SPF (Spruce-Pine-Fir) is the most common framing lumber species group in North America. 2x4 stud grade is the workhorse of residential and light commercial wall framing. Design values: Fb (bending) 675 psi, Ft (tension) 350 psi, Fc (compression) 1150 psi, E (modulus) 1.2M psi per NDS. Moisture content: KD19 (kiln-dried to 19% max). Grade stamps: ALSC accredited agencies. Fire rating: unprotected wood has 0-hour rating. 1-hour rating achievable with 5/8 Type X gypsum each side. Spacing: 16 inches OC typical, 24 inches OC for some applications. Code reference: IBC 2308 for conventional light-frame construction.