9.3 Wood Framing, Species/Grades, and Fasteners

9.3 Wood Framing, Species/Grades, and Fasteners

Wood framing is CSI Division 06 10 00 (Rough Carpentry). Your references are IBC Chapter 23, the AWC National Design Specification (NDS) for Wood Construction, and the AWC Wood Frame Construction Manual (WFCM). Allowable spans come from IBC span tables and the International Residential Code (IRC) for light-frame work. Know nominal vs. actual lumber sizes cold.

Nominal vs. Actual Dimensions

Lumber is sold by nominal size but installed at actual (dressed, S4S) size:

Trap: a 2x10 joist is 9-1/4 in deep, not 10 in. Section properties for span checks use the actual depth.

Species, Grades, and Design Values

Common framing species/groups: Douglas Fir-Larch (DF-L), Southern Pine (SP), Spruce-Pine-Fir (SPF), Hem-Fir. Visual grades from best to worst: Select Structural, No. 1, No. 2, No. 3, Stud, Construction, Standard, Utility. No. 2 is the typical framing default.

Design values from the NDS include Fb (bending), Fv (shear), Fc-perp (compression perpendicular to grain), and E (modulus of elasticity). DF-L No. 2 2x10 has higher Fb than SPF No. 2, so it spans farther.

Span Example and Spacing

Floor joists carry live + dead load and are sized from IRC/IBC tables by species, grade, size, spacing, and deflection limit L/360 for floors (L/240 for roofs with plaster, L/180 typical roof).

Worked example: DF-L No. 2 2x10 joists at 16 in o.c., 40 psf live / 10 psf dead span roughly 16 ft 1 in per the IRC table. Increase spacing to 24 in o.c. and the same joist drops to about 13 ft 1 in. Wider spacing = shorter allowable span. Always confirm against the printed table — do not interpolate from memory.

Header sizing over openings follows IRC Table R602.7: a built-up (2) 2x10 DF-L header spans roughly 7-8 ft carrying one floor plus roof, depending on snow load and building width. Notching and boring limits protect framing: a joist notch may not exceed 1/6 the depth and must avoid the middle third of the span; bored holes are limited to 1/3 the depth, kept 2 in from edges (IRC R502.8). Over-notching a joist for plumbing is a classic field and inspection failure.

Pressure-Treated Lumber and Moisture

Wood in contact with concrete/masonry or exposed to weather must be preservative-treated per AWPA U1 with retention by use category (e.g. UC4A ground contact ~0.40 pcf for ACQ). Treated lumber is corrosive to steel, so fasteners must be hot-dip galvanized (ASTM A153) or stainless — plain or electro-galvanized fasteners fail prematurely. A sill plate on a foundation must be treated and anchored.

Fasteners and the IBC Fastening Schedule

Nails are specified by penny (d) size: a 16d common nail is 3-1/2 in long, a 10d is 3 in, an 8d is 2-1/2 in. The IBC Table 2304.10.1 (Fastening Schedule) dictates nailing — e.g., a stud to top/bottom plate is end-nailed with 2-16d or toe-nailed with 4-8d. Anchor bolts for sill plates are typically 1/2-inch min, 6 ft o.c. max, within 12 in of plate ends (IBC/IRC), with a minimum of two bolts per piece, and embedded at least 7 inches into concrete or grouted masonry.

Moisture content at installation should be 19% or less ('S-DRY' or 'KD' stamp) for dimension lumber; higher MC ('S-GRN') leads to shrinkage, nail-pop, and crowning. Read the grade stamp for mill number, species/group, grade, MC, and grading-agency mark (e.g. WWPA, SPIB). A worked board-foot takeoff: board feet = (thickness-in x width-in x length-ft) / 12. A 2x10 x 16 ft = (2 x 10 x 16) / 12 = 26.7 bd ft; multiply by the joist count for the order.

Common Exam Traps

Don't use nominal dimensions in a structural calc — a 2x12 is 11-1/4 in. Remember wider spacing shortens allowable span, and a stronger species/grade (DF-L No. 2 over SPF No. 2) lengthens it. Match treated lumber with hot-dip galvanized or stainless fasteners, and pull nailing quantities from the IBC fastening schedule, not from habit.

Nominal vs. Actual and Fastener Schedules

Lumber is sold by nominal size but is smaller actual (dressed): a 2×4 is actually 1-1/2 × 3-1/2 in; a 2×10 is 1-1/2 × 9-1/4 in. Nailing follows the IBC/IRC fastening schedule — e.g., a stud to plate uses 2-16d end-nailed or 4-8d toe-nailed; sole plate to joist 16d at 16 in o.c. Use hot-dipped galvanized or stainless fasteners with pressure-treated (ACQ) lumber, because ACQ corrodes plain steel.

Common Exam Traps

• Trap: A 2×4 measures 2×4 in. Actual is 1-1/2 × 3-1/2 in.
• Trap: Plain steel fasteners in treated lumber — ACQ corrodes them; use HDG/stainless.
• Trap: Toe-nail and end-nail counts are interchangeable — follow the fastening schedule.
• Trap: Higher grade number = stronger. Lumber grades like Select Structural > No.1 > No.2 run the other way.