These are approximate maximum spans for common pressure-treated lumber at 40–60 psf live load with a 10 psf dead load, spaced 12–24" on centre. They're a planning guide only — deflection, cantilevers, snow load and local building regulations (UK: BS 5268 / Eurocode 5, and approved document A) all affect the final size. A joist at 16" centres spanning 12 ft is a typical starting point for a domestic deck.