HOW-TO GUIDES

How to Install and Space Deck Joists for Composite Decking

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These step-by-step instructions from our How-To Team will show you how to build a sturdy frame that will ensure your composite decking lasts for years.

Disclaimer: Instructions provided by Trex® are based on the build specifications that Trex® demonstrated in the Trex® Academy video series, and are not universal to all homeowners’ projects. A homeowner’s build may vary from Trex’s instructions based on the homeowner’s local municipality requirements, individual design preference and project specifications. For questions or assistance needed during your project, please reach out to howto@trex.com

8-12 Hours with a Team of Two*
*Timing is estimated for a 14'x22' deck with two workers with intermediate carpentry skills. Estimate includes installation, hardware, and blocking for 27 joists. Times may vary based on the complexity of the shape and features of the deck.
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For any home project, be certain to follow local code and permitting requirements.
© 2022 Trex Company, Inc.
01. Intro 00:00
02. Crowning of Joists 00:18
03. Installing the Rim Joist 01:11
04. Installing First Beam Block 03:10
05. Installing Joist Hangers 04:45
06. Joist to Blocking Connection 08:46
07. Joist to Beam Connection 10:27
08. Installing Beam Blocking 10:42
09. Breaker Board Framing 12:35
10. Planning and Cutting the Cantilever 13:37
11. Front Rim Joist Install 18:38
12. Checking for Flatness 21:01
Step-by-Step Instructions
Chapter 01: Determining and Marking Joist Locations
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So you're ready to get started? Watch our how-to tutorial before you start your build.

Before you begin any DIY project, wear the appropriate personal safety equipment. Eye protection, ear protection, gloves, long pants, a long-sleeved shirt, and reinforced toe shoes are recommended. Always make sure you have a first aid kit nearby. For any home project, be sure to follow local code and permitting requirements.

Proper deck joist spacing is essential for building a strong, durable deck that can safely support foot traffic, furniture, and environmental loads. Incorrect spacing can result in a weak or sagging deck surface, potentially creating safety hazards such as trip risks, structural instability, or premature wear of decking materials. Additionally, failing to follow recommended deck frame spacing guidelines may lead to inspection failures or noncompliance with local building codes, requiring costly corrections before the deck can be approved for use.

While some materials, such as thicker wooden decking boards, may allow for 24-inch on center (OC) spacing in specific applications. When installing composite decking, joists should generally be spaced no more than 16 inches OC for standard applications. If you plan to install your decking diagonally relative to the joists, 12-inch spacing is required. Installing joists 12 inches apart or less may also be required for joists that carry heavier loads, have longer spans, or utilize intricate deck patterns. Because of the additional force of stepping down, you should install your stair stringers to the specifications of the type of tread material you are using, which is usually 12 OC or less. If you are replacing wood decking with composite decking and find that the existing joists are spaced 24 inches OC, adding one new joist between each of the existing joists is a simple fix.

It’s best to mark both the ledger board and the rim joist at the same time. Marking both boards before you install the ledger will not only ensure accuracy, it will also help you identify the areas to avoid when installing your ledger board fasteners, so they don’t interfere with the joist or joist hangers.

Step 1

Once they are cut to size, set both the rim joist and the ledger, side by side, on a pair of sawhorses.

Step 2

Hook your tape measure on the end of either board and make a mark every 16 or 12 inches (depending on your application).

Step 3

Before installing the ledger board, double check that the joists won’t interfere with any wall protrusions, railing post locations, or any other obstacle that a joist location might interfere with.

Step 4

Once you have confirmed that the joist spacing will work, use a square to draw one line that crosses both boards at those locations you marked. You can mark the location of each side of the joists or just mark one side and draw an “X” on the side of the line where the joist will be installed.


Tools

Bar Clamps

Chalk Line

Circular Saw

Drill

Drill Bits

Drill Tips

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Materials

3" Exterior-Grade Wood Screws

Simpson Strong Tie SD Connector Screws

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