How the deck joist calculator works
Spacing, span, and stiffness
Joist spacing (12/16/24" on center) sets how many joists run across the deck; joist span (the deck width) sets how deep the lumber must be. A 2x8 PT joist spans ~10.5 ft at 16" OC; a 2x10 reaches ~14. Over-spanning shows up as trampoline bounce within a season — the calculator's warning line is the check inspectors run first.
Composite changes the framing
Composite decking weighs more and flexes more than PT pine, so most manufacturers cap joist spacing at 12–16" — a deck framed at 24" for wood may need doubled joists for composite. The material decision happens twice: once at the register, once in the framing plan. Check the brand's spec sheet before framing.
The connection hardware that matters
Ledger flashing (where deck meets house) is the #1 rot source; joist hangers need structural screws (not deck screws); posts need rated post bases, not concrete burial. Framing lumber is commodity — the hardware is where code, safety, and longevity actually live, and it prices in at ~10% of framing cost.