What Is the Proper Joist Spacing for Composite Decking?
Choosing the right joist spacing for composite decking determines how solid your deck feels, how well it holds up over time, and whether it meets manufacturer and building code requirements. Composite boards are not structural, so the frame underneath does all the work. Correct spacing ensures the boards stay flat, secure, and aligned for years.
Quick Answer:
Most composite decking requires 16 inches on center for standard installations and 12 inches on center when boards are installed diagonally. These measurements apply to nearly all leading composite decking brands. This guide explains the correct spacing for straight and diagonal decking, brand-specific guidelines, the difference between on-center measurements, how to correct older 24 inch layouts, stair spacing requirements, and common mistakes to avoid.
Why Joist Spacing Matters
Composite boards are more flexible than wood. If joists are spaced too far apart, the boards can flex, sag, or feel soft underfoot. Incorrect spacing also affects:
Long-term board performance
Appearance of the deck surface
Warranty coverage
Inspection and code approval
Lifespan of the frame and decking
Following spacing guidelines prevents these issues and ensures the deck performs as intended.
Standard Joist Spacing for Composite Decking
Straight Installation
For most composite brands, the correct spacing is:
16 inches on center.
This is the most common requirement and is supported by building codes and manufacturer instructions.
Diagonal Installation
Boards installed at a 45 degree angle must be supported more frequently because the span increases.
The correct spacing is:
12 inches on center.
High Load or High Traffic Decks
Decks that support heavy furniture, outdoor kitchens, hot tubs, or commercial use benefit from tighter spacing. In these cases:
12 inches on center improves stiffness and reduces deflection.
Brand-Specific Joist Spacing Guidelines
| Brand | Straight Spacing | Diagonal Spacing | Notes |
|---|---|---|---|
| Trex | 16 in. OC | 12 in. OC | Standard across all collections |
| TimberTech and AZEK | 16 in. OC | 12 in. OC | MAX boards allow extended spans |
How to Measure On Center (OC)
Joist spacing is always measured from the center of one joist to the center of the next. This is different from measuring the space between joists.
For example, a typical joist is 1.5 inches thick. When joists are spaced 16 inches on center, the actual gap between them is roughly 14.5 inches. This ensures the deck boards are supported evenly across the frame.
What If Your Existing Deck Has 24 Inch Joist Spacing
Many older wood decks were framed at 24 inches in the center. Composite decking cannot span this distance.
The solution is straightforward:
Add a new joist between each existing joist to reduce the spacing.
This brings the frame into compliance with composite decking requirements and avoids sagging, flexing, or voided warranties.
Joist Spacing for Stairs
Composite stair treads flex more than wood, so stair framing requires tighter spacing. Most manufacturers require:
12 inches on center or less.
Wider spacing will cause stair treads to bend under normal foot traffic.
Additional Spacing Considerations
Joist Size and Maximum Span
The structural load on a joist increases with the distance between supports. The size of the joist determines its maximum span.
Common recommendations include:
2 x 6: short spans only
2 x 8: standard for most residential decks
2 x 10 and 2 x 12: longer spans for larger decks
Tighter joist spacing increases allowable span and reduces movement. Make sure you consult with your engineer.
Deck Board Patterns
Decorative layouts such as picture frames, herringbone, chevron, or angled borders require more support.
Use 12 inch spacing under any board that changes direction or meets at a seam. Add blocking anywhere boards intersect.
Blocking
Install blocking every 4 to 6 feet to prevent joists from twisting and to support transitions, seams, and borders. Blocking helps maintain a level surface as the deck ages.
Common Joist Spacing Mistakes
Avoid the following issues when building with composite decking:
Using 16 inch spacing on diagonal decking
Failing to reinforce older 24 inch layouts
Not leveling high or crowned joists
Skipping blocking
Misaligned joist hangers
Forgetting to confirm spacing with product installation instructions
These mistakes reduce deck stability and shorten the lifespan of the structure.
Summary: Correct Joist Spacing for Composite Decking
Use 16 inches on center for straight decking.
Use 12 inches on center for diagonal decking.
Use 12 inches on center or less for stairs.
Adjust spacing for borders and deck patterns.
Reinforce existing 24 inch layouts by adding new joists.
Correct spacing creates a stronger frame, prevents deflection, and supports long-term performance. Always consult with your engineer.
Need Help Planning Your Deck? Visit OK Lumber Co.
OK Lumber Co. supplies:
Composite decking from leading brands
Pressure treated framing lumber
Joist tape and flashing
Joist hangers and fasteners
Tools and accessories for deck construction
If you need help determining the right spacing for your deck or choosing the right composite material, our team is ready to help. Visit us or call for a material estimate.
