centre for the floor joists. Draw an X on the side of the line where you want your boards to go. This can help plan where the hardware for hanging things will go.

Step 3: Toenail a joist to the rim joist

Make sure the top of the joist is even with the header, and then use screws or nails to attach the rim joist to the first joist. Using a support beam keeps the joist in place while the connection is made. Remember that toenail boards can't handle winds of 75 mph or more. In areas with a lot of wind, you'll need to secure each one with hurricane clips.

Step 4: Use a chisel to notch beams to create a level frame

Notching a joist to fit over a beam. All the other joists were 1/4" lower than this one. To make the top of the deck frame level, you can cut out a section of the joist or add shims to a narrow joist. Be careful not to cut a notch that is bigger than the maximum size allowed in different places along the beam spans.

Step 5: Always fill all the nail holes for deck hardware

Structural screws with a coating that keeps them from rusting hold things together much better than nails. The hole made by the nail can get bigger over time because the wood can grow and shrink as it gets wet and dry. Read the label on the box of structural screws to make sure they go with the right joist hanger. Simpson Strong-Tie makes a full line of hangers and structural screws that have been approved. Don't skip any of the nail hole spaces, as they are all important for the stability, safety, and longevity of your deck. If you don't do this, it could also affect your warranty.

Step 6: Inspect the board and trim off the rougher edge

Measuring the length of the treated 2x10 joists so they can be cut. Use a speed square to put your joists in a straight line. Pay attention to the top of the board to figure out where to put it.

Step 7: Install Joists Crown Side Up at Centre

Install more pressure-treated 2x10 joists in order, starting with the first one. Connect beams every 16 inches (or your required spans as determined above). Make sure to put the crown side of the joists up, so that any bowing in the board faces up. Don't forget to use your knees to lift the joists!

Step 8: Install Hurricane Tie to Each Beam Connection

Use a Simpson Strong-Tie H2.5Z Hurricane Tie or something similar to connect all of the joists to the beams. Again, instead of nails, use Simpson structural screws. Remember that hurricane ties can't take the place of solid blocking. And if you're putting ties on plated trusses, don't finish the fastening from behind the truss plate, because that can hurt how well the hardware works.


Building a safe frame can be hard work because even the smallest mistakes need to be fixed. If you find professional deck builders near you, you can be sure that your project will be done according to all local rules, on time, and to the highest standards.


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