The Backbone of Your Deck
Joists are the repeated, parallel structural members that provide the rigid framework for your deck surface. If the concrete piers are the foundation and the beams are the load-bearers, the joists are the spine. They carry the weight of the decking boards, the furniture, and the people standing on the deck, transferring that load safely down to the beams.
Properly spacing your joists is non-negotiable. If joists are spaced too far apart, your deck boards will sag, bounce, and eventually break under pressure.
Understanding Joist Spacing (On Center)
In construction, repetitive framing members are almost always measured "On Center" (O.C.). This means the measurement is taken from the exact center of one joist to the exact center of the next joist—not the empty space between them.
The most common joist spacing for residential decks is 16 inches On Center (16" O.C.). This provides a strong, stiff feel for standard 5/4-inch wood decking or 1-inch composite decking.
If you are laying deck boards diagonally (at a 45-degree angle), the span between joists becomes physically longer. To prevent the boards from sagging, building codes typically require you to reduce the joist spacing to 12 inches On Center.
How to Calculate the Number of Joists
To figure out exactly how many joists you need to order to build your deck framework, you must account for the entire length of the deck plus the "starter" joist.
The Formula
- Convert the total Deck Length from feet to inches (multiply by 12).
- Divide the total deck length in inches by your desired Joist Spacing (e.g., 16 inches).
- If the result is a decimal, round up to the next whole number.
- Add 1 to the final number. (This accounts for the very first joist at the starting edge of the deck, often called the rim joist).
Quick Example: 16 ft Deck at 16" O.C.
Imagine you are building a deck that is 16 feet long and you are spacing your joists at the standard 16 inches on center.
- Convert length to inches:
16 × 12 = 192 inches - Divide by spacing:
192 ÷ 16 = 12 - Add the starter joist:
12 + 1 = 13 joists
You will need to order 13 joist boards to frame this section of the deck.
Blocking and Bridging
Even with perfect 16" O.C. spacing, deep joists (like 2x10s or 2x12s) can twist or warp over time as the wood dries out. To prevent this, builders install "blocking."
Blocking consists of short, perpendicular pieces of wood installed tightly between the joists, usually running in a staggered line down the center of the deck span. This locking grid ties all the joists together, forcing them to share heavy point loads and drastically reducing the "bounce" you feel when walking on the deck.