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 the eight NDS adjustment factors for bending (Table 4.3.1). CD, CL, CF, CM, Ct, Ci, Cfu are not, and shear and bearing are not checked. So treat the results as planning-level estimates for preliminary review; they can differ from the IRC tables. Leaving out a factor that raises the allowable bending stress (snow CD, or size factor CF, which is 1.3–1.1 for 2x6–2x10) makes a bending-governed span shorter, while CD does not apply to E, so a deflection-governed span is unchanged. Leaving out a factor below 1.0 (wet service, high temperature, incising) makes it longer. The tool's base design values are simplified from the NDS tables, so the result can land on either side of a full NDS calculation. Have a structural engineer re-run the design with the full NDS factors before ordering materials.

Bottom line

Deflection beats bending. Snow zones change everything. This tool gives planning-level estimates, not final design values. Use the span calculator for first-pass sizing, then have a structural engineer verify.