Deck Joist Calculator
Calculates pressure-treated framing joists (spaced 12" OC or 16" OC), double rim joists, ledger board, Simpson Strong-Tie LUC/HUS joist hangers, structural ledger screws, and mid-span solid blocking.
Formula
Joists = Ceil((Length in Inches ÷ Spacing) + 1); Hangers = Joists × 2; Rim Boards = 2 × (Length + Span)
1. Joist Count = Ceil((Deck Length ft × 12) ÷ Spacing in) + 1 2. Field Joists = Joist Count × Joist Span ft 3. Ledger & Rim Boards = 2 × Deck Length + 2 × Joist Span 4. Joist Hangers (Simpson LUC210) = Joist Count (at ledger) + Joist Count (at flush beam) 5. Mid-Span Solid Blocking = Joist Count pieces of 14.5" block
Inputs
- Deck Length (Ledger span along house)
- Joist Span (Projection out from house)
- Joist On-Center Spacing
- Lumber Size
Worked example
Framing a 20-ft ledger with 16-ft joist spans using 2x10 pressure-treated lumber at 16" OC.
Outcome: Joist count is (20 × 12 ÷ 16) + 1 = 16 field joists (each 16 ft long). Ledger & outer rim require four 20-ft 2x10s and two 16-ft 2x10s (total 22 structural 2x10 boards). Joist hangers: 16 Simpson LUC210 hangers. LedgerLOK structural screws: 40 screws.
Why is 12" on-center joist spacing required for composite decking installed on a diagonal?
When deck boards are laid at a 45-degree angle, the effective unsupported distance between joists increases by ~41%. Spacing joists at 12" OC keeps the diagonal span under 17", preventing composite boards from feeling spongy or sagging under foot.