Building a sturdy, long-lasting deck starts with understanding the foundation—specifically, the spacing and sizing of your deck joists. While the question "how far apart should deck joists be?" seems simple, the answer dictates the stability, safety, and overall performance of your finished outdoor space. At A to Z Supply, we know that proper framing ensures your decking material won't sag or fail under pressure.
Understanding the Deck Joist: What is a Joist on a Deck?
A deck joist is a horizontal framing member that supports the decking boards above. Joists run between beams or from a beam to the ledger board attached to the house. Together, they:
- Carry the weight of people, furniture, and the deck surface
- Transfer that load into the beams, posts, and footings below
- Help keep the deck surface flat and resistant to bounce or sag
Most residential deck joists are made from pressure‑treated lumber, sized and spaced according to span, local building code, and the decking material you plan to use.
What Is a Rim Joist on a Deck?
A rim joist (also called a band joist) runs around the outer edge of the deck framing. It:
- Caps the ends of the joists and ties them together
- Helps keep the frame square and rigid
- Provides fastening support for perimeter deck boards and often for railing posts
Because it plays such a key structural role, the rim joist should be the same size and grade as the main joists and securely fastened with the proper structural hardware.
How Far Apart Should Deck Joists Be?
Deck joist spacing is usually expressed as “on center” (O.C.), meaning the distance from the center of one joist to the center of the next. Common spacings are: 12 inches O.C., 16 inches O.C., 24 inches O.C. This spacing provides excellent support for most common decking materials. However, this spacing is often determined by the material you plan to use for the surface:
- 16 Inches O.C. : This is the minimum requirement for standard 5/4" deck boards (like treated lumber or standard composite) to prevent noticeable bounce or sagging over time.
- 12 Inches O.C. : If you are installing premium composite decking, PVC, or interlocking tile systems, manufacturers often require 12 inches O.C. spacing. Installing joists closer provides maximum rigidity and guards against warranty voidance due to insufficient support.
- 24" O.C. : This is generally too wide for most modern deck boards unless the manufacturer specifically allows it.
Planning Layout Tip
Pick your decking brand and board orientation first, then:
- Confirm the maximum joist spacing allowed.
- Sketch your deck framing with joists at that spacing.
- Make sure all deck board ends land on a joist, not in between.
This simple planning step prevents headaches and expensive re‑work later.
Where to Buy Decking Materials
If you are planning a deck in the Joplin area and wondering where to buy decking materials, A to Z Supply is your local one‑stop source. We offer:
- Pressure‑treated framing lumber for joists, beams, and rim joists.
- Wood and composite decking with clear spacing and installation guidance.
- Structural fasteners, hangers, and hardware rated for outdoor use.
A to Z Supply sells far more than just gutters—contractors and homeowners rely on us for decking, roofing, siding, windows, doors, trusses, fencing, sheet metal, garage doors, and more! Contact A to Z Supply today for help planning joist spacing and selecting the right materials for a strong, comfortable deck.
Next Up
Keep an eye out for "Part 2: What Size Joists Does Your Deck Need?”