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.

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?

Your joist span will depend on the size of the lumber joists. See the below span chart as well as the local codes and guidelines for your jurisdiction. 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

You can also use Decks.com’s Joist Span & Spacing Calculator to determine how long your deck joist should span.

Deck Joist Span Chart

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'-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"

 

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 ledger board. Be particularly cautious to follow an approved ledger connection method to the house. If you identify a crown in the board, you should always install it upwards. The crown will likely eventually settle after completing construction and should settle 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.

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 maximum 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 9" 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.

How to Space and Lay Joists For Decking

As they serve an integral role in the structural integrity of your deck, taking the time to learn how to space floor joists is essential to the safety of the final result. Start by ensuring any existing structures are sound.

You’ll also need your deck plan ready, including dimensions and layout for the railing, especially if your railing installation method would otherwise interfere with the blocking of your frame. With a game-plan and a little hard work, figuring out how to lay joists for decking can be relatively simple.

Materials & Tools Needed

You may need:

• Miter Saw
• Pneumatic palm nailer / Hammer
• Tape measure
• Electric drill
• Chalk Line
• Level
• Pencil/market
• Joist tape
• Wood screws
• Fasteners
• Spare Lumber
• Safety Glasses

Step 1: Lay out the first joist position on an angle.

Start your layout from one end of the rim joist and work towards the other. The position of your first joist will help guide the placement of the rest, and make placing each subsequent board simpler. For an angled corner, you can use a scrap piece of wood as a guide to situate the first joist into position.

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

Mark out floor joist spacing locations at 16" on center spacing on the header with a pencil, marker, or chalk line. Draw an X on the side of the line where you want your boards to set. This can help plan the layout for the location of hanging hardware.

Step 3: Toe nail the joist to the rim joist.

Ensure the top of the joist is flush with the header, then connect the rim joist and first joist using screws or nails. Using a support beam can keep the joist in position while the connection is made. Keep in mind that toe nailing boards are inadequate for 75+ mph winds. In high wind areas, you’ll need to secure each with hurricane clips.

Step 4: 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. Be careful to not exceed the maximum allowed notch size in different locations of the beam spans.

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

Using a palm-nailer or hammer to install joist hangers with 1-1/2" teco nails to the ledger board. Don’t skip any of the available nail hole spaces, as each is necessary for the stability, longevity, and safety of your deck. Failing to do so may also affect warranties.

Step 6: 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. Pay attention to the crowning of the board for placement.

Step 7: Install Joists Crown Side Up at Center

Continuing from your first joist, install additional 2x10 pressure treated joists in sequence. Connect beams at 16" on center (or your required spans as determined above). Be sure to install the joists crown side up, ensuring any bowing in the board is facing upwards. Don’t forget to lift joists with your knees!

Step 8: Install Hurricane Tie to Each Beam Connection

Use a Simpson Strong Tie H2.5Z Hurricane Tie, or equivalent, on every joist-to-beam connection. A pneumatic palm nailer is adequate for fastening them, but keep in mind that hurricane ties are not a replacement for solid blocking. And if you’re installing ties on plated trusses, don’t complete the fastening from behind the truss plate, as doing so can compromise the performance of the hardware.

Building a safe frame can be challenging work because even the smallest mistakes need to be remedied. Finding professionals deck builders near you can ensure your project is completed in accordance with all local codes, constructed on-schedule, and up to the highest standards. You can also find retailers for the most appropriate materials or kits that make installation easier.