Using the deck joist span calculator is straightforward:

1. Select Wood Species: Choose the type of wood you are using for your deck joists. Different species have different strengths and therefore different allowable spans.
2. Select Joist Size: Choose the nominal size of your joist lumber (e.g., 2x8, 2x10). Larger joists can span longer distances.
3. Select Joist Spacing: Choose your on-center spacing for the joists. The closer the spacing, the longer each individual joist can span.
4. View the Result: The calculator will automatically display the maximum allowable horizontal span for that combination of materials and spacing.

• Joist: A horizontal framing member that supports the deck boards.
• Span: The distance a joist covers between two support points (e.g., between a ledger board and a beam, or between two beams).
• On-Center Spacing: The distance from the center of one joist to the center of the next. Common spacings are 12", 16", and 24".
• Live Load: The weight of movable objects on the deck, such as people, furniture, and snow. The standard for decks is 40 pounds per square foot (PSF).
• Dead Load: The static weight of the decking and framing materials themselves, typically assumed to be 10 PSF.

"Don't automatically choose the maximum possible span. Building with slightly shorter spans or closer joist spacing results in a much stiffer, more solid-feeling deck. While the code might allow a 2x8 to span 12 feet, bringing it in to 11 feet can make a noticeable difference in bounce." - Experienced Carpenter

"These span tables assume standard conditions. If you're planning to put a heavy feature on your deck, like a hot tub or a large outdoor kitchen, you must use a much more robust framing plan. Those are 'concentrated loads' and require specific engineering."

• Confusing Joist Span with Beam Span: Joists are the smaller members that support the decking. Beams are the larger members that support the joists. They have different span tables. This calculator is for joists only.
• Ignoring Cantilevers: The amount a joist can overhang past a beam (a cantilever) is also limited. A common rule of thumb is that a joist can cantilever up to one-fourth of its backspan.
• Using Undersized Lumber: Never be tempted to stretch the span of a smaller joist. A 2x6 is not a substitute for a 2x8, even if it's cheaper. A bouncy or sagging deck is a sign of an undersized frame.
• Not Using Pressure-Treated Lumber: All deck framing lumber must be pressure-treated for ground contact to resist rot and insects.