How far can a 2x10 SPF span?

A house I quoted in Yangyang last year had a 4.2m living room. I picked SPF #2 2x10 at 400mm spacing per the Canada Wood table. A year later: noticeable rafter sag at the eaves. Reason? Yangyang is in Gangwon Yeongdong — snow load 3.0 kN/m², six times the typical 0.5. I'd sized for 0.5.

Bending vs deflection — which hits first?

Two checks: bending (won't break) and deflection (won't feel bouncy). For residential members, deflection almost always governs. Bending could go further but deflection cuts you short.

Korean snow load is the wildcard

Most of Korea: 0.5 kN/m². Gangwon Yeongdong (Sokcho, Yangyang, Gangneung, Donghae, Samcheok) and Ulleung Island: 3.0. Six-fold difference. Sites in those regions need rafters stepped up one or two sizes.

What this calculator omits

Honestly, only Cr (repetitive member factor) is applied from NDS's seven adjustment factors. CD, CL, CF, CM, Ct, Ci, Cfu are not. Results run about 10–15% conservative vs the IRC tables. Safe side, but a structural engineer should re-run with full NDS for cost optimization on larger jobs.

Bottom line

Deflection beats bending. Snow zones change everything. This tool is conservative. Use the span calculator for first-pass sizing, then have a structural engineer verify.