If you’re installing composite decking, it’s essential to get the joist spacing right from the start. Improper framing can lead to soft spots, sagging boards, or long-term structural issues that are costly to fix.
We recommend spacing joists no more than 12 inches apart to support the weight and flexibility of composite materials. Even though some brands allow wider spacing, tighter placement delivers better performance and durability. The right framing approach helps your composite deck stay smooth, sturdy, and safe for years to come.
What’s the Recommended Joist Spacing for Composite Decking?
Joist spacing refers to how far apart each framing board (joist) is placed under your deck boards. This spacing affects how solid your deck feels when you walk on it, and how well it holds up over time.
Why 12 Inches on Center is Best
We always recommend 12 inches on center for composite decking. This means each joist is spaced 12 inches apart from the center of one joist to the center of the next.
Here’s why:
- Composite decking isn’t as stiff as wood
- Narrow spacing adds strength and support
- Less movement means fewer issues with uneven boards
If you go with 16 inches on center, which is allowed by some manufacturers, your deck may still meet code, but the boards may feel softer underfoot and could show more movement over time.
What If My Decking Runs at an Angle?
If your deck boards are going to run at an angle (like a diagonal pattern), we space our joists 8 inches on center. Running decking at an angle increases the span between joists, so tighter spacing is needed for support.
Key Takeaway: Always reduce your joist spacing when installing composite boards at an angle. We recommend 8 inches on center for best results.
Need expert help with composite decking? Contact DW Decks for a free consultation. We’ll make sure your joist spacing is right, your frame is strong, and your boards are lined up straight. Your deck will look great and last for years.
What Happens If Joist Spacing Is Too Wide?
If you space your joists too far apart, you risk:
- Sagging boards in the middle of the span
- Movement or bounce when walking across the deck
- Waves or unevenness that show up over time (called “undulation”)
Some brands say you can go up to 16 inches on center, but we’ve seen more issues with long-term performance in those builds.
More joists mean more structure working together to keep the deck steady. It also reduces the chance that a single joist moving out of place will create visible dips or waves in your boards.
Do You Need Ladder Blocking Between Joists?
Yes, and we always recommend it.
Ladder blocking means adding short boards between the joists. These boards help stop the joists from twisting or moving over time. This keeps your deck strong and flat.
Here’s what we suggest:
- Put ladder blocking every 10 feet
- Add blocking between each joist bay
- Make sure the frame stays firm and square
Pro Tip: Add blocking before you lay the deck boards. It helps keep everything in place and stops the frame from shifting later.
Summary: Our Best Practices for Composite Decking Joist Spacing
Here’s a simple checklist to build a strong composite deck:
- Use 12 inches on center for most deck boards
- Use 8 inches on center if boards run at an angle
- Add ladder blocking every 10 feet to stop joist movement
- Avoid 16 inches on center unless the brand allows it and it’s well supported
These steps help stop sagging, bending, and uneven spots. They keep your deck safe, solid, and looking good for years.
Ready to Build a Strong, Long-Lasting Composite Deck?
At DW Decks, we build composite decks that are made to last. From proper joist spacing to high-quality materials and expert installation, we don’t cut corners. If you want a smooth, solid, and safe outdoor space, we’re ready to help.
Call DW Decks today to schedule your free quote and get started on your dream deck.
